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Cadmium photosensitized reactions S. Tsunashima and S. Sato

Department of Applied Physics, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo

Contents

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 II. Experimental methods . . . . . . . . . . . . . . . . . . . . . . . . . 202

IH. The role of metastable atoms, Cd(SPo) . . . . . . . . . . . . . . . . . . 204 IV. Quenching cross-sections . . . . . . . . . . . . . . . . . . . . . . . . 206

A. Chemical method . . . . . . . . . . . . . . . . . . . . . . . . . 206 B. Physical method . . . . . . . . . . . . . . . . . . . . . . . . . . 207

1. Quenching of the resonance line . . . . . . . . . . . . . . . . . . 207 2. Flash photolysis of dimethyl cadmium . . . . . . . . . . . . . . . 209 3. Phase shift studies . . . . . . . . . . . . . . . . . . . . . . . . ' 210

V. Cadmium-photosensitized luminescence . . . . . . . . . . . . . . . . . 210 A. Ammonia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210 B. Amines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213 C. Water, alcohols, and ethers . . . . . . . . . . . . . . . . . . . . . 216

VI. Cadmium-photosensitized reactions . . . . . . . . . . . . . . . . . . . 216 A. Hydrogen and saturated hydrocarbons . . . . . . . . . . . . . . . . 216

I. Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . 216 2. Saturated hydrocarbons . . . . . . . . . . . . . . . . . . . . . 217

B. Unsaturated hydrocarbons . . . . . . . . . . . . . . . . . . . . . 218 1. Ethylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219 2. Propylene . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 3. Butenes . . . . . . . . . . ~ . . . . . . . . . . . . . . . . . 221 4. Fluorinated ethylenes . . . . . . . . . . . . . . . . . . . . . . 221 5. Acetylene . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 6. Benzene . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224

C. Carbonyl compounds . . . . . . . . . . . . . . . . . . . . . . . . 225 1. Acetone . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225 2. Cyclopentanone . . . . . . . . . . . . . . . . . . . . . . . . . 226 3. Aldehydes . . . . . . . . . . . . . . . . . . . . . . . . . . . 227

VII. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228 VIII. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228

1. Introduction

Studies of chemical reactions induced by excited a toms have played i mpor t a n t roles in the fundamenta l unders tand ing of chemical react ions3 N u m e r o u s investigations of mercury photosensi t izat ion have been published, and several excellent reviews are now avai lable3 ~

C a d m i u m belongs to the I Ib group of the periodic table and has an energy level d iagram (cf. Figure 1) similar to that of mercury. By absorpt ion of the resonance line at 326.1 nm, the cadmium a tom is excited to the relatively long- lived aP x level ( r = 2.4 ~s). 5 This excited a tom can give rise to a photosensi t ized

© 1979, by Verlag Chemie International, Inc.

201

202 S. Tsunashima and S. Saw

reaction with a substrate in a manner similar to that of triplet mercury. Com- parison of photosensitized reactions by the two species will provide information on the difference in the reactions attributable to the different excitation energies.

Since Bates and Taylor investigated the Cd-photosensitized reactions of a mixture of ethylene and hydrogen, e several compounds have been subjected to cadmium photosensitization; ethane, 7 propane, 8-1° ethylene, 1°-13 propylene, 13 butenes, 13 and acetylene. 14 Steacie compared the reactions of ethylene induced by triplet cadmium with those induced by other excited metal atoms. 15.16 Decomposition of ethylene into hydrogen and acetylene was observed with mercury photosensitization but was not detected with triplet cadmium photosensitization. This difference was explained by the smaller excitation energy available in cadmium photosensitization. In the higher energy Cd(1P1) - photosensitization, on the other hand, the formation of acetylene, propylene, and a polymer was observed. T M

After the comparison made by Steacie, virtually no publications appeared on triplet cadmium photosensitization until 1967. The reasons may be summarized as follows:

(1) The vapor pressure of cadmium is much less than that of mercury, so that it is necessary to elevate the temperature of the reaction system to conduct experiments.

(2) An intense cadmium lamp cannot be made as easily as a low pressure mercury lamp.

(3) Since the excitation energy of triplet cadmium is only 87.7 kcal tool -I, the number and yield of products was expected to be small.

In 1962 Ishikawa and Noyes reported the benzene-photosensitized emission of biacetyl. 19 Since then, the interest of many investigators has been attracted to reactions photosensitized by organic compounds. 2° The excitation energy of triplet benzene (3B1~) is 84.4 kcal tool- 1, a value which is close to that of triplet cadmium. In 1967 our laboratory showed that the cis-trans isomerization of 2-butene is efficiently sensitized by triplet cadmium. 21 This observation initiated the revival of the study of cadmium-photosensitized reactions.

This paper will briefly review the results obtainedin cadmium-photosensitized reactions which appeared until 1977. In this article, the expression "cadmium photosensitization" means the reaction induced by triplet cadmium, unless otherwise stated.

H. Experimental methods

The technique used for the investigation of cadmium-photosensitized reactions is essentially the same as that for mercury-photosensitized reactions, except for the temperature and light source. The temperature of the reaction system has to be raised to between 200 and 300°C in order to get sufficient cadmium in the vapor phase, i.e., 10-3-10 -2 Torr. 22

Cadmium Photosensitized Reactions 203

Pyrex glass can be used as a filter to eliminate the shorter wavelength resonance line at 228.8 nm. Kalra and Knight used a commercially available cadmium spectral lamp and obtained an intensity of about 4 × 10 -3 t~E rain -x at 326.1 nm. 23 We have made quartz and Pyrex glass discharge lamps, in which a few pieces of cadmium metal and a few Torr of argon were sealed. These lamps were operated at 200-300°C with 1000 V AC and gave about 0.5 t~E rain-1. 24 Breckenridge and Callear constructed a multielectrode cadmium resonance flash lamp 25.26 which had a flash duration of 40 ms, when a 1.0 t~F condenser was powered at 20 kV. Morten et al. used a microwave powered discharge lamp for their phase shift studies. 27 The details were not given. We have also constructed a microwave-powered discharge lamp with Pyrex glass, which contained 10 Torr of helium. 2s To avoid deposition of cadmium metal on the lamp wall, the emitting zone was covered with another Pyrex glass tube and the annular space was evacuated. The intensity of the lamp was lower and less stable compared with the AC powered discharge lamp.

The cis-trans isomerization of 2-butene can be used as a conventional actinometer for the resonance line at 326.1 nm. The quantum yield of isomer formation was first assumed to be 0.5, 24 and this value was later confirmed by Hunziker. 29

The intensity of the light at 326.1 nm that is absorbed by cadmium vapor in a reaction vessel depends on the pressure of reactant because of pressure broadening of the resonance line2 ° The absorbance of the resonance line may be given by relation [I]:

A = 1 - f F(v) exp[ - k(v)/] dv f F(v) dv [hl]

Here, v is the frequency, F(v) the profile of the emission line from the lamp, k(v) the profile of the absorption line, and l the length of the light path.

Umemoto et al. calculated the pressure broadening of the resonance line at 326.1 nm due to argon and ammonia by using relation [I]. sx They assumed that the profile of the emission line depends only on Doppler broadening and that of the absorption line on both Doppler and pressure broadening. Since the hyperfine structures due to the isotopes of cadmium overlap each other, they assumed that the hyperfine structures are composed of two or three imaginary isotopes. On this assumption, they estimated the collision broadening cross-sections of argon and ammonia. The results are shown in Table 1. This table also contains the collision broadening cross-sections obtained by Barger, 32 who used the Hanle effect and the method of modulated light double resonance.

Hence, for an estimation of the absorbance of the reaction system, we have to know the absolute values of many factors such as the collision broadening cross-section, the temperatures of the emitting zone in the lamp and of the reaction cell, and the vapor pressure of the cadmium. Experimentally, however, a conventional method is often used in which the reactant is diluted with an excess amount of inert gas, keeping the total pressure constant because the difference in

204 S. Tsunashima and S. Sato

Table 1. Collision broadening cross-sections (10 - I n m 2) for Cd(aPx).

S u b s t r a t e = ~ e d

He 51 52 Ne 53 52 Ar 77 86 Kr 121 116 Xe 156 174 NHa

73 49

153 100

* Obtained with the Hanle effect, see Reference 32. Obtained with the method of modulated light double

resonance, see Reference 32. * The hyperfine structure is assumed to consist of two

imaginary isotopes in the ratio 92: 8, see Reference 3I. a The hyperfme structure is assumed to consist of three

imaginary isotopes in the ratio 16: 76: 8, see Reference 3 I.

the pressure-broadening cross-sections between reactant and inert gas is much smaller than that of the quenching cross-section for the excited atoms. 33 This method makes the absorbance independent o f changes in the pressure of reactant.

