structure, harmonic force field and hyperfine coupling constants of nitrosyl chloride

J. CHEM. SOC. FARADAY TRANS., 1995, 91(19), 3347-3355 3347

Structure, Harmonic Force Field and Hyperf ine Coupling Constants of Nitrosyl Chloride

Bethany Gatehouse, Holger S. P. Muller,? Nils Heinekingt and Michael C. L. Gerry* Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, B.C., Canada V6T IZI

The pure rotational spectra of five isotopic species of nitrosyl chloride were measured using a cavity pulsed microwave Fourier-transform spectrometer. Some a-type transitions of all five isotopomers, and some weak b-type transitions of four of these isotopomers were measured in the 4-26 GHz frequency range. Precise values for the rotational constants and the quartic centrifugal distortion constants were obtained. The rotational constants were used in structure determinations and the centrifugal distortion constants were used in a refinement of the harmonic general valence force field. A harmonic central valence force field was also calculated.

Hyperfine structure in these transitions arising from quadrupole and spin-rotation coupling interactions was also observed. Diagonal and off-diagonal quadrupole coupling constants and diagonal spin-rotation coupling constants of both the chlorine and nitrogen nuclei were determined. The principal quadrupole coupling constants were evaluated and used to calculate the approximate ionic character of the N-CI bond. The spin- rotational coupling constants were used to calculate the diamagnetic shielding factor for the nitrogen nucleus ; the magnitude of this value indicates a fairly ionic N-CI bond.

The N-C1 bond of nitrosyl chloride is very long and weak, and rapidly undergoes photolysis' to yield the products C1 and NO. Although nitrosyl chloride is only a minor component in the atmosphere, it is of interest to environmental chemists because atomic chlorine catalyses the conversion of 0, to 0,. Atomic chlorine is an important contributor to the depletion of the ozone layer. It is therefore of interest to study nitrosyl chloride in order to elucidate information about the N-Cl bond.

Although nitrosyl chloride has been well studied, the analysis is by no means complete. Previous microwave studies, with the exception of one investigation2 of 35Cl 14N l60 (hereafter referred to as ClNO; isotopic substi- tutions will be indicated by using superscripts), have reported only a-type transitions, and have thus been able to determine only two of the three rotational constants. Quartic centrifugal distortion constants have been determined,3 at least in part, for only two isotopomers: ClNO and 37ClN0. Some hyperfine structure due to quadrupole interactions has been resolved and quadrupole coupling constants of the 35/37Cl and I4N n u ~ l e i ~ , ~ have been evaluated. However, these constants have not been determined with much precision, and there has been no resolution of further hyperfine splittings nor any recogni- tion of deviations in the hyperfine structure due to the magnetic interaction between the spins of the C1 and N nuclei with the overall rotation of the molecule. There have also been many low-resolution studies of various isotopomers of nitrosyl chloride6 and high-resolution studies of ClN0,7-9 in the IR region. These data have been combined with microwave data in order to determine an equilibrium structure and force field for the m~lecu le .~ With the harmonic wavenumbers known to only a moderate degree of precision and with very few centrifugal distortion constants known, however, only five of the six quadratic force constants could be determined; the sixth constant had to be held fixed to a value determined by an SCF ab initio study.

The primary goal of this study was to investigate the hyperfine structure of some a-type transitions in order to

t Present address: Jet Propulsion Laboratory, California Institute

$ Present address: Humbolt-Universitat zu Berlin, Berlin, of Technology, Pasadena, CA 91 109, USA.

Germany.

determine the spin-rotation coupling constants of the chlorine and nitrogen nuclei, and to determine more precisely the known quadrupole coupling constants. These constants can be used to obtain information about the type of bonding that is involved in the molecule. During the course of this work, it was found that there were deviations in the hyperfine structure of some of the transitions because of near degeneracies between some of the rotational energy levels. Consequently, the scope of the study was extended to search for some b-type lines so that both the off-diagonal quadrupole coupling ccn- stants and the A rotational constants could be unam- biguo us1 y determined.

This study was also seen as an opportunity to test the T o ,

r,-based, and r, structure determination methods proposed or improved by and to compare the results with those of previous structure ana ly~es .~ A ground-state average, rZ, structure was also evaluated. To calculate the rz structure, a harmonic force field had to be calculated; this was done by refining the previously determined force field through the inclusion of the ground-state inertial defects of four of the isotopomers studied, the inertial defects of the (1, 0, 0), (0, 1, 0), and (0, 0, 1) excited vibrational states of the ClNO isotopomer, and the centrifugal distortion constants determined in this study.

Experimental The experiments were carried out in the 4-26 GHz frequency range using the Balle-Flygare type14 cavity pulsed microwave Fourier-transform (MWFT) spectrometer described earlier.' In this instrument, gas-phase samples entrained in an inert carrier gas are introduced into the cavity via a super- sonic expansion through a General Valve nozzle; because the nozzle is positioned such that the molecular beam is coaxial with the microwave propagation, all observed lines are split into two Doppler components. The widths of single, well resolved lines are ca. 7 kHz (fwhm) and the line positions are accurate to 1 kHz. In the case of unresolvable lines, only those components with an intensity greater than 25% of the strongest component were included in the fits; those with a theoretical splitting of less than 1 kHz were given an uncertainty of 1 kHz while those with a larger theoretical splitting, and thus an increased linewidth, were given an

Publ

ishe

d on

01

Janu

ary

1995

. Dow

nloa

ded

by U

nive

rsity

of

Cal

ifor

nia

- Sa

nta

Cru

z on

27/

10/2

014

02:2

9:57

. View Article Online / Journal Homepage / Table of Contents for this issue

http://dx.doi.org/10.1039/ft9959103347

http://pubs.rsc.org/en/journals/journal/FT

http://pubs.rsc.org/en/journals/journal/FT?issueid=FT1995_91_19

3348 J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91

uncertainty of + 2 kHz. In order to avoid distortions due to overlap effects in power spectra, frequencies of closely spaced lines were determined by fitting to the time-domain signals.16

The samples were prepared by mixing together gaseous nitric oxide and chlorine according to the reaction C1, + 2NO+2ClNO; at room temperature, the equilibrium for this reaction lies far to the right ( K = 2.6 x lo7 at 20°C). The nitrosyl chloride formed was reactive toward metal so a glass sample reservoir was used. The composition of the samples used was adjusted according to the energies of the rotational levels involved in the observed transitions. Leaner mixtures result in a lower temperature being achieved in the molecular beam; only the lowest rotational energy levels are populated. For this reason, low-J transitions were measured using samples consisting of roughly 1% C1, and 2% N O in 2 atm of Ne and high4 transitions were measured using a more concentrated mixture containing ca. 10% C1, and 20% N O in 2 atm Ar.

