Indian Journal of Chemistry Vol. 43A. March 2004. pp. 481-486

h >

UV /visible lun1inescence from photo products in infrared multiphoton dissociation of aery lonitrile

K K Pushpa, Awadhesh Kumar, P D Naik & A V Sapre*

Radiation Chemistry and Chemical Dynamics Division, Bhabha Atomic Research Centre, Mumbai 400 085. India Email : pdnaik @apsara.barc.ernet.in

Received J I Janua ry 2003: revised 9 January 2004

Infrared multi photon dissociation (IRMPD) studies arc cartied out on acrylonittile at pressures of- 10 torr by irradiation at 976 em·' by a TEA CO, laser. The stable products on irradiation have been found to be H, . C, H, and C,H. as measured by gas chromatography and FfiR. With increased laser energy, the radical pathways compete with the molecular elimination channels leading to extensive decomposition of acrylonitrile. A broad visible luminescence (350-750 nm) has been observed which is assigned to the electronically excited carbenc intermediates. Results on IRMPD are compared with those on UV laser induced di ssociation of acrylonitrile.

Cyano-substituted unsaturated hydrocarbons are found to be present in the interstellar medium. A lot of work has been devoted to acrylonitrile (ACRN), the smallest member of this group to obtain the effect of cyano group on the dissociation mechanism. Infrared Multi photon Dissociation (IRMPD) of ACRN has been studied quite extensively to obtain the dissociation channel from the ground state potential without the interference from the surface effect

1.5

• It was found that the pressure has a strong influence on the emission observed in the UV/visible region, in IRMPD of ACRN by CO, laser excitation. At low pressure ( <100 mtorr) the IRMPD of ACRN

1 results into two broad band

structureless features: one at 390 nm and the other in the IR region. These emissions are difficult to assign to any known species. At higher pressures of > 100 mtorr an additional emission from excited CN was observed. ACRN was shown to be a good and efficient source of both C2 and CN radicals

2-4

. The C2 radical is formed in the ground electronic state, while CN radical is formed both in ground and a

31t" excited state

3·

4• At

193 nm photolysis of ACRN both CN and C 2 are formed in the electronicaliy excited state

6• Carbene

radical is also formed by photolysis of ACRN at 193 nm. In the earlier work from our laboratory on the IRMPD of halocarbons, visible emission from carbene radicals was obtained 12

-14

•

In the present work, the detailed product analysis on IRMPD of acrylonitrile was carried out to elucidate the dissociation mechanism. The transient chemical species formed in the excited state were detected employing time resolved luminescence technique.

Meterials and Methods IRMPD studies on the ACRN were carried out in

the gas phase under static conditions at torr level pressures in a cross-four-window Pyrex glass cell having two pairs of polished KCI end windows, one pair for irradiation and the other for IR spectral measurements

12-

14• The phot~lysis cell was evacuated

to pressures of <0.0 1 ton·. Th: sample was degassed by several freeze-pump-thaw cycles and was filled into the photolysis cell through an all glass vacuum system.

A grating tuned (914 to 990 em·' and 1019-1092 em-') multimode transversely excited atmospheric (TEA) C02 laser was used (Lambda Physik: model EMG 201-E) for irradiations. The 976 em·' [10R(20)l line of the C02 laser was used for photolysis. The energy of the laser output was measured using a pyroelectric power meter (Gentec, ED-200). The temporal profile of the pulse was measured with the help of a home made MCT detector and a typical laser pulse consisted of an initial spike lasting 100 ns followed by few j..ls tail.

482 INDIAN 1 CHEM, SEC A, MARCH 2004

FfiR and GC techniques were used to identify the stable photoproducts. A FfiR spectrometer (Mattson, Cygnus-1 00, USA) equipped with deuterated triglycine sulphate detector was employed to measure the I~ absorption spectra in the range of 4000 to 500 em with a resolution of0.4 em·'. For GC analysis, the gas after irradiation was transferred from the photolysis cell to a GC sampling device, by a modified Toeppler pump assembly. By this procedure large volume of the gas at lower pressure was compressed to a known small volume at a known pressure for quantitative a nalysis. A homemade GC was used for the characterization of the photolysed products. Three columns namely Silica Gel, Porapak Q and Molecular Sieve 5A were used to analyze the photoproducts. Silica Gel and Porapak Q columns were used in conjunction with flame ionization detector, while with molecular sieve column thermal conductivity detector was used. Nitrogen was used as a carrier gas in all the cases .