IT/. The role of metastable atoms, Cd(SPo)

The Cd(SP0) state is located at 86. I kcal t oo l - t above the ground state and 1.55 kcal tool - t below the aP t state (Figure I). The energy difference between the Cd(3Pt) and (3P0) states corresponds to the thermal energy, 3/2 kT, at 247°C. Since cadmium-photosensitized reactions are usually studied at 200-300°C, it is

kcolimol "! 180

160

140

120

100

80 60

40 20

0

3D 3 3D 2 3D~

Ipt - ' ~ • --,-- ~,..,\ ..~' ,..~."~,.~/

/ • e. P2" P1 PO

1S o

Figure 1. Energy level diagram of cadmium.


expected that the equilibrium between the (aPz) and (3Po) states is readily established in the presence of a foreign gas

Cd(3Pz) + M ~ Cd(aPo) + M

Breckenfidge and Callear observed the formation of Cd(aP0) atoms along with Cd(aP1) atoms in the presence of helium, nitrogen, or methane (100-750 Tort) at 235°C upon irradiation with the resonance line at 228.8 rim. 28 Young et al. flash photolyzed dimethyl cadmium at 25°C and observed Cd(aPz) and Cd(3P0) atoms by the absorptions at 346.6 and 340.4 nm, respectively. They also found that the decay rates of both states were the same in the presence of an excess amount of argon, methane, or isobutane. This observation led them to conclude that the equilibrium between the two states was easily established under their experimental conditions, a~.ss

Takaoka et al. observed the Cd(aPo) state in cadmium-photosensitized reactions under steady illumination and obtained the following results : as'aT

(1) Equilibrium between (3Pz) and (aPo) states is established in the presence of 1 Torr of helium, neon, argon, krypton, or xenon at 250°C.

(2) The efficiency of nitrogen in generating the Cd(aPo) state from Cd(aPt) is more than 10 a higher than that of noble gases, and ammonia is more efficient than nitrogen.

(3) The pressure dependence of the concentration of the Cd(aP0) state can be explained by the following reaction mechanism:

Cd + hv(326.1 nm) > Cd(aPz) (1)

Cd(apz) , Cd + hv (2)

Cd(aPz) + M . " Cd(aPo) + M (3)

Cd(aPo ) w.a> Cd (4)

Cd(aPt) + M > quenching (5)

Cd(aPo) + M > quenching (6)

where M denotes a foreign gas. This reaction mechanism is essentially the same as that proposed by Kimbell and LeRoy in mercury photosensitization, except for the absence of the radiative decay of the Hg(aPo) state. 38

The energy difference between the 3P t and 3P o states of mercury is 5.1 kcal mol- t, about three times that of cadmium. Consequently, equilibrium between the two states in mercury photosensitization is not readily established at room temperature, making it possible to investigate the difference in reactivity between these two states. In cadmium photosensitization, however, it is very difficult to distinguish between the reactions of the two states because of the equilibrium established between them. Using a flash resonance lamp, Brecken- ridge et al. measured the concentrations of Cd(aPz) and Cd(aP0) in the presence of helium, argon, nitrogen, and methane, and estimated the lower limit of the


rate constant for the forward direction of reaction (3) to be 6 x 10 e I mol -~ s-1 for helium and 2 × 107 1 mo1-1 s -x for argon, nitrogen, and methane, s9'4°

IV. Quenching cross-sections

For quantitative interpretation of the results obtained in cadmium photosensitization, it is essential to know the absolute values of the quenching cross- sections of the gases used. These values are important requirements for elucidating the mechanism of energy transfer and the process of energy dissipation.

Quenching cross-sections for Cd(3P1) have been estimated by measuring the pressure dependence of the intensity of the resonance fluorescence. This method was used by Bates, .1 Bender, *~ Lipson and Mitchell, 43 and Steacie and LeRoy. .4 In this method, the effective radiative lifetime of the triplet cadmium has to be estimated since the lifetime is prolonged by radiation "imprisonment" which depends on the temperature and the pressure of the reaction system.

A purely chemical method which is not affected by imprisonment has been proposed to estimate values of relative quenching efficiencies; it involves the cis-trans isomerization of 2-butene as a competitive reaction. 2x,45 When the pressures of 2-butene and quencher are properly chosen, the method is not affected by pressure broadening.

A. Chemical method

Cis-trans isomerization of 2-butene occurs readily in cadmium photosensitization, at a rate which is independent of the ratio of the two isomers. 21"2~ The formation of products other than the two isomers was found to be negligibly small. The relative quenching efficiencies of several olefins and acetylene were measured by this competitive method31.~5 The estimation is based on the following reaction mechanism:

Cd + hv(326.i rim) --> Cd*

Cd* + B---> Cd + B* [kb]

Cd* + Q --> quenching [k,]

B* + B (or Q) ---> 1/2 cis + I/2 trans + B (or Q)

where B and Q represent 2-butene and a quencher, respectively, and the asterisk indicates the species in the excited triplet state. Steady-state treatment gives the following relation:

Ro kq[Q] 1 + kb[B] [Iq

where Ro and R are the rates of isomerization of 2-butene in the absence and in the presence of the quencher molecule, respectively. The relative quenching


efficiency, kJkb, can be estimated from the slope of the linear relationship between Ro/R and [Q]/[B]. The quenching efficiencies thus obtained are shown in the second column of Table 2, where the efficiency of 2-butene is assumed to be unity.

When 2-pentene is used as quencher, two cis-trans isomerizations occur. The relative quenching efficiency calculated by using the cis-trans isomerization of 2-pentene agreed with that calculated by using that of 2-butene) s

Using this chemical method, we could show that the quenching efficiencies of all the olefins examined are almost the same and are about 10 a times that of propane, aL4S Tsunashima and Strausz showed that the quenching efficiency of fluorinated ethylene decreases with an increase in the number of fluorine atoms. 46 This trend is the same as that obtained in mercury photosensitizationY

Since 2-butene is reactive to radicals, this method cannot easily be applied to quencher molecules which produce radicals upon cadmium photosensitization. Yamamoto et al. measured the relative quenching efficiency of carbon dioxide using the 2-butene method and then used carbon dioxide as the standard for the estimation of the relative quenching efficiencies of acetone and three aldehydes, 4a since these compounds decompose into radicals upon cadmium photosensitization. The values obtained are also listed in the second column of Table 2.

B. Physical method

1. Quenching of the resonance line

Steacie and LeRoy estimated the quenching cross-sections of several gases by measuring the pressure dependence of the fluorescence intensity. ~4 This method was based on the following reaction mechanism:

Cd + hv(326.1 nm) --* Cd(aP1)

Cd(3P1) --> Cd + hv 1/r

Cd(aP1) + Q -+ quenching kq

Here Q denotes a quencher molecule. By the steady-state treatment relation [III] can be obtained,

I0/I = l + 7kq[Q] [III]

where Io and I are the emission intensities of the resonance line at 326. l nm from the reaction vessel in the absence and in the presence of quencher, respectively. The quenching efficiency kQ can be determined from the slope of the Stern- Volmer plots, using the effective radiative lifetime ~- of Cd(aP1). Steacie and LeRoy used the natural lifetime of Cd(aP1) as the effective radiative lifetime. Since their measurements were carried out at 213°C, the error due to neglect of imprisonment may be ignoredJ 4

The quenching rate can be converted to the quenching cross-section, q2, using the following relation:

kq = (~2(8*rkT/g) 1~2 [IV]


Table 2. Relative q~enching efficiencies and quenching cross-sections (10 -~ nm ~) for triplet cadmium.

Relative Quenching cross-section Quencher quenching efficiency ~ (1) b (2) e

He < 0.0002 j, < 0.0003 ~ He < 0.0008 k Ar < 0.0008 j, < 0.0008 ~ H2 0.8 a 0.67 h, 3.541 4.3 j, 3.9 ~ HD 4.3 ~, 3.9 ~ D2 0.19 h, 1.8 ~ 2.3 j, 2.0 ~ N~ 0.021 ~ 0.0084 j, 0.0076 ~, 0.014 l NO 18 ~ NHa 0.041 h, 0.052 ~ 0 .044 ~ CO 1.7 k, 2.2 l CO2 0.16" 12 ~, 12.5 ~ SO~, > 40 ~ SFe 2.3 j, 2.6 ~ CH4 0.012'


where/z is the reduced mass and other symbols have their conventional signifi- cance. The quenching cross-sections obtained by Steacie and LeRoy are also listed in the third column of Table 2.