An isotopically enriched sample was necessary to measure some a-type transitions of 37Cl "NO and some weak b-type transitions of both C1 "NO and 37Cl "NO. Labelled nitric oxide was prepared from a 99% 15N sodium nitrite sample obtained from Isotech. The sodium nitrite was reacted with sulfuric acid in order to liberate "NzO3 which subsequently decomposed to yield the products "NO and "NO,. These products were separated by fractional condensation.

Observed Spectra and Analysis Nitrosyl chloride is a near-symmetric prolate molecule with C, symmetry. It has a large dipole component along the a axis and a small dipole component along the b axis and, thus, its rotational spectrum is expected to exhibit strong a-type and weak b-type transitions. Some of these transitions will show hyperfine structure arising from both quadrupole and spin-rotation interactions. The splittings due to the chlorine nucleus are much larger than those due to the nitrogen nucleus so the serial coupling scheme, J + ZcI = FcI; FcI + IN = F, is used.

Because this molecule has been extensively studied in the microwave region in the past, a-type rotational transitions having J < 2 of the ClNO, 37ClN0, C1 15N0, and ClN isotopomers were easily found; a-type transitions of 37Cl 5 N 0 were observed later using a "N-labelled sample. The data for the first four isotopomers were fitted to the rotational constants, the quartic centrifugal distortion constants, the quadrupole coupling constants of chlorine and nitrogen where appropriate, and the spin-rotation coupling constants of both chlorine and nitrogen. The program used was Pickett's exact least-squares fitting program SPFIT.' Watson's S reduction is generally more suitable for fitting data of a near-symmetric top such as ClNO ( K = -0.991); however, the fits were done using the A reduction because it was used in previous studies, and because there was no advantage to using the S reduction since only a-type and b-type transitions having K , d 1 were measured.

While these preliminary data fitted fairly well for most isotopomers, with standard deviations around 1 kHz, those of C1 'NO did not. There were several hyperfine components of the 212-1 transition that were shifted from their calculated positions by as much as 94 kHz. These deviations were attributed to the fact that the only non-zero off-diagonal quadrupole coupling constant, X a b , of the quadrupolar chlorine nucleus had not been accounted for. In the case of an interaction between energy levels of the correct symmetry, AF =0 , A J = O , f1 , f 2 ; K , K , =eet*oe or eo-oo for ClNO, the off-diagonal quadrupole coupling constants can have an effect on the hyperfine structure of transitions involv-

ing any of the interacting levels. These effects are usually only noticeable when the levels are separated by a small energy difference. This is the case for C1"NO; the 2,, and 404 energy levels are separated by 254.2 MHz.

Since off-diagonal quadrupole coupling effects had been seen in transitions involving interacting energy levels of the C1 "NO isotopomer, it was of interest to look for similar effects in the spectra of other isotopomers. Near degeneracies of less than 2.5 GHz were found between the 404 and 212 levels of 37C115N0 (2158.4 MHz), the 716 and 808 levels of ClNO (1807.7 MHz), and the 716 and 808 levels of 37ClN0 (25.6 MHz).

Some a-type transitions involving these near-degenerate energy levels were measured. These transitions were seen to show a significant dependence on Xab, and, surprisingly, transitions involving energy levels of the correct symmetry separated by as much as ca. 6500 MHz were also seen to exhibit observable (ca. 3 kHz) off-diagonal effects. With these data included in the fits, Xab of 35/37C1 and 14N were fitted as free parameters. However, the A rotational constant was highly correlated with xaa when only a-type lines were included in the fits. Thus, in order to determine both A and Xab uniquely, some b-type transitions had to be measured.

The A constant of ClNO had previously been determined,' and some b-type transitions were easily found. For the other isotopomers, however, finding b-type transitions could have been difficult since these transitions are much weaker than the a-type transitions, and since the A rotational constants for these other isotopomers had not yet been determined. Furthermore, the A rotational constants of nitrosyl chloride have a significant vibrational contribution and are therefore difficult to predict. As a means of predicting the unknown A constants and, thus, limiting the search range, an r I , e structure was determined. This type of structure determination, described in the Discussion section and in ref. 10 and 11, allows a partial cancellation of the vibrational effects, and is particularly well suited to the prediction of unknown rotational

Using this program, estimates for the unknown A rotational constants were obtained. Subsequently, some b-type transitions of 37ClN0 were predicted using the calculated A constant, and the lines were found within 3 MHz of the prediction. These new lines were then included in a spectral fit, and the resulting rotational constants were used in a new rl , structure determination. The inclusion of the additional A rotational constant in the structure fit resulted in a better prediction of the remaining unknown rotational constants, and this in turn led to a better prediction for the b-type transitions of these isotopomers. Using this iterative procedure, b-type transitions were found, first for C1 "NO, and then for 37C1 ''NO. Ultimately, b-type transitions of four isotopomers, ClNO, 37ClN0, Cl 15N0 and 37Cl I5NO, were found. The transitions of the former two isotopomers were measured in natural abundance, and those of the latter two isotopomers were measured using a "N-labelled sample. An example b-type transition of 37ClN0 which shows a significant dependence on the off-diagonal quadrupole coupling constants of 37Cl and 14N is shown in Fig. 1. The largest shifts in the hyperfine structure, caused by the interaction between the 716 and 808 energy levels, are ca. 1600 kHz.

Because of its low natural abundance, the l80 isotopomer exhibited extremely weak a-type lines, and so none of the weaker b-type transitions were looked for. The value for the A rotational constant of ClN was determined from the final r l , E structure fit. In this fit, the experimentally determined rotational constants of all five isotopomers studied were included. From a consideration of the differences between the observed and calculated rotational constants of

Publ

ishe

d on

01

Janu

ary

1995

. Dow

nloa

ded

by U

nive

rsity

of

Cal

ifor

nia

- Sa

nta

Cru

z on

27/

10/2

014

02:2

9:57

. View Article Online

http://dx.doi.org/10.1039/ft9959103347

J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91 3349

h 9637.8 MHz 9641.4 MHz

measured positions I II I I II I

9637.8 MHz I I \ 19641.4

9637.8-MHz 9641.4 MHz

Fig. 1 Example b-type transition of 37ClN0 that is strongly affected by xab. Upper trace: composite drawing of the measured 8,,-7,, transition. The hyperfine structure is due to quadrupole and spin-rotation interactions. 600 averaging cycles were used to measure each component. Note that each line is split into two Doppler components. Centre trace: a stick diagram of the Doppler-averaged measured positions of the hyperfine components. Lower trace: stick diagram of the line positions that are calculated when the off- diagonal quadrupole coupling constants of both the chlorine and nitrogen nuclei are not included.

the other isotopomers, the uncertainty (la) in the value determined for A of C1N l80 was estimated to be 0.5 MHz.