Time resolved luminescence was used to monitor the transient species. The laser beam was focused into the cell using a Ge lens (f= I 0 em). The detection system consisted of PMT, power supply, digital oscilloscope (Tektronix TDS 220, 100 MHz) and the

13

750

Wavelength (Microns)

14 15

HCN

•

700 650 -I

Wavenumber (em )

16 l7

600

Fig. 1- ln frared absorption spectrum of acrylonitrile ( I 0 torr) before and after irradiation for 600 pulses at the laser energy of 450 mJ/pulse.

related electronics. The signal was averaged over 20 pulses to get better signal to noise ratio.

Results and Discussion

Stable product analysis

Figure 1 shows the relevant p01tion of lR spectra of ACRN before and after irradiation . TheIR spectrum of the irradiated sample shows strong new peaks at 729 cm·'and 713 em·'. The peak at 729 em·' is attributed to acetylene. The two weaker peaks obtained at 3317.7 and 3331.3 em·' (not shown in the figure), due to C-H bond stretching of CH=CH confirms the assignment. The peak at 713 em·' is assigned to the HCN product.

GC analysis of irradiated ACRN was performed with Porapak Q, silica gel and mol ecular sieve columns. Hydrogen, acety lene and ethylene were found to be the major products. In Table 1, the yields of hydrogen, acetylene and ethylene are tabulated as a function of pulse energy. The yields are normalized with respect to H2 yield, obtained at the pulse energy of 302 mJ. At low energies, the main products are H2

and HCN, formed via the molecular elimination channel. The other thermodynamically feasible channel, involving radical pathways, starts competing at energy (>487 mJ) , thus reducing yields of the molecular products. These results are in agreement with UV light induced dissociation of ACRN

15• The

products are mainly H 2 and HCN (i.e. molecular elimination) at A = 213.9 nm, with a decrease in the yield at lower wavelengths because of the onset of the radical channels.

Table I- Products yields obtained on the IRMPD of

acrylonitrile at different pulse energies

Pressure: 9.0 ± 0.5 to1T, nom1alized to 9 torr

Energy(mJ/pulse) H2 C2H2 C2H.

545 2.78 3.33 0.62

505 3.85 3.35 0.76

487 6.56 7.25 1.40

451 4.41 3.24 1.38

378 2.89 2.20 0.87

352 1.54 I. 22 0.28

3 16 1.04 1.30 0.22

302 1.00* 1.12 0.21

*Normalized with respect to this value of H2: 9.44 ~tmol observed

PUSHPA eta/.: INFRARED MULTIPHOTON DISSOCIATION OF ACRYLONITRILE 483

0.10

0.08

0.06 q) (.) c c-;j

..Cl 0.04 ....

0 Vl

..Cl ~

0.02

0.00

0 200 400 600 800 1 000 1200

Number of laser pulses

Fig. 2- Plot of absorbance of C, H 2

at 729 em·' versus number of laser pulses with energy 450 mJ/pulsc.

To understand whether C2H2 undergoes further dissociation, effect of number of laser pulses on its yield is studied. The sample cell containing ACRN at a pressure of 10 torr was irradiated with increasing number of laser pulses of the same energy (450 mJ/ pulse) and the increase in absorbance due to acetylene at 729 cm-

1 was measured by FTIR. The plot of

absorbance against the number of pulses is given in Fig. 2, which indicates that acetylene once formed does not decompose further and also it does not reach saturation under the photolysis conditions used.