Recently Breckenridge et aL have measured the quenching cross-sections of several compounds at 280°C, using a flash resonance lamp. The reaction vessel was filled with an excess of nitrogen, argon, helium, or methane, a9.4° They calculated the ratio of the effective lifetime to the natural lifetime of Cd(3P1) by using the modified Milne theory. In the case of argon at a pressure of 400 Torr at 280°C, the ratio was 1.21. At 10/zs after the flash, they measured the relative concentrations of CdH, Cd(3Po), and Cd(aP1) by plate photometry of absorption bands at 248.3, 340.4, and 346.6 nm, respectively and estimated the quenching cross-sections from these data. Under their experimental conditions, the equilibrium between Cd(aPx) and Cd(aPo) is established; therefore, the observed quenching cross-sections correspond to the sum of the cross-sections for Cd(aPt) and Cd(3Po), i.e., o~(aP~)+ Ko2(3po) where K is the equilibrium constant for the reaction

kt Cd(apx) + M . " Cd(3Po) + M

kr

Their results are shown in the fourth column of Table 2.

2. Flash photolysis of dimethyl cadmium

Because of the low vapor pressure of cadmium, cadmium photosensitization was usually carded out at elevated temperatures. In order to obtain quenching eti~ciencies at lower temperatures, Young et aL used the flash photolysis of dirnethyl cadmium as the source of cadmium vapor and measured the time dependence of the concentration of Cd(aPz) and Cd(aPo) in the absence and in the presence of quencher molecules, a4.as Since their method can be applied over a wide range of temperature, they could estimate activation energies for the quenching cross-sections. The results are shown in Table 3. Their experimental quenching efficiencies correspond to (kf/(kt + kr))k(3Pa) + (kr/(kt + kr))k(3Po), where k(aP1) and k(aPo) are the quenching etiiciencies for Cd(aPz) and Cd(aPo), respectively. Their quenching efficiencies are all one or two orders of magnitude smaller than those reported in the literature for quenching Cd(ap1). Young et al.

• K. Yamamoto, S. Tsunashima, and S. Sato, Bull. Chem. Soc. Japan, 46, 3677 (1973). i S. Tsunashima, O. Oosawa, C. Takahashi, and S. Sato, Bull. Chem. Soc. Japan, 46, 83

(1973). g S. Tsunashima and O. P. Strausz, unpublished results.

H. C. Lipson and A. C. G. Mitchell, Phys. Rev., 48, 625 (1935). t E. W. R. Steacie and D. J. LeRoy, J. Chem. Phys., 11, 164 (1943). J W. H. Breckenridge and T. W. Broadbent, Chem. Phys. Lett., 29, 421 (1974). k W. H. Breckenridge, T. W. Broadbent, and D. S. Moore, J. Phys. Chem., 79, 1233

(1975). S. Yamamoto, M. Takaoka, S. Tsunashima, and S. Sato, Bull. Chem. Soe. Japan, 48,

130 (1975).


Table 3. Arrhenius parameters for the quenching of Cd(3Po). 35

log A, E,, Quencher in I mol-1 s- i units kcal mol-x

H2 11.6 5.4 C2H~ 10.6 2.4 C2H~ 10.6 2.4 cis-2-C~Hs 10.3 1.8 C2H3F 11.1 4.0 C~F4 10.7 4.5

tried to interpret the difference by the factor kr/(kf + kr) of k(aPa) in their quenching efficiency.

3. Phase shift studies Recently Morten et aL applied the phase shift method to determine the

radiation trapping time of Cd(1P1) and the quenching cross-sections of several gases for the resonance line at 228.8 nm. ~9 They also made one measurement foi" the quenching of Cd(SP0 by ammonia and found that the quenching cross- section thus obtained agreed with that obtained by other methods. Incidentally, their definition of the quenching cross-section is different from that of relation [IV] by a factor of ~r.

V. Cadmium-photosensitized luminescence

In order to elucidate the nature of the intermediate in mercury photosensitization, Penzes et aL investigated the sensitized emission processes of ammonia, water, and paraffins, s° Since then, the results of many investigations on the mercury-photosensitized luminescence have appeared and several reviews are now available. 51'~ Penzes et aL classified the mercury-photosensitized luminescence into three types: a, b, and c bands. The a band appears at a few angstroms on either side of the 253.7 nm resonance line (noble gases, SF6, CH~, and C2H6). The b band appears as a broad shoulder of the resonance line (noble gases and cycloparaffins). The c band is also broad and structureless and is separated from the resonance line (NHa, amines, water, alcohols, and ethers). 5a

Recently it was shown that band fluorescence similar to the c band can be observed in the cadmium-photosensitized reactions of ammonia, 27"as's4 amines, 5s water, 56 alcohols, 56 and ethers. 56 In this article we will briefly summarize the results of cadmium-photosensitized luminescence.

A. Ammonia

In 1972, Morten et al. observed a band emission at 430 nm in the cadmium- photosensitization of ammonia at 287°C. 2v By analogy with the mercury photosensitization case, they assigned the band fluorescence to emission from the

Cadmium Photasensitized Reactions 211

I I I ! I I

1.0

0.5 ®

0 i " ' " 380 400 420 440 460 480 Wavelength (nm)

Figure 2. Emission band obtained in the cadmium-pbotosensitized luminescence of ammonia at various pressures. (O): 0.8 Torr, (0): 2.9 Torr, (A): 21.5 Torr + 185 Torr (Ar), (O): 93 Torr, (A): 263 Torr.

complex formed between Cd(aPo) and ammonia. In order to explain the experimental results of the phase shift study, they assumed a reaction mechanism similar to that proposed for mercury-photosensitized luminescence and estimated the lifetime of the complex and the bimolecular rate constant for the formation of the complex.

Our laboratory independently investigated the cadmium-photosensitized

I ! I

43 N H 3 J

I I I 0 10 20 30

Pressure of Ammonia (Torr) Figure 3. Stern-Volmer plots for the quenching of the 326.1 nm resonance line by

ammonia and deuterated ammonia.


luminescence at 250°C. 54 Using an AC powered lamp, we found a broad band emission having a maximum of 432 nm and a band width of about 100 nm, as shown in Figure 2. A few years later, we reinvestigated the same system in the presence of an excess of argon, which was added to reduce the effect of pressure broadening, aa Even in the presence of argon, the Stern-Volmer plot for the quenching of the resonance line at 326.1 nm was found to be nonlinear at low pressures of ammonia, as shown in Figure 3. For an explanation of this result, the following reaction mechanism was proposed:

Cd + hv(326.1 tun) --> Cd(aPt) I~

Cd(aPz) --> Cd + hv ko

Cd('~Pt) + Ar(or A) ~- Cd(aP0) + At(or A) kz, k-z (1)

Cd(aPz) + A --> quenching ka (2)

Cd(aP0) + A --~ quenching ka (3)

Cd* + A ,~ CdA* k4, k_ 4 (4)

CdA* + A(or Ar) -+ CdA* + A(or Ar) ks(k'5) (5)

CdA*--> Cd + A + hvt ke (6)

CdA* + A(or At)--> Cd + A + A(or Ar) kT(k'7) (7)

where A denotes an ammonia molecule. CdA* and CdA* are respectively an unstabilized and a stabilized complex formed between Cd* and A. Cd* denotes Cd(aPt) and/or Cd(3Po). In this mechanism, the unstabilized complex CdA~* is assumed not to fluoresce.

The steady-state treatment on the assumption that the equilibration reaction between the aP t and aPo states of cadmium is very fast, gives the following relation with respect to the quenching of the resonance radiation:

I°28.z/I3a6.1 = I + a[A] + b[A][(l + I/(c[A + Ar] + d[A])) IV]

Here I°26.1 and Ia26.1 are the emission intensities at 326.1 nm in the absence and in the presence of ammonia respectively. The parameters a, b, c, and d are expressed as follows: when Cd* -- Cd(3Pz), a = (ka + kzka/k-~)/ko, b = k4/ko, c = k~/k_ 4, and d = (k~ - k'5)/k_ 4, while in the case of Cd* = Cd(3Po), k4 is replaced by (kz/k_z)k4. The curved Stern-Volmer plots shown in Figure 3 were well explained by assuming appropriate values for the parameters a, b, c, and d in relation V. 33 The Stern-Volmer plots obtained with ammonia-d3 showed a similar trend. 5v

The overall quantum efficiency of the band emission defined as the number of photons emitted from the intermediate complex per quenched Cd(°P~) was measured to be 0.67 + 0.10 in the ammonia pressure range of 5-50 Torr and the total pressure range of I00-200 Tort. 5s In the mercury-photosensitized luminescence of ammonia, the quantum efficiency was reported to be 0.70 _+ 0.14. ss


In the case of mercury photosensitization, there is another emission band at a shorter wavelength and it is clearly observed when the pressure of ammonia is lowered by below 1 Torr. 59'8° Recently the phase difference between the two emission bands was measured. 61 In the case of cadmium photosensitization however, no emission band other than that at 432 nm can be observed. ~7

B. Amines

As in the case of ammonia, cadmium-photosensitized luminescence can be observed with methylamine, ethylamine, n-propylamine, n-butylamine, sec- butylamine, tert-butylamine, dimethylamine, diethylamine, trimethylamine, and triethylamine at 220°C. 5s,62 The band shape of the luminescence was similar to that observed with ammonia, but the wavelengths of peak intensity were shifted to the red. The peak wavelengths and the quantum e~ciencies of the band emissions are listed in Table 4. For comparison, the peak wavelengths are plotted in Figure 4 against those obtained in mercury-photosensitized luminescence. As Figure 4 shows, the order of the peak wavelength is almost the same as that observed in mercury photosensitization. The peak wavelengths are also plotted as a function of the ionization potentials of amines in Figure 5. This trend

Tsble 4. Wavelengths at the peak of the emission band (,~m,x), quantum efficiencies of the emission (~om), relative values of k4, half-quenching pressures (1t. Q.P.), and quenching cross-sections (~2) obtained in cadmium-photosensitized luminescence.