In total, 488 lines of five isotopomers of nitrosyl chloride were measured. The numbers of hyperfine components observed in each of the measured rotational trasitions are listed in Table 1. All observed hyperfine components of b-type transitions and of transitions involving any of the closely interacting energy levels are listed in Table 2. A complete listing of all observed lines has been submitted to the database MOGADOCT; it may also be obtained by con- tacting the authors.

The values for the spectroscopic constants of all five isotopomers were determined using SPFIT. Initially, fits were done keeping the centrifugal distortion constants AK , 6 , and, in the case of 37Cl "NO and ClN l8O, 6, fixed; the spin- rotation coupling constants C,,(N) of C1 "NO, and Cb,(N) and C,(N) of 37Cl "NO and C1N l80 were also fixed. The resulting values obtained for the determined constants seemed reasonable; however, the uncertainties in the rota-

? Jurgen Vogt, Sektion fur Spektren und Stukturdokumentation, Universitat Ulm, Postfach 4066, D-89069 Ulm, Germany; e-mail : juergen.vogt @chemie.uni-ulrn.de.

tional constants were unreasonably small. In order to obtain more realistic error limits for the rototional constants, the fixed distortion constants were given uncertainties which were propagated through the fit. The input uncertainties for the fixed centrifugal distortion constants of ClNO were obtained from ref. 3. For the other isotopomers, the values for the fixed distortion constants were calculated using the ratio of the parameters predicted by the force field to the constants determined for ClNO in ref. 3 and the uncertainties were assumed to be four times those given for the corresponding quantities of ClNO. The A rotational constant of C1Nl80 was also treated as a constrained parameter for which the value and uncertainty were determined from the final r,, structure fit. Because the uncertainty of A was small, it had negligible effects on the uncertainties of the other constants. Also, for C1N l8O, the 2,, and 404 levels are moder- ately close (ca. 4874 MHz). Inclusion of x,b(c1) = a 29.9 MHz and Xab(N) = f 1.8 MHz as fixed values showed that none of the hyperfine components of the 2,2-111 transition were significantly affected. Because neither A nor x a b could be determined precisely, and because all other constants were only affected within their uncertainties, the off-diagonal quadrupole coupling constants were omitted from the final fit of the C1N '80 data. Some of the spin-rotation coupling constants of C1 "NO, 37Cl "NO and C1N l80 were indeterminate with respect to their uncertainties. Thus, these spin-rotation coupling constants were retained as fixed parameters; no uncertainties in these parameters were propagated through the fits because the uncertainties in these quantities did not affect any of the other determinations. The results of the final fits are presented in Table 3.

Discussion and Conclusions Structure Determination

In this study, precise rotational constants of five isotopomers of ClNO were determined. This information was used to test the structure models proposed by Rudolph, and to calculate a ground-state average structure for the molecule.

The programs RU111J and RU238J were used to calculate ro-typel**' and r,-type', structures, respectively. These programs use least-squares procedures to fit the structural parameters of the molecule to the rotational constants, B,, to the moments of inertia, I , , to the planar moments, P , , or to the isotopic differences between these data.

Vibrational effects can be partially accounted for in the ro- type determinations using two different fitting methods. lo*'

In the first method, an rl, or rp, structure is determined; the

Table 1 Measured transitions of nitrosyl chloride

number of hyperfine components measured

transition 3 5 c 1 1 4 ~ 1 6 0 3 7 ~ 1 1 4 ~ 1 6 0 3 5 c 1 15N 1 6 0 3 7 ~ 1 1 5 ~ 1 6 0 3 5 ~ 1 1 4 ~ 1 8 0

14 16 19 23 12 11 11 12 12

11 12 14

12 13 15 14 14 16 11 16 11

12 12 9

6 14 13 10

8 8

2 4

~

14 16 9 5

Publ

ishe

d on

01

Janu

ary

1995

. Dow

nloa

ded

by U

nive

rsity

of

Cal

ifor

nia

- Sa

nta

Cru

z on

27/

10/2

014

02:2

9:57


http://dx.doi.org/10.1039/ft9959103347


Table 2 Selected transitions of nitrosyl chloride

3 5 ~ 1 1 4 ~ 1 6 0 37C1 14N 1 6 0

obs. - calc. obs. - calc.

obs. UnC. with xub without Xub obs. unc. with Xub without xab J ; o K , - J ~ o K , F& F Fc, F /MHz /kHz /kHz /kHz /MHz /kHz /kHz /kHz

7.5 7.5 6.5 6.5 8.5 8.5 7.5 5.5 6.5 8.5 5.5 7.5 5.5 6.5

8.5 7.5 8.5 8.5 7.5 7.5 8.5 9.5 6.5 9.5 6.5 7.5 9.5 6.5

3.5 6.5 3.5 6.5 3.5 6.5 5.5 4.5 4.5 5.5 4.5 5.5

2.5 5.5 2.5 5.5 5.5 3.5 4.5 3.5 4.5 3.5 4.5 2.5

6.5 6.5 4.5 4.5

2.5 2.5 5.5 5.5 3.5 3.5 4.5

6.5 8.5 5.5 7.5 7.5 9.5 7.5 6.5 6.5 8.5 5.5 6.5 4.5 6.5

7.5 6.5 7.5 9.5 8.5 7.5 8.5 8.5 5.5

10.5 7.5 7.5 9.5 6.5

3.5 6.5 4.5 7.5 2.5 5.5 5.5 4.5 5.5 6.5 3.5 4.5

2.5 5.5 3.5 6.5 4.5 3.5 4.5 4.5 5.5 2.5 3.5 1.5

7.0 6.0 5.0 4.0

3 .O 2.0 6.0 5.0 4.0 3.0 5.0

7.5 6.5 7.5 8.5 6.5 5.5 6.5 7.5 8.5 7.5 8.5 9.5 7.5 7.5 5.5 6.5 6.5 6.5 8.5 8.5 5.5 5.5 6.5 6.5 5.5 4.5 7.5 6.5

6.5 6.5 6.5 5.5 7.5 6.5 7.5 8.5 6.5 7.5 6.5 6.5 7.5 7.5 8.5 7.5 5.5 4.5 8.5 9.5 5.5 6.5 7.5 6.5 8.5 8.5 5.5 5.5

4.5 4.5 7.5 7.5 4.5 5.5 7.5 8.5 4.5 3.5 7.5 6.5 6.5 6.5 5.5 5.5 5.5 6.5 6.5 7.5 5.5 4.5 6.5 5.5

3.5 3.5 6.5 6.5 3.5 4.5 6.5 7.5 6.5 5.5 4.5 4.5 5.5 5.5 4.5 5.5 5.5 6.5 4.5 3.5 5.5 4.5 3.5 2.5