The possible primary channels of dissociation3

· 11

are the following :

C,H,CN ~ H, + C,HCN 111-1' = 176 kJ/mol ... ( I )

C,H,CN ~ HCN + C,H, Ml' = 182 kJ/mol ... (2)

C,H,CN ~ H + C,H,CN Ml' = 450 kJ /mol ... (3)

C,H,CN ~ CN + C,H, Ml' =532 kJ/mol ... (4)

where fl.H0

is taken at 298 K. The same primary dissociation channels have been identified

6 in the photo

dissociation of ACRN at 193 nm6

. This indicates that at 193 nm excitation, dissociation occurs on the ground electronic surface, following internal conversion from the initially optically prepared state

10•

The contribution of different channels in IRMPD depends on the extent of energy available with the vibrationally excited molecule. However, reaction (4) cannot compete with reaction (3) at the lower level excitation conditions of our experiments. However, Wittig et al.

3 have observed this channel , as the

fluences used in their experiments are very high (30-150 J cm.2)

1•3

• Even in the UV laser photolysis of ACRN at 193 nm, the CN elimination channel (reaction 4) is a minor one (<1%)

10. From fl.~ values it is clear

that the reactions (1) and (2) should be favoured over (3) under the low fluences used in the present work . However, activation energies of these reactions govern the relative yields of the product channels. A recent ab initio molecular orbital calculation

16 predicts lower

activation barrier for both molecular channels, H2

(reaction 1) and HCN (reaction 2) elimination , in comparison to the radical channels (reactions 3 and 4). Three-centre (1 ,1) elimination of HCN producing singlet vinylidene

16 (£ .. = 421.7 kJ/mol) is preferred to

the four-centre elimination (£ .. = 458.6 kJ/mol). Also, three-centre elimination of H2 (£ .. = 402.5 kJ/mol ) is predicted to be the lowest energy channel. Thus, molecular elimination (reactions 1 and 2) predominates over bond scission (reactions 3 and 4).

In addition to primary reactions 1 - 4. secondary channels are also possible. The observed product ethy lene cannot be formed by direct channel. Two possibilities exist for its formation via reactions 5 and 6.

C2H, + H ~ C21-1•

C, H, + H, ~ C2H4

M f = -450 kJ/mol

11H' = -166.8 kJ/mol

... (5)

.. (6)

Both reactions (5) and (6) are thermodynamically feasible. The reaction 5, radical-radical combination reaction, which is kinetically unfavourable at very low pressure, may be quite feasible at pressure of about 10 torr used in the present studies. The reaction 5, which has four-center transition state, appears to have a very high activation barrier.

C2H2 can be produced in the primary reaction (2) involving a,f)-HCN molecular elimination. Alternatively, it can be generat' d after rearrangement of H 2C=C: carbene produced via a ,a-HCN elimination. The activation barrier for the a,a-HCN elimination being lower, this is the most probable route for C2H2 formation .

Effect of lluence

For studying fluence dependence, the sample cell

484 INDIAN J CHEM, SEC A, MARCH 2004

containing ACRN at a pressure of 10 torr was irradiated with the same number of pulses (600) at different tluences. The tluence was varied both by changing high voltage of the laser power supply and by putting polythene sheets as attenuator in the path of the laser beam.

The yield '<!> (P)' of a product and the tluence 'I' are related by the expression

<!> (P) = kl"

Hence, log <1> (P) = log k + n log I

So log <1> (P) versus log I plot will be linear with slope 'n', where 'n' represents the number of photons absorbed by the molecule and k is a constant depending on experimental conditions. Since absorbance is proportional to yield, log absorbance versus log I plot will also be linear with the same slope 'n' (Fig. 3). The slope and hence the photonicity of the IRMPD process is found to be three from Fig. 3. The ACRN molecule absorbs more than 30 IR photons before it dissociates. The observed photonicity of 3 probably implies that absorption of 3 photons brings the ACRN molecule to the quasi-continuum region of the vibrational manifold.

-0.4 v u c:: c<l

.D .... 0 V)

.D -0.6 c<l '-" 00 .2

-0.8

0.75 0.80 0.85 0.90 0.95 1.00 1.05

log I

Fig. 3- Plot of log (absorbance) of C2H

2 at 729 em·• versus log I

(i.e f1l.'ence in Jcm·2).