,~=~x H.Q.P. Substrate (nm) ~,= k~" (Torr) (10 -2 nm 2)

1 NHa 432 0.67 (1.0) - - - - 2 NDa 432 - - 1.0 - - - - 3 CHaNH2 449 0.58 10.7 0.885 1.2 4 CaH~NH2 452 0.26 15.8 0.350 3.5 5 n-CaHTNH2 452 0.20 24.9 0.215 6.2 6 n-C~HaNH2 453 0.18 28.0 0.177 8.1 7 sec-C4HoNH2 453 0.12 17.8 0.108 13.0 8 tert-C4HoNH~ 454 0.04 53.3 0.029 49.0 9 (CHa)2NH 450 0.43 10.0 0.740 1.6

I0 (CHa)aN 435 0.46 5.1 1.13 1.2 11 (C2Hs)2NH > 460 - - 43.1 0.181 7.8 12 (C2Hs)aN >460 - - 15.3 0.191 8.1 13 H20 385 0.0012 0.035 5.38 0.16 14 D20 385 0.0057 0.038 31.5 0.028 15 CHaOH 395 0.0031 0.081 2.4 0.44 16 CHaOD 395 0.0096 0.075 7.7 0.14 17 CDaOH 395 0.0034 0.073 2.7 0.41 18 CDaOD 395 0.0096 0.073 8.0 0.14 19 C2HsOH 400 0.0015 0.091 0.82 1.5 20 CHaOCH3 390 0.048 0.044 92.6 0.013 21 CaHsOC2Hs 398 0.011 0.139 ll.2 0.13 22 tetrahydrofuran 398 0.085 0.189 26.8 0.053

* The value for ammonia is assumed to be unity.


" ~ i i i i i

460

".= 440 ,,o~4 s

o 420 ,.."

.o" ~0 380 . - 13

C I I I I I 280 300 320 340 360

E Xmax in Mercury-Photosensitization (nm) ,,< Figure 4. Plots of the wavelengths of the peak intensity of the emission band ob-

tained in cadmium photosensitization against those obtained in mercury photosensitization. The numbers correspond to the compounds listed in Table 4.

suggests that the complex formed between the triplet cadmium and amine is of the charge transfer type.

The quantum efficiency of the band emission decreased with increasing molecular complexity, which was observed in mercury photosensitization;

460 A

E .~c 440 ¢,.-

c 420 0 ) .

~ 4 0 0 0 Q

a. 380

i i i ,%

0 12 0 I1 ,~ 8 0 0 ~ 4

9 0 6 O ' o 3

0 1 0 5 0 1 \

2t ~ 2 2 0 0 ~ 9 ° 13

15 - - - 0 -

I I I I I 7 8 9 10 11 12 13

Ionization Potential (eV) Figure 5. Plots of the wavelengths of the peak intensity of the emission band

obtained in cadmium photosensitization versus ionization potentials. The numbers correspond to the compounds listed in Table 4.


however, the values obtained in cadmium photosensitization are much larger than those obtained in mercury photosensitization. 6a

The pressure dependence of the luminescence intensities at 326.1 nm and at the peak wavelengths of the band emissions were analyzed in terms of the reaction mechanism proposed for the photosensitized luminescence of ammonia, ss The Stern-Volmer plots for amines, however, were linear in contrast with those for ammonia. This is probably because, in the case of amines, the reverse process of reaction (4) is not important compared with reaction (5). From the slopes of the straight lines, half-quenching pressures were estimated and are given in Table 4. The half-quenching pressure (H.Q.P.) is defined as the pressure at which the intensity of the resonance line is reduced to one half of the intensity obtained in the absence of quencher, i.e.

H.Q.P. = 1/[k2 + (kl/k-1)ka + k4]r = 1/~-kq [VI]

Here kq is the quenching efficiency. If the radiative lifetime of Cd(aPx) in this system is known, it is possible to estimate the quenching cross-section from the half-quenching pressure. Since the measurements on amines were made at 220°C, the radiative lifetime of Cd(aP1) should be close to the natural lifetime, 5s and the calculated quenching cross-sections calculated on this basis are listed in Table 4. As this table shows, the quenching cross-sections increase with increasing molecular complexity and with a decrease in the quantum efficiency of luminescence.

The relative values of k4, the rate constant for complex formation, were also calculated and are listed in Table 4. In Figure 6 the relative values of k4 are plotted as a function of the ionization potential of the amines. The relative values increase with a decrease in ionization potential, which again suggests that the complex is of the charge transfer type.

2.0 \1! = \ o OI v I I ~.0 ' ~ 6

o \ 1.0 12 X o 9 7 o?

z

~ 0.0

0 o 2[

--J -1.0 \ 1 9 o . 0 1 5 . . . . . . 1 3 0 , \ -2 .0 J i J =

7 8 9 10 11 12 13

I on i za t ion P o t e n t i a l ( e V ) Figure 6. Plots of k4/k~(NHa) versus ionization potential. The numbers correspond

to the compounds listed in Table 4.


C. Water, alcohols, and ethers

Cadmium-photosensitized luminescence has also been observed with water, methanol, ethanol, dimethylether, diethylether, and tetrahydrofuran, but not with ethylene oxide) 6 The experimental results were analyzed in a similar way to the case of amines. The observed half-quenching pressures, the peak wavelengths, the quantum efficiencies, and the relative values of k4 are included in Table 4 and in the plots of Figures 4, 5, and 6. The band shape observed with these oxygen-containing compounds is a little narrower than that of nitrogen- containing compounds, and the wavelength at peak intensity shifts to shorter wavelengths. The dependence on molecular complexity and ionization potential is similar to that observed with amines, as shown in Figures 4, 5, and 6. The quantum efficiencies of these compounds, however, are much smaller than those of amines. This is in marked contrast to the case of mercury photosensitization. 64

Using CHaOH, CHaOD, CD3OH, and CDaOD, the isotope effect of quenching of the resonance line at 326.1 nm and of the band emission process has been studied. 6s,66 The results are also listed in Table 4. The band shape was not affected by the substitution of hydrogen atoms in methanol with deuterium atoms. These results were explained by assuming that the triplet cadmium reacts with methanol in two ways:

Cd* + CHaOH-+Cd--H---OCHa (1)

/ H --~ Cd--O

\CH~ (a)

Process (1) corresponds to abstraction of hydrogen atom and is subject to a large isotope effect. Process (2) corresponds to complex formation which is followed by emission, and its isotope effect is expected to be small because it is secondary. The observed kinetic isotope effect agreed well with that calculated by simple theory on the basis of the above mechanism. 65'66 A similar conclusion was reached by comparison of the photosensitized reaction of NHa with that of NDa. s~ In the case of methanol, (I) is the main process and (2) is minor, while in the case of ammonia, reaction equivalent to process (2) is the main process.

VI. Cadmium-photosensitized reactions

A. Hydrogen and saturated hydrocarbons

1. Hydrogen Decomposition of hydrogen is brought about by cadmium-photosensitiza-

tion. When cis-2-butene is present in the system at elevated temperatures, methane, propylene, n-butane, and 1-butene are produced together with trans-2- butene. Products other than trans-2-butene increase with an increase in the


fraction of hydrogen in the reaction mixture. 4s The reaction mechanism may be written as follows:

Cd* + H2--~ CdH + H

Cd* + cis-C~Ha--~ Cd + C,H*

C~H* + M -+ 1/2 cis + 1/2 trans + M

CdH -+ Cd + H

H + C4Ha -+ C~H9

C4H9 --~ CHa + C.~H6

2C4H9 -+ C4HIo + C4Ha

-+ C,HIs

Formation of CdH has to be assumed because the hydrogen bond dissociation energy is larger than the excitation energy of Cd(aP1). The energy available for the abstraction of the hydrogen atom by Cd(3P0 is 104.2 kcal mol- ~ (= 87.7 + 16.5 kcal mol-X). 22 From the quantum yields of propylene and n-butane, the relative quenching efficiency of hydrogen can be estimated. The value obtained was 0.8 that of cis-2-butene.