101 12.6547 101 12.9924 101 13.0413 101 13.5540 10114.9213 10 1 15.2692 101 15.6730 10115.8195 10116.1351 101 18.2236 101 18.6396

11920.5579 11921.5054 11921.5440 11921.6456 11921.6738 11922.9152 11922.9828 11923.1935 11923.2164 11923.3544 11923.4485 11923.9019 11924.7590 11924.7914

12499.5743 12499.60 10 12500.8532 12501.0112 12501.1979 12501.2210 12502.0320 12502.0575 12503.2041 12503.3117 12503.42 16 12503.4562

24476.9545 24477.0303 24478.1298 24478.4288 24478.6777 24480.0922 24480.1254 2448 1.1097 24481.3270 2448 1.3482 2448 1.4908

1 .o 1 .o 1 .o 1 .o 1.0 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o

1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1.0 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o

- 0.2 0.0 0.3 0.3

0.3 0.3 0.2

-0.6 -0.1 - 0.4

-0.3

- 0.3 0.4

- 0.9 0.8

- 1.1 -0.1 - 0.6

0.3 0.0 0.7 0.5

- 0.2 0.5

-0.1

-0.1 0.0

-0.1 0.4

- 0.4 - 0.2 - 0.2

0.0 0.1 0.4 0.1

-0.1

0.0 - 0.2 - 0.8

0.2 0.7

-0.5 0.1 0.1 0.3 0.2

- 0.2

- 1.1 - 0.8 - 24.0 - 22.0 - 17.9 - 17.6 - 2.2

0.0 - 23.1 - 19.4 - 0.6

17.7 1.8

17.5 19.3 0.2 1 .o

19.3 0.6

24.6 0.8

24.8 1.3 0.7

22.8

1.9 - 0.3

2.1 0.1 1.8

- 0.5 1.4

-0.3 -0.1

1.8 -0.1

1.4

0.6 - 0.7

0.0 - 0.3

0.2 - 1.5

0.8 - 0.9

1 .o - 0.7

0.4

7.5 8.0 9153.9255 1.0 0.1 - 0.2 7.5 7.0 9153.9314 1.0 - 0.6 - 0.8 5.5 6.0 9156.2175 1.0 1.1 1 .o 5.5 5.0 9156.2218 1.0 - 0.6 - 0.8

3.5 4.0 3.5 3.0

6.5 6.0 21054.9376 4.0 1.2 0.7 4.5 5.0 4.5 4.0 5.5 6.0

6.5 7.0 21054.9262 4.0 - 1.3 - 1.7

9662.9722 9663.2581 9664.7540 9665.1968

9665.9739 9665.465 1 9667.5495

9668.2886 9662.3 188 9664.9 198 9668.2026

9638.2287

9638.4054 9639.6164

9639.6164 9638.0202 9639.7862 9638.294 1

9641.1895 9639.8515

141 77.8741 14177.8804 141 79.1524 14179.2928 14179.4975 141 79.5047 14179.8165 141 79.902 1 141 81.0200 141 8 1.1020 141 8 1.2297 14 1 8 1.2 1 20

25861.6332 25861.6667 25862.8072 25863.065 1 25863.3 159 25864.1798 25864.13 17 25865.1705 25865.3404 25865.3973 25865.4707 25863.2556

1 .o 1.0 1 .o 1 .o

1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o

1 .o

1 .o 2.0

2.0 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1.0 1.0 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o

- 2.3 - 2.3 -0.3 -0.1

- 1.3 - 0.7 - 0.3

0.0 - 2.4 - 0.2 - 0.4

1.5

1.7 1 .o

- 0.7 - 1.6 - 0.7 -0.1

- 0.9 0.2

1 .o - 0.7

0.0 0.0 1.1

- 0.3 0.0 0.1 0.3

- 0.2 -0.1 - 0.4

- 0.9 0.0 0.0

-0.1 - 0.6

0.0 0.5 0.1 0.6 0.4

0.4 - 0.9

65.1 61.4

1580.7 1435.5

173.0 13.9

1330.9

16.5 64.8 - 0.2

1330.9

- 62.8

- 52.5 - 37.8

- 8.7 - 1598.9

- 0.6 - 1556.6

- 6.4 - 1348.2

2.0 - 0.9

1.1 - 0.2

2.3 - 0.5

0.9 - 0.2

0.1 0.6

- 0.2 0.4

- 0.6 - 0.4

0.3 - 0.5 - 1.0 - 0.8

0.9 -0.7

1 .o - 0.3 - 0.6

0.8

3 7 ~ 1 1 5 ~ 1 6 0

22453.6424 4.0 - 1.9 - 1.2 22453.6538 4.0 - 0.8 - 0.2 22453.8089 4.0 - 1.2 - 1.6 22453.8222 4.0 1.5 1.1 22456.0388 4.0 1.5 0.9 22456.0486 4.0 1.7 1.1 22456.2097 4.0 - 1.1 - 0.6

4.5 4.0 5.5 5.0 22456.22 1 1 4.0 0.4 0.9

Publ

ishe

d on

01

Janu

ary

1995

. Dow

nloa

ded

by U

nive

rsity

of

Cal

ifor

nia

- Sa

nta

Cru

z on

27/

10/2

014

02:2

9:57


http://dx.doi.org/10.1039/ft9959103347


Table 24ont inued

Table 2 Selected transitions of nitrosyl chloride

obs. - calc. obs. - calc.

obs. unc. with xab without zob obs. unc. with xab without Xab

JkoKc-JLaKc F& F" Fb F /MHz /kHz /kHz /kHz /MHz /kHz /kHz /kHz

2,,-111 2.5 3.0 1.5 2.0 2.5 2.0 1.5 1.0 1.5 2.0 1.5 2.0 1.5 1.0 1.5 1.0 2.5 2.0 2.5 2.0 2.5 3.0 2.5 3.0 3.5 3.0 2.5 2.0 3.5 4.0 2.5 3.0 1.5 2.0 0.5 1.0 1.5 1.0 0.5 1.0 1.5 1.0 0.5 0.0 0.5 1.0 0.5 1.0 0.5 0.0 0.5 1.0 0.5 1.0 0.5 0.0

2165 1.6264 2 165 1.6264 21655.0902 2 165 5.0902 21658.9427 21658.9575 2 1663.9774 2 1663.992 1 21 668.3999

21668.4371

2.0 2.0 2.0 2.0 1 .o 1 .o 2.0 2.0 1 .o 1 .o

2.9 - 2.2 1.6

- 0.4 0.1 0.1

- 2.6 1.8

-0.1

- 0.7

54.5 21 153.1646 49.4 21153.1646 1.5 21155.9308

51.6 21158.9428 51.5 21158.9613 88.9 21 162.8866 93.3 21162.9013 0.0 2 1 166.4506