UV/visible luminescence

Transient species formed during IRMPD process were monitored by their luminescence properties. We ascertained that the luminescence originated in the focal region of the C02 beam and it is dependent on the IR wavelengths of excitation. Hence, it was not due to the electrical breakdown. Wittig et al. had also shown emission from IRMPD of ACRN

1• The identity

of the emitting species was not clear at lower pressure ( < 100 mtorr). At pressures > 100 mtorr they could get emission from CN* whose spectral features are quite distinct. In the present work the total emission was collected at a pulse energy of 500 mJ and the spectrum was taken at intervals of -10 nm in the wavelength range of 350-650 nm. The spectrum is shown in Fig. 4 and the quenching rate constants as well as the life times obtained at different wavelengths are shown in Table 2. A representative temporal profile of luminescence at 380 nm using a filter is given in inset the Fig. 4. Natural life times obtained (Table 2) at different wavelengths indicate that there are at least two types of species, which are responsible for the emission. We cannot rule out some contribution from c2 * to visible luminescence since total fragmentation

30

25

---. 20 .... c:: :::l

..ci .... ~ 15 ~ ·v; c:: ~

E 10 -5

• I • c ·u; c Q) ...... c

• 0 2 4 6 8

I Time

• I • I fit

350 400 450 500 550 600 650 700

Wavelength (nm)

Fig. 4- UV/visible luminescence spectrum of acrylonitrile at - 10 torr and laser pulse energy 500 mJ . Inset: Temporal profile

of luminescence at 380 nm.

PUSHPA eta/.: INFRARED MULTIPHOTON DISSOCIATION OF ACRYLONITRILE 485

Table 2- Quenching rate coefficients obtained from visible

luminescence studies at different wavelengths

Wavelength (nm)

405 480 520 580

Quenching rate coefficient

(em' molecules· 1 s· 1)

(x 10 ·12)

7.1

3.2

3.6

3.5

I d I. . 19

can ea to t 11s spectes .

Natural life ti me

( ~lS)

2.1 2.2

3.0

In our earlier IRMPD 12•

14 studies of 1,2-dibromo-

1, 1-d ifluoroethane, 2-bromo-2-chloro-1, 1, 1-tritluoroethane and 1 ,2-dichloro-1, 1-ditluoroethane a strong UV/visible luminescence was observed and was assigned to the formation of photoproduct carbenes. In the present study also, UV/visible emission can be attributed to the electronically excited carbenes, such as H2C=C: and :C=CHCN, which are generated after a,a-elimination (three-center) ofHCN and hydrogen, respectively during the IRMPD of ACRN (as discussed earlier). Electronically excited triplet vinylidene radicals (CH2=C:) have been observed following the vacuum UV photolysis of C 2H 3CN , C 2H" and C H 1. 11. 1s H I Bl k. 1 1o -

2 4 · . owever, recent y an et a . observed the formation of singlet vinylidene radical instead of the triplet from photolysis of ACRN at 193 nm.

There exists a possibility of other carbenes, a­cyanoethylidene (:C(CN)CH3) after the shift of the a­H (Ea = 299.6 kJ/mol) and ~-cyanoethylidene (:C(H)CH2CN) after the shift of the CN group (E, =36 1.5 kJ/mol) or the ~-H (Ea = 407.5 kJ/mol)

16• Even

at the semi-empirical PM3 level, we could calculate the transition state structure for a-H shift giving :C (CN)CH3 with the activation energy of 312.5 kJ/mol.

Thus, energetically the most probable carbene is a-cyanoethylidene and hence the most probable candidate for the observed emission in our experiment. Most of this carbene returns to the parent molecule because of the low activation batTier (22.2 kJ/mol) for the reverse reaction

16• This carbene can also undergo

di ssociation eliminating molecular H 2 (£,1=426.3

kJ/mol) and HCN (£"=397 .5 kJ/mol ) with overall activa tion barrier comparable to that of direct molecular elimination channels.