CdH formation in the cadmium-photosensitized reactions of several hydrocarbons had already been recognized by Be~,der, 6~ Olsen/8 and Steacie and LeRoy. 1° They measured the resonance fluorescence of CdH at 431,450, and 457 nm by using a discharge as exciting source in a mixture of cadmium and hydrogen. Breckenridge and Callear confirmed these observations by using a multielectrode flash lamp. 26 They measured the absorption of CdH and CdD in the wavelength region 200-250 nm and estimated the relative rates of formation of CdH or CdD in the reactions of Cd(aP0 with H2, HD, and D~. Breckenridge et aL calculated the half life of CdH to be 150 t~s in the presence of an excess of argon. The yield of CdH was then used as a product "marker" in the measurement of the quenching cross-sections of hydrocarbons, a~'~°

2. Saturated hydrocarbons

The cadmium-photosensitized decompositions of ethand and propane a-l° were studied by Steacie et aL Recently studies on ethane, 69 cyclopentane, 7° and cyclohexane 23 have appeared. In every case hydrogen formation was observed and abstraction of hydrogen atom by the triplet cadmium was assumed to be the initial process.

Steacie et aL measured the quantum yield of hydrogen in the cadmium- photosensitized decomposition of propane) After correcting for incomplete quenching of Cd(aP1) by propane, they concluded that the quantum yield is unity. In the case of cyclopentane, Kalra and Knight ~° observed that the quantum yield of hydrogen decreased with reaction time, for example, it was 0.8 after an irradiation period of 8 h and 0.2 after 36 h. This is mainly due to the


scavenging of hydrogen atoms by olefins eventually formed in the system. Since the quenching cross-section of olefins and hydrogen are much larger than those of saturated hydrocarbons, only a few percent of olefms or hydrogen formed during the reaction, or introduced into the system as an impurity, are enough to compete with saturated hydrocarbons in quenching Cd(aPt).

The cleavage of the C--C bond is energetically possible in cadmium- photosensitized reactions. McAlduff and Yuan observed the formation of methane in the cadmium-photosensitized decomposition of ethane at 260°C. 69 However, they showed that the formation of methane is reasonably explained in terms of the atomic cracking reaction

H + C2Hs -'+ (Call6)* --," 2CH8

and that there is no evidence for C-C bond cleavage of ethane in a primary step. In conclusion, the cadmium-photosensitized decomposition of saturated

hydrocarbons is essentially the same as that in mercury photosensitization at elevated temperatures.

B. Unsaturated hydrocarbons

Since unsaturated hydrocarbons have low-lying triplet states whose energies are smaller than the excitation energy of Cd(aPx), energy transfer from Cd(aPt) to the unsaturated hydrocarbon is expected to occur. In fact, the quenching cross-sections of unsaturated hydrocarbons are very large.

The present chapter briefly reviews the reactions of ethylene, 7t'72 propylene, 7a

lOOt I- , , , ,

80

60

u_ 4 0 0

2o

0 60 120 180 240 Reoction Time (min}

Figure 7. Variation of the mole fractions of cis-(O), trans-(f~), and asym-(©) dideuteroethylenes with irradiation time from the cadmium-photosensitized isomerization of trans-dideuteroethylene at 275°C. [Reproduced with permission from the National Research Council of Canada, Canadian Journal of Chemistry, 46, 995-998 (1968).]


butenes, 24 fluorinated ethylenes, 74 acetylene, 75 and benzene 2a'Ts induced by cadmium photosensitization.

1. Ethylene

Before 1967, it was thought that ethylene did not react upon cadmium (aP1) photosensitization, although this compound was known to quench triplet cadmium efficiently. In mercury photosensitization, it is well known that ethylene decomposes into acetylene and hydrogen. 2-4 When partially deuterated ethylene is used, hydrogen atom scrambling is observed. 77-70 The results have been explained in terms of the "two excited state mechanism."

We investigated the cadmium-photosensitized reaction of 1,2-dideuteroethylene in 1967 and observed the formation of cis and trans isomers and of 1,1- dideuteroethylene (hereafter abbreviated to asym) with practically no decomposition products/1 Figure 7 shows the irradiation time dependence of the mole fractions of cis-, trans-, and asym-dideuteroethylene. The initial reactant was 3 Torr trans isomer and the temperature was 275°C. The results were explained by the following reaction mechanism, which resembles the "two excited state mechanism" proposed for the mercury-photosensitized decomposition of ethylene.

Cd* + E - ~ C d + E *

E* -+ E**

..+ E t

E* + M--~ 1/2 cis + 1/2 trans + M

E** + M -,'- 1/3 cis + 1/3 trans + 1/3 asym + M

E t -~ I/2 cis + 1/2 trans

(1) (2) (3) (4) (5) (6)

Here, E* and E** are excited ethylene molecules similar to those considered in mercury photosensitization. E* is probably the twisted ethylene triplet and E** is the ethylidene radical. E t is another excited state of ethylene, which stabilizes to either the cis or trans isomer, and is probably a vibrationally excited ground state. In the case of mercury photosensitization, E** is considered to decompose into acetylene and hydrogen. In cadmium photosensitization, this process can be omitted from the mechanism because no hydrogen formation was observed.

Using various organic compounds as photosensitizer, we studied the isomerization of dideuteroethylene a°'al and estimated the relative rates of reaction (2) by measuring the pressure dependence of the products. The relative values of k2 thus obtained decreased with decreasing triplet energy of the photosensitizer and were well explained by the RRK relation

k = A(1 - E°/E) S-1 [VII]

Here E ° is the minimum energy required for reaction (2) and may be approxi- mated to be the difference between the energy of the lowest triplet ethylene and


that of the ethylidene radical. E is the total energy available for reaction (2) and is estimated to be the energy difference between the triplet state of the sensitizer and the lowest triplet state of ethylene, s is the effective number of oscillators in ethylene and is assumed to be 9 in our calculation. 8x'82

A theoretical treatment similar to that for organic sensitizers was applied to an estimation of the relative values of k2 in the cadmium-photosensitized reaction of dideuteroethylene at various temperatures/x Taking into account the heat capacity of the vibrations in ethylene molecule, the experimental values of k2 were well explained by relation [VII]. The consistency thus obtained suggests that reaction of the excited ethylene depends only on the energy transferred from the sensitizer and is not affected by the nature of the sensitizer.

Hunziker made a similar investigation on the cadmium-photosensitized reaction of dideuteroethylene/2 In order to explain his experimental results, he derived a reaction mechanism almost the same as that proposed by us.

2. Propylene

When propylene is photosensitized by cadmium, small amounts of decomposition products are obtained. 13 Hirokami and Sato investigated the cadmium- photosensitized isomerization of propylene-l,3,3,3-d4 at 275°C and compared the results with those obtained in photosensitization by mercury, benzene, and benzene-d8 at room temperature! a Since propylene is a homolog of ethylene, cis-trans isomerization and hydrogen atom scrambling were expected to occur. Propylene-l,3,3,3-d4 is a compound whose cis-, trans-, and hydrogen-scrambled isomers can be distinguished by infrared absorption spectroscopy. When c/s- or trans-propylene-l,3,3,3-d~ was photosensitized by mercury, cadmium, benzene, or benzene-d6, it was found that eis-trans isomerization occurred efficiently in every case. However, the formation of propylene-2,3,3,3-d4, the product of hydrogen scrambling, could not be observed, and the quantum yield of hydrogen scrambling was determined to be less than 10 -a. The ratio of the formation of cis and trans isomers was unity in every case examined. The largest extent of decomposition was observed in mercury photosensitization where the products were 4-methyl-l-pentene, 1,5-hexadiene, 2,3-dimethylbutane, propane, ethane, ethylene, hydrogen, and methane. In the case of cadmium photosensitization, only very small amounts Of these products were observed and none were observed in benzene photosensitization. The initial steps for these reactions may be written as follows:

S* + CDaCH---~CHD--~ S + CDaCH--CHD*

CDzCH=CHD* ~ D + CD2CH--CHD

CDaCH~-------CHD* + M ~ 1/2 cis + 1/2 trans + M

where S* represents the sensitizer in the excited state. In cadmium photosensitization, the last reaction is dominant.