21 166.4677

2 1 170.3468 21 170.3673 21 170.3673

-0.4 21 155.9308

-0.7 21166.4876

2.0 2.0 2.0 2.0 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 2.0 2.0

2.3

1.5 - 2.8

- 0.3 - 1.3

1.1 - 1.6 1.8 1.2

- 0.5 - 1.8 1.2

- 1.8 0.5

6.8 1.6 1.5

-0.3 3.0 5.4 7.0 10.3 1.2

- 0.4 - 1.8

0.8 - 2.2

0.0

vibrational effects are taken into account by describing the ground-state moments of inertia by the equation 1; = 1: + E ~ ,

where g = a, b, c, and are the vibration-rotation interaction parameters. As a first-order approximation, the E~ are assumed to be isotopomer independent. The r l ,E and rP ,& structures are identical when the same input data are used; the difference in nomenclature signifies which type of data has been used in the fit. Vibrational effects are taken into account in the second method by fitting to the isotopic differences between the input data. Because isotopic differences are used in this type of fit, the vibrational effects will cancel to the extent that they are isotopically invariant. Structures determined in this manner are designated as rAl or r A p structures. The structural parameters determined using either technique

are identical when the same input data are used; however, each methodology is different. The r l ,E and rp ,& determinations are especially suitable for predictive purposes, and the rA, or rAp determinations may be used to account for Costain's error.l**l The r l , structure determination method was used in this study to predict values for the A rotational constants of four isotopomers of ClNO. This is discussed in the previous section.

Vibrational effects are also partially accounted for in the rs structure determination. In this type of structure calculation, the isotopic differences between the moments of the parent and the substituted molecule are, in principle, the most important parameters,' and, therefore, the vibrational effects partially cancel. It was found empirically that the r , structural

Table 3 Rotational, centrifugal distortion and hyperfine constants of nitrosyl chloride"

87374.4480 (31) 5737.78058 (15) 5376.23181 (15)

6.3444 (21) -61.019 (76) 4394.3 (27)'

0.501940 (57) 40.300 (59)'

-49.05967 (78) 9.38484 (90)

0.98115 (111) f 29.00 (23)

- 8.56752 (78) f 1.85 (65)

42.82 (33) 8.367 (69) 5.308 (73)

1.598 (80) 1.119 (84)

0.45

42.61 ( 5 5 )

~

87269.458 (10) 5601.33053 (41) 5255.87213 (41)

6.0709 (21) - 60.144 (78) 4373.8

0.469805 (57) 38.65 (20)d

- 38.73042 (104) 7.46419 (94)

0.98629 (91) f22.6341 (36)

- 8.57268 (85) f 1.710 (12)

35.17 (37) 6.741 (68) 4.263 (72)

1.501 (82) 1.072 (85)

0.60

42.63 (66)

83459.619 (12) 5693.93966 (47) 5322.28942 (48)

6.253 (10) - 54.25 (19) 3975.7 (100)'

0.51038 (99) 39.61 (20)d

-49.3627 (23) 9.6947 (28)

f28.51 (19) -

83356.002 (45) 5556.24830 (90) 5201.38800 (92)

5.96 (10) - 53.45 (20) 3956.4 (100)'

37.98 (20)' 0.4766 (20)d

-38.9718 (20) 7.7105 (29)

f 24.0 (22) -

84433.00 5439.56510 (50) 5 102.99503 (50)

5.641 (33) - 60.62 (1 3) 4178.8

0.4444 (20)' 35.85 (20)'

-48.3588 (15) 8.6909 (38)

0.9399 (17) -

-8.5232 (50)

40.19 (54) 8.38 (17) 5.28 (20)

-58.4 (13) -2.65 (42) - 1.55f

0.78

33.52 (62) 6.69 (31) 3.85 (54)

- 62.3 (16) -2.17f - 1.52f

1.09

42.23 (66) 7.71 (49) 5.08 (58) 40.3 (15) 1 .521 1.061

0.53 ~~ ~

a Constants determined using the I' representation; uncertainties (la) are given in parentheses. Value and uncertainty are determined by a least-squares fit to the rotational constants of the other isotopomers; see text. Values and uncertainties are those given in ref. 3. Values obtained from a harmonic force field calculation, using ratios of the predicted distortion constants compared to those known for 35Cl 14N l60;

uncertainties estimated as ca. four times those for the corresponding values in ref. 3. Spin-rotation coupling constants are designated by C as recommended in ref. 18. Fixed at values calculated from those determined for 35Cl 14N l60, using ratios of the nuclear g-factors, the nuclear spins and the rotational constants.

Publ

ishe

d on

01

Janu

ary

1995

. Dow

nloa

ded

by U

nive

rsity

of

Cal

ifor

nia

- Sa

nta

Cru

z on

27/

10/2

014

02:2

9:57


http://dx.doi.org/10.1039/ft9959103347


parameters should be equivalent to those of the rAI or rAp structure. An r, structure determination can also use non- trivial first and second-moment conditions in order to determine better the coordinates of atoms which are close to an inertial axis or which have little or no isotopic substitution data.'

Following the recommendation of Rudolph, the ro , r,, and rAp structures, but not the and rz structures, of nitrosyl chloride were calculated by fitting to the principal planar moments with P , omitted because of the planarity of the molecule.13 Costain's error also has been applied in the r, and rAp structure determinations. In the determination of the r, structure, the first and second-moment conditions in the ab plane were used to help determine the position of the oxygen atom; this atom had only a single isotopic substitution for which the A rotational constant had not been experimentally determined. For both the r,-type and the r,-type structure determinations, the data were weighted according to the inverse squares of their uncertainties. At first, the uncertainties were input as those given in Table 3. However, the experimental uncertainties do not reflect the quality of the data. In order to give more appropriate weightings, the uncertainties of ClNO were increased by a factor of three, and those of 37Cl 15N0 were decreased by a factor of two over those given in Table 3; all other uncertainties were kept the same as those given in Table 3.

To evaluate the ground-state average, rz, structure, a harmonic force field was calculated as is described in the next section. The ground-state average rotational constants were obtained by subtracting the harmonic contributions to the a constants from the measured rotational constants. These data were weighted as for the r,-type and r,-type structure determinations. Isotopic variations in the bond lengths were accounted for using the equation:"

(1) where ( u 2 ) and K are the zero-point mean-square amplitude of a given bond and its perpendicular amplitude, and a is the Morse parameter for the bond. Both ( u ' ) and K are where (u ' ) and K are the zero-point mean-square amplitude of a given bond and its perpendicular amplitude, and a is the Morse parameter for the bond. Both ( u 2 ) and K are obtained from the force field, and the Morse parameters are approximated from the corresponding diatomics.' The Morse constant for the N=O bond, 2.549 A- ', was obtained from the tabulated values in ref. 21, and the Morse constant for the N-Cl bond, 1.952 A- ', was calculated using the data for the NCl radical obtained from ref. 22. The rz structure was evaluated using the program MWSTR to fit to the calculated ground-state average rotational constants.