The generation of electronically excited carbenes

during IRMPD can be explained by the absorption of additional photons from the laser pulse by nascent carbenes. If carbenes are produced in the quasi­continuum they can have resonant or near-resonant absorption ofiR radiation of any frequency. Therefore , these highly vibrationally excited carbenes formed in the ground electronic state can relax to a higher electronic state from which UV /v isible emission occurs. Alternatively, the carbenes can be produced in the higher electronic state in the primary step itself acquiring sufficient energy from the partitioning of the available energy with the parent molecule. This possibility appears to be difficult on the bas is of a large energy requirement.

Study with scavengers

Bromine and hydrogen chloride are commonly used as scavengers to trap radical species formed during photolysis and were added to ACRN sample to get an evidence for the free radical pathways. It was found that they react with ACRN before photolysis (dark reactions). Hence the studies with scavengers are inconclusive.

Conclusion

IRMPD of acrylonitrile was studied using IR pulses from a C02 laser at 976 em·'. ACRN produces hydrogen, acetylene and ethylene as stable products in good yields. Linear dependence of acetylene yield on laser energy and number of pulses indicates that acetylene does not undergo secondary dissociation within conditions of our experiment on irradiation of ACRN. Visible luminescence is observed in the wavelength range of 350 to 700 nm, which is assigned to the electronically excited intermediates. The relative yields of the products depend strongly on laser energy. At lower energies ( <487 mJ/pulse) the molecular channels involving elimination of H 2 and HCN predominate. However, at increased energies the radical channels involving C-H and C-CN bond cleavages start competing with the molecular channels. A suitable reaction scheme has been proposed to explain the observed products.

Acknowledgement

Thanks are due to Shri Bibeka Nanda Kar for carrying out some preliminary experiments. The authors acknowledge the help rendered by Dr. V S Kamble while using FfiR spectrometer.

486 INDIAN J CHEM, SEC A. MARCH 2004

References

Yu M H, Reisler H. Mangir M & Witti g C. Chem Phvs Lett, 62 ( 1979) 439. .

2 Reisler H, Mangir M & Wittig C. J Clwn Phys, 71 ( 1979) 2109.

3 Yu M H. Levy M R & Witti g C, J Chem Phys. 72 ( 1980) 3789.

4 Miller C M & Zare R N. Chem Phys Lett. 71 ( 1980) 876.

5 Ashfold M N R, Hancock G & Hardaker M L, J Plwtochem, 14 ( 1980) 85.

6 Nishi N, Shinohara H & Hanazaki I, J Chem Phys. 77 ( 1982) 246.

7 Fahr A & Laufer A H, J Phys Chem, 96 ( 1992) 4217.

8 Bird C A & Donaldson D J. Chem Phys Lett. 249 ( 1996) 40.

9 North S W & Hall G E, Chem Phys Le11. 263 ( 1996) 148.

10 Blank D A. Suits A G. Lee Y T, North S W & Hall G E. J

Chem Phys. I 08 (1998) 5784.

II Guo J, Canington T & Filseth S V. J Chem Phys. 115 (2001 )

8411.

12 Pushpa K K, ~~umar A, Naik PD. Rao K A. Parthasarathy V, Sarkar S K & M ittal J P, Chem Phys Lell. 279 ( 1997) 172.

13 Pushpa K K, Kumar A, Naik P D, Rao K A, Mittal J P. Parthasarathy V & Sarkar S K. Chem Phys Leu, 240 ( 1995)

489.

14 Pushpa K K, Kumar A, Naik P D. Rao K A, Mittal J P.

Parthasarathy V & Sarkar S K. Chem Phys l ... ?tl, 249 ( 1996)

167.

15 Gandini A & Hackett P A, Can J Chem, 56 ( 1978) 2096.

16 Derecskei-Kovacs A & North S W, J Chem Phys. II 0 ( 1999) 2862.

17 Laufe r A H, Chem Phys Letf, 94 ( 1983) 240.

18 Fahr A & Lauffer A H, J Pho10chem, 34 ( 1986) 261 .

19 Lupo D W & Quack M, Chem Rrv. 87 ( 1987) 181.