3. Butenes Steacie and LeRoy examined the cadmium-photosensitized reaction of 2-

butene and found that while this compound efficiently quenched the luminescence from Cd(3P1), only very small amounts of products were formed. 18 We reinvestigated this reaction and found that cis-trans isomerization occurred efficiently. 2~ As stated previously, we used this isomerization reaction as the standard for the determination of the relative efficiencies of other compounds in quenching triplet cadmium.

Products other than the cis-trans isomers were found to be hydrogen, methane, and propylene; however, the yields were less than 10 -s times that of the isomer. 24 The initial rate of isomerization was constant at pressures higher than 1 Torr and were the same for the cis and trans isomers in the temperature range 275-350°C. The photostationary ratio of the cis to trans isomer, obtained after prolonged irradiation, was unity. This is similar to that obtained in benzene photosensitization. 8s-ss This reaction may be used as a conventional actinometer for cadmium photosensitization.

When the quenching efficiency of 1-butene was measured, methylcyclo- propane was carefully searched for among the products since this compound was one of the products formed in the mercury-photosensitized decomposition of 1-butene; s however, no evidence was obtained for the formation of methyl- cyclopropane.

4. Fluorinated ethylenes An ethylene molecule in which one of the hydrogen atoms is replaced by

fluorine atom is still expected to decompose into hydrogen fluoride and acetylene upon cadmium photosensitization because of the lower energy required for decomposition:

C2H~ --* H2 + C2H2 AH = 41.7 kcal tool -1

C2H3F --~ HF + C2H2 AH = 18 kcal tool-1

The cadmium photosensitized decomposition of vinyl fluoride was in fact observed at 275°C and the quantum yield at zero pressure was estimated by extrapolation to be unity. 74 This result was compared with those obtained in mercury and benzene photosensitizations. The quantum yields of acetylene formation are plotted in Figure 8 as a function of the pressure of vinyl fluoride. The results of mercury photosensitization were rationalized in terms of the "two excited state mechanism" considered in the mercury-photosensitized decomposition of ethylene:

S* + E - + S + E* (1) E* --~ E** (2)

E* + M - + E + M (3)

E** -+ HF + C2H2 (4) E** + M - + E + M (5)

222 S. Tsunashiraa and S. Sato

| I I I ! I

1.

3= _ U Q8 0

~ 0.6 . m

~ " o °

E 0.4

(2/ 0.2 o -

I I I I 0.0 0 100 200 300 400 500 600

Pressure of Vinyl Fluoride (Torr) Figure 8. Pressure dependence of the quantum yield of acetylene formation in

mercury (27°C), cadmium (275°C), and benzene (27°C) photosensitizations.

Here E denotes vinyl fluoride, E* and E** are the triplet vinyl fluoride and triplet 2-fluoroethylidene radical respectively, and S and M refer to the sensitizer and the third body which deactivates the excited molecule.

In the cadmium- and benzene-photosensitizations on the other hand, the experimental results apparently followed the kinetics predicted by the one excited state mechanism. This discrepancy was explained by assuming that k2/k~ is large in these cases.

When the RRK relation [VII] is applied to the isomerization reaction (2), the Kassel factor E ° can be estimated to be approximately 6.0 kcal mol-l , a6 By combining this value with the rate constant obtained in mercury photosensitization, the value of 9 × 10 l° s -1 can be deduced for the A factor of reaction (2). On the other hand, the rate constant of the decomposition reaction (4) can also be estimated independently by the RRK relation [VII]. On the basis of the experimental data shown in Figure 8, the Kassel parameters are estimated as follows: A = 2.1 x 1012s -z, E ° = 22.4 _+ 1.7kcalmo1-1, and s = 9.5. As seen from these results, the rate constant of the isomerization reaction (2) is insensitive to the excitation energy of the sensitizer but the rate constant of the decomposition reaction (4), owing to the higher activation energy involved, is highly sensitive to the energy content of the sensitizer and becomes the sole rate-controlling factor in cadmium- and benzene-photosensitized reactions.

In the case of difluoroethylene, the two excited state mechanism was again applicable to the results obtained in the mercury-photosensitized reaction, a6 In the cadmium-photosensitized reaction of difluoroethylene, cis-trans isomerization and hydrogen atom scrambling were also observed; however, the quantum yield of cis-trans isomerization increased with an increase in pressure, from about


unity at 100 Torr to about 7 at 700 Torr at 275°C. a7 Obviously chain reactions were involved.

5. Acetylene

It is well known that benzene is one of the main products in the mercury- photosensitized reaction of acetylene. Others are hydrogen and a polymer called cuprene. 2-~ In order to explain benzene formation, two reaction mechanisms have been proposed: one is a radical mechanism in which the intermediates were assumed to be radicals such as C2H and C2H3, and the other is an excited state mechanism in which the intermediate is an excited molecule. 88-g2

The energy available in cadmium-photosensitization is smaller than the C-H bond dissociation energy in acetylene, even if CdH formation is taken into account. If benzene is formed only via the radical mechanism, it should not be formed in the eadmium-photosensitized reaction of acetylene. We investigated the reactions of acetylene photosensitized by cadmium at 270°C and observed the formation of benzene and vinylacetylene. ~s The pressure dependence of the products is shown in Figure 9. Neither hydrogen nor polymer was detected. The following reaction mechanism was proposed:

Cd* + A ---> CdA* (1)

CdA* ~ Cd + A* (2)

CdA* + A ~ Cd + 2A (3)

A* + A ~ A* (4)

--~ 2A (5)

A* + A --~ A* (6)

A* --* 3A (7)

C4H4 + A (8)

C~Hs (9)

A* + A - + C6H8 + A (10)

where A*, A*2, and A* are the excited monomer, dimer, and trimer of acetylene, respectively. The quantum yields of benzene and vinylacetylene formation from acetylene-d2 were smaller than those from acetylene-do. This is probably due to the isotope effect in reactions (8), (9), and (10). The isotopic hydrogen atom distribution in benzene produced in a mixture of acetylene-do and -d2 could not be determined because intermolecular hydrogen-deuterium exchange occurred even in the dark at 270°C. In order to check whether benzene formation was initiated by radicals, hydrogen was added to the reaction system. This resulted in increased benzene formation along with the formation of ethylene and 1,3- butadiene.


.~ 0.12

0.08

g o.o4

I I I

0.00 0 100 200 300

Pressure of Acetylene (Torr) Figure 9. Pressure dependence of benzene and vinylacetylene formation in the

cadmium-photosensitized reaction of acetylene at 270°C. [Reproduced with permission. 75]

6. Benzene The lowest triplet state of benzene (8B1~) is located 84.4 kcal mol-1 above

the ground state. Consequently energy transfer from Cd(aP1) to benzene is expected to occur:

Cd* + Be-+ Cd + Be* (1)

where Be = benzene. This energy transfer was confirmed by Hunziker 29 and Tsunashima et al. 76 in the cadmium-photosensitized reaction of a mixture of benzene and c/s-2-butene.

Hunziker assumed the following reactions were occurring, in addition to reaction (I):

Cd* + Bu-+ Cd + Bu*

Be* --> Be

Be* + Bu-+ Be + Bu*

Be* + Be --~ 2Be

Bu* + M --, 1/2 c/s + 1/2 trans + M

(2) (3) (4) (5) (6)

Here Bu and M denote 2-butene and the third body respectively and the asterisk represents the molecule in the triplet state. Hunziker estimated the lifetime of the triplet benzene (1/ks) to be about 0.3/~s at 317°C and calculated an upper limit of 6 kcal tool-1 for the activation energy of reaction (3) by using a room temperature lifetime of 26/~s. 83

On the basis of a detailed analysis of the pressure dependence of the cis-trans isomefization of 2-butene in the presence of benzene, we found that the following reaction should be included in the above reaction mechanism.

Cd* + Be-=,'-Cd + Be' (7)


Here Be' stands for an excited benzene which cannot sensitize the isomerization of 2-butene, presumably a vibrationally excited ground state of benzene. By including reaction (7) in the mechanism, we estimated the lifetime of the triplet benzene to be of the order of 10 t~s at 270°C for both benzene-do and -ds, which is close to the value obtained at room temperature. We also estimated the quenching efficiency of benzene to Cd(3PI) to be 1.5-2.5 times that of cis-2- butene.

C. Carbonyl compounds

The direct photolysis of carbonyl compounds has been extensively studied for fifty years. ~'~*'gs The first absorption band of carbonyl compounds extends from 350 nm to 200 rim. By the absorption of light in this wavelength region, carbonyl compounds are excited to the n, ~r* or ,r, ,r* singlet states and triplet states are then produced by intersystem crossover from these excited singlet states. If decomposition can take place from both the excited singlet and triplet states, it is difficult to distinguish which state is responsible for the decomposition in direct photolysis experiments.