All structures determined in this study are presented in

6r, = $aS(u2) - 6K

Table 4. The structural parameters agree with one another within their stated uncertainties, and, moreover, those of the rAp and rs structures are almost identical. Of these structures, r, is probably the most reliable since it shows the smallest correlations between parameters. This structure is also the best approximation to the equilibrium structure."

Table 4 also gives a comparison of the determined structures with two equilibrium structures calculated in a previous s t ~ d y . ~ These structures were obtained using different methods and are therefore subject to error in different manners. The first method used fit the structural parameters to Be and C, of ClNO and C1 15NO; the accuracy of this structure is limited by the small data set. The uncertainties quoted in Table 4 for this structure are those that were based on the experimental errors in the equilibrium rotational constants; see ref. 3 for details. In the second method, equilibrium rotational constants of a large set of isotopomers were derived from the ground-state values using a cubic force field; the reliability of this method is difficult to assess because the results are dependent upon the accuracy of the force constants, some of which had to be held fixed to the SCF ab initio value^.^^^^ The N-Cl bond length and the ClNO angle of the r,, TAP and rs structures agree with the previously determined equilibrium structures within their uncertainties. This comparison shows that the r,-derived and r,-type structure determination methods proposed by Rudolph give a good estimate of the equilibrium geometry of the molecule.

Force Field

In order to determine a ground-state average structure for ClNO, and to derive values for the undeterminable centrifugal distortion constants, a harmonic force field had to be calculated. This was done by using the program NCAZ4 to refine a previously determined force field3 where the interaction constant fNo, CINO was barely determined, and, thus, was kept fixed to the value determined from an SCF ab initio study. With several centrifugal distortion constants and inertial defects included in the fit, all six quadratic force constants were determined.

The data were weighted according to the inverse squares of their uncertainties. The harmonic wavenumbed of eight isotopomers of ClNO were included in the fit. With the exception of o1 of ClN l80, which was omitted because of a large Fermi interaction between v 1 and 3 v , , the harmonic wavenumbers were given initial uncertainties of 1 cm-'. The ground-state inertial defects of ClNO, 37ClN0, Cl 15N0, and 37Cl "NO, and the inertial defects of the excited (100), (OlO), and (001) vibrational statesg of ClNO, were also used as input data. The uncertainties of the inertial defects were

Table 4 Structural parameters' of nitrosyl chloride

this work

10 rI, E Is ' A P 'z

r(N=O)/A 1.14029 (57) 1.13644 (23) 1.13858 (40) 1.13859 (77) 1.1471 (80) r(N-Cl)/%i 1.97510 (58) 1.97214 (41) 1.97286 (19) 1.97286 (38) 1.9690 (79) L ClNO/degrees 113.291 (30) 113.564 (15) 113.413 (27) 113.413 (52) 11 3.72 (36)

previous study

reb ' eC r(N=O)/A 1.1336 (17) 1.135710 (68) r(N - Cl)/A 1.9745 (17) 1.972626 (67) L ClNO/degrees 113.320 (87) 113.4053 (34)

a See text for details. fit to cubic force constants.

Ref. 3; fit to Be and C , for 35Cl I4N l 6 0 and 35Cl 15N l 6 0 . See this reference for an explanation of uncertainties. Ref. 3;

Publ

ishe

d on

01

Janu

ary

1995

. Dow

nloa

ded

by U

nive

rsity

of

Cal

ifor

nia

- Sa

nta

Cru

z on

27/

10/2

014

02:2

9:57


http://dx.doi.org/10.1039/ft9959103347


obtained by propagating the errors from the rotational constants and the conversion factor BI = 505379.07 (43) MHz u

The refinement of the force field was done by first fitting the previously determined values and then using an iterative procedure, whereby the force constants were adjusted slightly and fitted again in order to reproduce the experimental data better. Preliminary fits showed that the harmonic wavenumbers were poorly reproduced ; their weights, therefore, were increased by a factor of 1000 and those of the centrifugal distortion constants were lowered by a factor of 4. The final force constants and the potential-energy distribution (PED) for the vibrational wavenumbers of the main isotopomer are given in Table 5 together with a comparison with the force constants determined in ref. 3 and with those calculated in high-level ab initio s t ~ d i e s . ~ ~ ~ ' ~ As is shown in this table, the determined constants are in good to very good agreement with both the previously determined and ab initio values. The

A'.

Table 5 energy distribution of the general valence force field

Quadratic Force Constants (100 N m-') and potential-

force constants

f N O f C l N f C I N 0 f N 0 , CIN fN0, ClNO fCIN, ClNO

this work 15.282 1.257 1.248 1.249 0.260 0.109 ref. 3 15.424 1.254 1.299 1.44 0.417" 0.1505 ab initiob 14.758 1.269 1.282 1.238 0.249 0.128 ab initio' 15.248 1.311 1.316 1.328 0.271 0.135

potential-energy distributiond

f N 0 f C l N f C I N 0 f N 0 . CIN fN0, ClNO &IN, ClNO

- - - - 1.027 - - 0.333 0.765 -0.053 - - 0.088

0 1

( 3 2 a 3 0.037 0.759 0.236 -0.096 - 0.074

" Fixed at a value obtained from an SCF ab initio calculation. CCSD(T)/TZ2P; ref. 23. ' CCSD(T)/cc-pVTZ; ref. 25. For

35Cl 14N '('0; only contributions greater than 0.03 are given.

experimental parameters are compared with those calculated from the force field in Table 6. Only the ground-state inertial defects, A*, have been included in this table; the measured and calculated excited vibrational state inertial defects of ClNO are given in a footnote. Considering the moderate precision of the observed harmonic wavenumbers, the agreement with the calculated values is mostly very good. As far as they have been determined, A j , AK , a,, and the ground-state inertial defects are reproduced quite well, and the agreement is reasonable for Aj,, dK, and the excited state inertial defects. Because a large set of input data was used, the present harmonic force field is probably more appropriate than that of ref. 3.

Using data from ref. 7, equilibrium centrifugal distortion constants for ClNO were estimated and included in a second fit. The distortion constants of all other isotopomers were omitted. The input data were not reproduced significantly better than they were in the previous fit.