In cadmium (aP1) photosensitization, only the triplet state is produced and the energy transferred to the compound is about the same as that available by intersystem crossing following absorption in the first absorption band. Thus, the study of cadmium (aP1)-photosensitized reactions of carbonyl compounds is expected to give an insight into the decomposition process.

Carbonyl compounds which have been subjected to cadmium-photosensitized reaction are acetone, 96'9~ cyclopentanone, 98 and three aldehydes, .8 acetaldehyde, propionaldehyde, and butyraldehyde. All these compounds quenched triplet cadmium as efficiently as 2-butene and gave carbon monoxide as one of the main products.

1. Acetone

The cadmium-photosensitized decomposition of acetone has been investigated independently by Kalra and Knight at 255°C 96 and by us at 270°C97; the measurements were mutually consistent. The main products were carbon monoxide, methane, ethane, and methyl ethyl ketone. The following material balance was found to hold.

2[CO] = [CH4] + 2[C2H6] + [C2HsCOCHa]

These products were exactly the same as those obtained in the direct photolysis at 313 nm. It is believed that decomposition of acetone takes place through the triplet state. ~

In order to estimate the contribution of direct photolysis, two series of experiments were carried out using two cadmium resonance lamps in which the pressure broadening of the resonance line is different, so that the ratio of the

226 S. Tsunashima and S. Saw

light intensity utilized for the cadmium-photosensitized reaction to that for the direct photolysis is different. From the acetone pressure dependence of the formation of carbon monoxide, the quantum yield of carbon monoxide was estimated to be 0.3 in the cadmium-photosensitized decomposition of acetone.

From detailed analysis of the pressure dependence of ethane formation, the fifetime of the energized ethane formed by the recombination of methyl radicals was estimated to be 2 x 10-as at 270°C. The calculation was based on the assumption that every collision of the energized ethane with the third body is efficient.

2. Cyclopentanone

The photochemistry of cyclopentanone was a subject of active investigations in the 1960s. 4,~4.95 It is well known that cyclopentanone decomposes into carbon monoxide, ethylene, and cyclobutane, or rearranges to 4-pentenal upon irradiation at 313 nm. The reaction intermediate, however, was not fully under- stood. Srinivasan 9~'99 and Mok I°° proposed a mechanism in which the excited singlet state having an excess amount of vibrational energy is responsible for the decomposition. On the other hand, Breuer and Lee proposed a mechanism in which the triplet state is the intermediate. 1°1 Frey and Lister investigated the photolysis of trans-2,3-dimethylcyclopentanone and suggested that the reaction intermediate is a biradical which can rotate freely3 °~

In a study of the cadmium photosensitization of cyclopentanone, we found that the reaction products are the same as those obtained in the 313 nm direct photolysis, i.e., carbon monoxide, ethylene, cyclobutane, and 4-pentenal) 8 The relative yields of the products were nearly constant at pressures higher than a few Tort. The quantum yield of carbon monoxide was about 0.18 at 273°C and increased with an increase in temperature. When an excess amount of cyclohexane was added, the ratio of 4-pentenal/carbon monoxide increased slightly, while the ethylene/cyclobutane ratio was not affected by pressure or temperature changes. These results were reasonably explained by the following reaction mechanism:

Cd* + S--> Cd + T

T --* CO + 2CaH~(or cyclo-C4Hs)

-~ C4HTCHO

--~S

T + M - ~ C4HTCHO + M

--~S + M

where S and T represent cyclopentanone in the ground and triplet states, respectively. On the basis of the similarities between the direct photolysis and the cadmium-photosensitized reaction, the importance of the triplet state in the direct photolysis was emphasized.


0 20 40 60 80 100 120 140 I ! I I I I I

o j / u 150 1"8 o

~: 100 ~ o ~ E 4 _0 c 50 °o • 2 ° y , , .

1 I I I I I I 00 100 200 300 400 500 600 0

Pressure of Aldehyde (Torr) Figure 10. Quantum yield of carbon monoxide formation v e r s u s the pressure of

aldehyde in cadmium photosensitization at 270°C. (©): acetaldehyde, (O): propionaldehyde, (©): n-butyraldehyde. [Reproduced with permission. ~8]

3. Aldehydes Chain reactions were observed in the cadmium-photosensitized decomposi-

tions of acetaldehyde, propionaldehyde, and n-butyraldehyde at 270°C. ~8 The quantum yields of carbon monoxide for three aldehydes are plotted in Figure 10 as functions of the reactant pressures. The main products other than carbon monoxide are RH(R = CH~, C2Hs, and n-C3HT) and small amounts of R2 and hydrogen. The pressure dependence of these products was well explained by the following sequence of reactions.

C d * + R C H O ~ C d + R + C H O (a)

--~ Cd + RH + CO (b)

R + RCHO ~ RH + RCO

RCO ~ R + CO

HCO ~ H + CO

H + RCHO--~ H2 + RCO

R + R --* R2 (or RH + Olefin)

On the basis of the pressure dependence of the quantum yield of carbon monoxide the ratio of the initial processes, a/b, was estimated to be ~2 for acetaldehyde, and ~0.3 for propionaldehyde and n-butyraldehyde.


VH. Conclusion

The experimental results obta ined in the c a d m i u m (aP1) photosensi t izat ion may be summar ized as follows:

( I ) Molecules which have low-lying triplet states, such as olefins, acetylene, benzene, and carbonyl compounds , quench tr iplet c a d m i u m efficiently and are excited to a tr iplet state.

(2) Molecules which contain an N or O a t o m but have no low-lying tr iplet state, such as ammonia , aliphatic amines, water, alcohols, and ethers, quench triplet c a d m i u m with modera te efficiency and f o r m an intermediate complex which can luminesce.

(3) Satura ted hydrocarbons quench triplet c a d m i u m inefficiently. The ma in quenching process is abstract ion reaction o f a hydrogen a t o m by the tr iplet cadmium.

VIIL References

(I) E. W. R. Steacie, "Atomic and Free Radical Reactions," Reinhold Publishing Co., New York, N.Y., 1954.

(2) H. E. Gunning and O. P. Strausz, Adv. Photochem., 1, 209 0963). (3) R. J. Cvetanovi~, Prog. React. Kinetics, 2, 39 (1964). (4) J. G. Calvert and J. N. Pitts, Jr., "Photochemistry", Wiley, New York, N.Y., 1966. (5) A. R. Schafer, J. Quant. Spectrosc. Radiat. Transfer, 11, 197 (1971). (6) J. R. Bates and H. S. Taylor, J. Amer. Chem. Soc., 50, 771 (1928). (7) E. W. R. Steacie and R. Potvin, J. Chem. Phys., 7, 782 (1939). (8) E. W. R. Steacie, D. J. LeRoy, and R. Potvin, J. Chem. Phys., 9, 306 (1941). (9) P. Agius and B. De B. Darwent, J. Chem. Phys., 20, 1679 (1952).

(10) E. W. R. Steacie and D. J. LeRoy, J. Chem. Phys., 12, 34 (1944). (11) E. W. R. Steacie and R. Potvin, Can. J. Research, B16, 337 (1938). (12) E. W. R. Steacie and R. Potvin, Can. J. Research, B18, 47 (1940). (13) D. J. LeRoy and E. W. R. Steacie, J. Chem. Phys., 10, 683 (1942). (14) D. J. LeRoy and E. W. R. Steacie, J. Chem. Phys., 12, 117 (I944). (15) E. W. R. Steacie, Ann. New York Acad. Sci., 41, 187 (1941). (16) D. J. LeRoy and E. W. R. Steacie, J. Chem. Phys., 10, 676 (1942). (17) E. W. R. Steacie and D. J. LeRoy, J. Chem. Phys., 10, 22 (1942). (18) S. Arai and S. Shida, J. Chem. Phys., 38, 694 (1963). (19) H. Ishikawa and W. A. Noyes, Jr., J. Chem. Phys., 37, 583 (1962). (20) For example: P. S. Engel and B. M. Monroe, Adv. Photochem., 8, 245 (1971). (21) S. Tsunashima and S. Sato, Bull. Chem. Soc. Japan, 40, 2987 (1967). (22) C.R.C. Handbook of Physics and Chemistry, 55, DI61 1974-1975. (23) B. L. Kalra and A. R. Knight, Can. J. Chem., 50, 2010 (1972). (24) S. Tsunashima and S. Sato, Bull. Chem. Soe. Japan., 41, 284 (1968). (25) W. H. Breckenridge and A. B. Callear, Chem. Phys. Lett., 5, 17 (1970). (26) W. H. Breckenridge and A. B. Callear, Trans. Faraday Soc., 67, 2009 (1971). (27) P. D. Morten, C. G. Freeman, M. J. McEwan, R. F. C. Claridge, and L. F. Phillips,

Chem. Phys. Lett., 16, 148 (1972). (28) H. Umemoto, S. Tsunashima, and S. Sato, Bull. Chem. Soc. Japan, 51, 1951 (1978). (29) H. E. Hunziker, J. Chem. Phys., 50, 1294 (1969). (30) A. C. G. Mitchell and M. W. Zemansky, "Resonance Radiation and Excited Atoms,'"

Cambridge University Press, Cambridge, 1934. (31) H. Umemoto, S. Yamamoto, and S. Sato, Bull. Chem. Soc. Japan, 48, 2760 (1975). (32) R. L. Barger, Phys. Rev., 154, 94 (1967). (33) S. Yamamoto, S. Tstmashima, and S. Sato, Bull. Chem. Soc. Japan, 48, 1172 (1975).


(34) P. J. Young, G. Greig, and O. P. Strausz, 3". Amer. Chem. Soc., 92, 413 (1970). (35) P. J. Young, E. Hardwidge, S. Tsunashima, G. Greig, and O. P. Strausz, J. Amer.

Chem. Soc., 96, 1946 (1974). (36) M. Takaoka, S. Yamamoto, S. Tsunashima, and S. Sato, Chem. Left., 343 (1974). (37) S. Yamamoto, M. Takaoka, S. Tsunashima, and S. Sato, Bull. Chem. Soc. Japan, 48,

130 (1975). (38) G. H. Kimbell and D. J. Lcgoy, Can. J. Chem., 38, 1714 (1960). (39) W. H. Breckenridge and T. W. Broadbent, Chem. Phys. Lett., 29, 421 (t974). (40) W. H. Breckenridge, T. W. Broadbent, and D. S. Moore, J. Phys. Chem., 79, 1233

(1975). (41) J. R. Bates, Proc. Nat. Acad. Sci., 14, 849 (1928). (42) P. Bender, Phys. Rev., 36, 1535 (1930). (43) H. C. Lipson and A. C. G. Mitchell, Phys. Rev., 48, 625 (1935). (44) E. W. R. Steacie and D. J. LeRoy, J. Chem. Phys., U, 164 (1943). (45) S. Tsunashima, S. Satoh, and S. Sato, Bull. Chem. Soe. Japan, 42, 239 (1969). (46) S. Tsunashima and O. P. Strausz, unpublished results. (47) A. R. Trobridge and K. R. Jennings, Proc. Chem. Soc., 335 (1964). (48) K. Yamamoto, S. Tsunashima, and S. Sato, Bull. Chem. Soc. Japan, 46, 3677 (1973). (49) P. D. Morten, C. G. Freeman, R. F. C. Claridge, and L. F. Phillips, J. Photochem., 3,

285 (1974--75). (50) S. Penzes, O. P. Strausz, and H. E. Gunning, J. Chem. Phys., 45, 2322 (1966). (51) L. F. Phillips, Ace. Chem. Res., 7, 135 (1974). (52) D. L. King and D. W. Setser, Ann. Rev. Phys. Chem., 27, 407 (1976). (53) O. P. Strausz, J. M. Cambell, S. DePaoli, H. S. Sandhu, and H. E. Gunning, J. Amer.

Chem. Soc., 95, 732 (1973). (54) S. Tsunashima, T. Toyono, and S. Sato, Bull. Chem. Soc. Japan, 46, 2654 (1973). (55) S. Yamamoto and S. Sato, Bull. Chem. Soc. Japan, 48, 1382 (1975). (56) S. Yamamoto, K. Tanaka, and S. Sato, Bull. Chem. Soc. Japan, 48, 2172 (1975). (57) S. Tsunashima, H. Umemoto, and S. Sato, to be submitted for publication. (58) R. H. Newman, C. G. Freeman, M. J. McEwan, g. F. C. Claridge, and L. F. Phillips,

Trans. Faraday Soc., 66, 2827 (1970). (59) T. Hikida, T. Ichimura, and Y. Mori, Chem. Phys. Left., 27, 548 (1974). (60) A. B. Callear and C. G. Freeman, Chem. Phys., 23, 343 (1977). (61) H. Umemoto, S. Tsunashima, and S. Sato, Chem. Phys. Lett., 53, 521 (1978). (62) S. Yamamoto, PhD Thesis, Tokyo Institute of Technology, 1976. (63) C. G. Freeman, M. J. McEwan, R. F. C. Claridge, and L. F. Phillips, Trans. Faraday

Soc., 67, 3247 (1971). (64) C. G. Freeman, M. J. McEwan, R. F. C. Claridge, and L. F. Phillips, Trans. Faraday

Soe., 67, 2567 (1971). (65) S. Tsunashima, K. Morita, and S. Sato, Chem. Left., 453 (1976). (66) S. Tsunashima, K. Morita, and S. Sato, Bull. Chem. Soc. Japan, 50, 2283 (1977). (67) P. Bender, Phys. Rev., 36, 1543 (1930). (68) L. O. Olsen, J. Chem. Phys., 6, 307 (1938). (69) E. J. McAlduff and Y. H. Yuan, J. Photochem., 5, 297 (1976). (70) B. L. Kalra and A. R. Knight, Can. J. Chem., 54, 77 (1976). (71) S. Tsunashima, S. Hirokami, and S. Sato, Can. J. Chem., 46, 995 (1968). (72) H. E. Hunziker, J. Chem. Phys., 50, 1288 (1969). (73) S. Hirokami and S. Sato, Bull. Chem. Soc. Japan, 43, 2389 (1970). (74) S. Tsunashima, H. E. Gunning, and O. P. Strausz, J. Amer. Chem. Soc., 98, 1690

(1976). (75) S. Tsunashima and S. Sato, Bull. Chem. Soc. Japan, 41, 2281 (1968). (76) S. Tsunashima, S. Satoh, and S. Sato, Bull. Chem. Soc. Japan, 42, 1531 (1969). (77) A. B. Callear and R. J. Cvetanovi6, J. Chem. Phys., 24, 873 (1956). (78) D. W. Setser, D. W. Placzek, R. J. Cvetanovid, and B. S. Rabinovitch, Can. J. Chem.,

40, 2179 (1962). (79) D. W. Setser, B. S. Rabinovitch, and D. W. Placzek, J. Amer. Chem. Sac., 85, 862

(1963). (80) T. Terao, S. Hirokami, S. Sato, and R. J. Cvetanovi6, Can. J. Chem., 44, 2173 (1966). (81) S. Hirokami and S. Sato, Can. J. Chem., 45, 3181 (1967).


(82) S. Sato, Pure Appl. Chem., 16, 87 (1968). (83) S. Sato, K. Kikuchi, and M. Tanaka, J. Chem. Phys., 39, 239 (1963). (84) M. Tanaka, T. Terao, and S. Sato, Bull. Chem. Soc. Japan, 38, I645 (1965). (85) M. Tanaka, M. Kato, and S. Sato, Bull. Chem. Soc. Japan, 39, 1423 (1966). (86) O. P. Strausz, R. J. Norstrom, D. Salahub, R. K. Gosavi, H. E. Gunning, and I. G.

Csizrnadia, J. Amer. Chem. Soc., 92, 6395 (1970). (87) S. Tsunashima and O. P. Strausz, unpublished results. (88) D. J. LeRoy, J. Chem. Phys., 45, 3482 (1966). (89) P. Kebarle, J. Chem. Phys., 39, 2218 (1963). (90) S. Shida, Z. Kuri, and T. Furuoya, J. Chem. Phys., 28, 131 (1958). (91) S. Shida and M. Tsukada, J. Chem. Phys., 44, 3133 (1966). (92) S. Shida, M. Tsukada, and T. Oka, J. Chem. Phys., 45, 3483 (1966). (93) C. S. Parmenter and B. L. Ring, J. Chem. Phys., 46, 1998 (1967). (94) R. Srinivasan, Adv. Photoehem., 1, 83 (1963). (95) R. B. Cundall and A. S. Davis, Prog. React. Kinetics, 4, 149 (1967). (96) B. L. Kalra and A. R. Knight, Can. J. Chem., 48, 1333 (1970). (97) S. Sato, C. Takahashi, and S. Tsunashima, Bull. Chem. Soc. Japan, 43, 1319 (1970). (98) S. Tsunashima, O. Oosawa, C. Takahashi, and S. Sato, Bull. Chem. Soc. Japan, 46, 83

(1973). (99) R. Srinivasan, J. Amer. Chem. Sac., 83, 4344, 4348 (1961).

(100) C. Y. Mok, J. Phys. Chem., 74, 1432 (1970). (101) G. M. Breuer and E. K. C. Lee, J. Phys. Chem., 75, 989 (1971). (102) H. M. Frey and D. H. Lister, J. Chem. Sac. A, 627 (1970).

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