In Table 7, the quadratic force constants determined for nitrosyl chloride are compared with those of some related compounds: FNO, BrNO, ClCO and C100. As one would expect, the diagonal force constants decrease from FNO to BrNO, and those of BrNO and ClNO are very similar; this trend holds to some extent even for the interaction force constants. The XN bonds of the nitrosyl halides are rather long

Table 7 Force constants" (100 N m-') of XEO Compounds (X = F, C1, Br; E = N, C, 0)

FNOb ClNO' BrNOd ClCO' C100"

f E 0 15.912 15.282 15.254 14.964 10.599 f X E 2.133 1.257 1.1011 1.173 0.509 fXE0 1.8414 1.248 1.0859 0.944 0.810 fE0, XE 1.902 1.249 1.15 1.413 0.604 f E 0 , XEO 0.323 0.260 0.294 0.026 0.294 fxE,xEo 0.2358 0.109 0.1031 0.122 0.032

" Deformation constants normalized to 100 pm bond length. 26. ' This work.

Ref. Ref. 27. " Ref. 30.

Table 6 Comparison of measured" and force-field-calculated parameters

3 5 ~ 1 1 4 ~ 1 6 0 35c1 15N 1 6 0 3 5 ~ 1 1 4 ~ 1 8 0 3Scl 1 . 5 ~ 1 8 0

obs. calc. obs. calc. obs. calc. obs. calc.

1835.6 603.2 336.4

6.3444 -61.019 4394.3

0.501940

0.13922 40.300

1835.7 603.2 334.9

-69.316 6.3093

43 14.0 0.502029

0.13931 35.994

1803.6 588.8 334.3

- 54.25 6.253

3975.8' 0.51038

0.14247 39.62'

1803.5 588.4 332.8

6.210 - 62.05 3903.3

0.50981

0.14259 35.38

1786.1b 595.5 329.3

5.641 - 60.62 4178.8'

0.4444'

0.14222' 35.85'

1786.3 596.3 327.3

5.628 - 68.28 4102.0

0.4440

0.141 68 32.03

1753.3 1753.2 579.8 580.1 323.2 321.4

5.30 - 60.32 3683.6

0.4227

0.14522 30.18

3 7 ~ 1 1 4 ~ 1 6 0 3 7 ~ 1 1 . 5 ~ 1 6 0 3 7 ~ 1 1 4 ~ 1 8 0 3 7 c 1 15N 1 8 0

obs. calc. obs. calc. obs. calc. obs. calc.

1835.6 602.2 332.3

6.0708 -60.144 4373.8'

0.469804

0.13929 38.65'

1835.6 602.1 330.8

6.0373 -68.161 4293.9

0.469708

0.13937 34.52

1803.4 587.8 330.3

5.96 - 53.45 3956.4'

0.4766'

0.14256 37.98'

1803.5 587.4 328.8

5.94 -61.04 3884.3

0.4765

0.14266 33.92

1753.4 1753.2 580.7 58 1.2 327.3 325.6

5.55 -61.41 3702.3

0.4532

0.14515 31.53

1786.3 1786.3 594.5 595.2 325.2 323.2

5.38 - 67.05 4082.2

0.4145

0.141 73 30.67

" o (cm-') from ref. 6, centrifugal distortion constants (kHz) from this study or from ref. 3, and A , (u A') from this study. exp (calc.) Ay of 35Cl 14N '('0: Acl,o,o, = 0.13469 (0.13178), A(o,l,o) = 0.34547 (0.33632), A(,,,,,, = 0.22916 (0.22846). Omitted from the fit because of a large Fermi interaction between v1 and 3v2. ' Derived values; not used in the fit.

Publ

ishe

d on

01

Janu

ary

1995

. Dow

nloa

ded

by U

nive

rsity

of

Cal

ifor

nia

- Sa

nta

Cru

z on

27/

10/2

014

02:2

9:57


http://dx.doi.org/10.1039/ft9959103347


Table 8 Quadratic force constants (100 N m-') and potential- energy distribution of the central valence force field

force constants

14.680 2.640 2.165 1.150 -0.046 -1.758

potential-energy distribution" - - - a1 0.987 -

a3 0.085 1.677 2.186 -0.140 - -2.816

-

a 2 - 0.656 0.053 - - 0.274

" For 35Cl 14N l 6 0 ; only contributions greater than 0.03 are given.

[ r = 1.7535 (20) A, f = 271.1 N m-']28,29 and NC1 [ r = 1.6610791 (19) A , f = 403.9 N m-1],22 whereas the N O bond lengths and force constants differ little from those of free NO. The bond length of free NO is greater but, remark- ably, the force constant is larger. The bonding situation for the nitrosyl halides is very similar to that of the more weakly bonded C100, and quite similar to that of 0 2 F , ClCO and FC0.30 Although the lowest vibrational modes, 03, of the nitrosyl halides have a predominant XN stretching character, they show large 16/180 isotopic shifts (see Table 6). According to ref. 30, these vibrations are better described as vibrations of the X atom against the rest of the molecule, vXPNO, rather than as vibrations of the X atom against the N atom alone, v ~ - ~ . Similarly, m 2 , a deformation mode where the X atom has little involvement in the vibration, can be viewed as an in-plane hindered internal rotation of the NO unit. As has been outlined in ref. 30, the situation is very similar for ClOO and ClCO.

A central valence force field (CVFF), with the coordinates rNO, rCIN and rcl...o, is sometimes preferred31 to a general valence force field (GVFF), with internal coordinates rN0, rCIN and L ~ ~ ~ ~ . The CVFF presented in Table 8 reproduced the input data as well as the GVFF did. And, while the CVFF better shows that the ClNO the N and the 0 atoms are quite similar in terms of molecular forces, the description of the vibrational modes is less instructive than in the GVFF.

Hyperfine Coupling Constants

Complete quadrupole coupling tensors were determined for the 35Cl, 37Cl and 14N nuclei of the ClNO, 37ClN0, C1 15N0 and 37Cl 5 N 0 isotopomers. The tensors were diagonalized to yield values for the principal quadrupole coupling constants, x, , and the angle between the principal quadrupolar z axis and the inertial a-axis, Oza. These results are shown in Table 9. As shown in the table, the principal quadrupole coupling constants of a given nucleus agree within the error limits, and, moreover, the ratio of the 35Cl Lo 37Cl quadrupole coupling constants (x:: : x:: = 1.26894 & 0.00013) agrees with the accepted value.32 The quadrupolar z axis of the I4N nucleus approximately bisects the ClNO angle (i.e. it lies in the expected direction of the non-bonded

2 b l

Fig. 2 nuclei

Principal quadrupolar axes of the chlorine and nitrogen

electron 'pair') and that of the chlorine nucleus lies approximately along the C1-N bond (within CQ. lo). Both findings are in agreement with simple bonding theories. The principal quadrupolar axes of the nitrogen and chlorine nuclei are shown schematically in Fig. 2.

Because the quadrupole coupling tensor of the chlorine atom aligns itself along the N-C1 bond, an approximate equation33 can be used to calculate the amount of x character and the total ionic character, i,, in the bond. A recent study of nitrosyl bromide34 has shown that the principal quadrupolar z axis here is also along the nitrogen-halogen bond, and so the ionic character of the N-Br bond can be calculated for comparison; see Table 10. It is interesting to note that, for both of these compounds, there seems to be a significant amount (ca. 12%) of n character in the nitrogen- halogen bond.

The spin-rotation coupling constants, C,, , of both chlorine and nitrogen have also been determined. As shown by F l~ga re ,~ ' c,, = agNBgg, where a is a proportionality factor. The proportionalities evaluated for the same nucleus in different isotopomers agree within their uncertainties. The spin- rotation coupling constants can be also be related to the electronic structure of the molecule. It has been shown that the spin-rotation coupling constants of a given nucleus are related to the magnetic shielding of that nucleus.36 The magnetic shielding, a, has two contributors : a paramagnetic term, a,, and a diamagnetic term, ad. The diamagnetic shielding term is related to the amount of electron density on the atom in question. A knowledge of the spin-rotation coupling constants and the geometry of the molecule allows a, to be determined, and the average shielding can be obtained from NMR data. Thus, the diamagnetic contribution to the shielding can be calculated.

The diamagnetic shielding terms calculated for the nitrogen nuclei of different nitrosyl halides are presented in Table 10. As is shown, the amount of electron density of the nitrogen atom increases from the fluoride to the bromide: a t (FN0) > ay(ClN0) > at(BrN0). This trend is reflected by the calculated ionic character of the N-X bond. Together, these data suggest that the chlorine atom is a better electron density acceptor than is bromine, as would be expected from

Table 9 Principal quadrupole coupling constants of chlorine and nitrogen

- 58.63 (14) 38.79 (14) 19.83742 (84)

- 18.26 (38)

-5.32 (39) 1.52 (39) 3.79319 (95)

106.4 (65)

-46.1306 (31) 30.4975 (31) 15.633 1 1 (99)

- 18.1051 (74)

-5.2485 (71) 1.4553 (71) 3.79321 (88)

105.34 (12)

-58.59 (11) 38.75 (1 1) 19.8340 (26)

- 17.93 (31)

-47.1 (14) 31.5 (14) 15.6307 (25)

- 18.7 (43)

Publ

ishe

d on

01

Janu

ary

1995

. Dow

nloa

ded

by U

nive

rsity

of

Cal

ifor

nia

- Sa

nta

Cru

z on

27/

10/2

014

02:2

9:57


http://dx.doi.org/10.1039/ft9959103347


Table 10 Ionic character of the N-X bond, diamagnetic contribution to the average shielding of the nitrogen nucleus and N-0 bond lengths of several nitrosyl compounds

i , aiN)/ppm r,(N=O) r,(N= 0)

FNO 497" 1.13155 (23)b 1.136 (3)b ClNO 29% 529' 1.1336 (17)d 1.13858 (40)' BrNO 24% 5 4 s 1.133 (20)g 1.146 (l)h NO 1.151'

dN) obtained from ref. 37; spin-rotation constants obtained from ref. 38. Ref. 26. dN) obtained from ref. 37. Ref. 3. ' This work.

uLN) obtained from ref. 37. g Ref. 27. Ref. 39. ' Ref. 40.

a consideration of the differing electronegativities of the two halogens.

The trend in the ionicity of the N-X bond of the nitrosyl halides is also backed up by a consideration of the N-0 bond lengths. For all nitrosyl halides this bond is consider- ably shorter than the usual N-0 double bond because of the halogen's acceptance of some of the electron density from the highest occupied anti-bonding molecular orbital of the N-0 group. Moreover, the bond length decreases from bromine to chlorine to fluorine, corresponding to an increase in the ionic character of the N-X bond. The N-0 bond lengths of several nitrosyl halides and of nitric oxide itself are presented together with the calculated ionic characters of the nitrogen-halogen bond and with the calculated diamagnetic contributions to the shielding of the nitrogen nucleus in Table 10.

We thank the NSERC of Canada for funding.

References W. M. Uselman, S. Z. Lmine, W. H. Chan and J. G. Calvert, in Nitrogenous Air Pollutants: Chemical and Biological Implica- tions, ed. D. Grosjean, Ann Arbor Science Publishers, Ann Arbor, Michigan, USA, 1979. G. Cazzoli, R. Cervellati and A. M. Mirri, J. Mol. Spectrosc., 1975,56,422. G. Cazzoli, C. Degli Esposti, P. Palmieri and G. Simeone, J . Mol. Spectrosc., 1983,97, 165. D. J. Millen and J. Pannell, J. Chem. SOC., 1961, 1322. M. A. Roehrig and S. G. Kukolich, Mol. Phys., 1992,76,221. L. H. Jones, R. R. Ryan and L. B. Asprey, J. Chem. Phys., 1967, 49, 581. J. K. McDonald, V. F. Kalasinsky, T. J. Geyer and J. R. Durig, J. Mol. Spectrosc., 1988, 132, 104.

8

9

10

11 12 13 14 15

16 17 18

19

20

21 22

23 24 25

26

27

28

29

30

31

32

33 34

35 36 37 38

39 40

M. Cheikh and C. Alamichel, Spectrochzm. Acta., Part A, 1989, 45, 153. C. Alamichel and J. Verges, Spectrochim. Acta., Part A, 1991, 47, 915. H. D. Rudolph, in Advances in Molecular Structure Research, ed. I. Hargittai and M. Hargittai, vol. 1, JAI Press, Greenwich, CT, USA. H. D. Rudolph, Struct. Chem., 1991,2,581. K. Epple and H. D. Rudolph, J. Mol. Spectrosc., 1992, 152, 355. H. D. Rudolph, J. Mol. Spectrosc., 1992,89,460. T. J. Balle and W. H. Flygare, Rev. Sci. Instrum., 1981,52, 33. Y. Xu, W. Jager and M. C. L. Gerry, J. Mol. Spectrosc., 1992, 151, 206. J. Haekel and H. Mader, 2. Naturforsch. A., 1988,43,203. H. M. Pickett, J. Mol. Spectrosc., 1991, 148, 371. E. Hirota, J. M. Brown, J. T. Hougen, T. Shida and N. Hirota, J. Mol. Spectrosc., 1995, 168,628. (a) C. C. Costain, Trans. Am. Crystallogr. Assoc., 1966,2, 157; (b) B. P. van Eijck, J. Mol. Spectrosc., 1982,91,348. K. Kuchitsu, J. Chem. Phys., 1968, 49, 4456; K. Kuchitsu, T. Fukuyama and Y. Morino, J. Mol. Struct., 1968, 1, 463; 1969,4, 41. K. Kuchitsu and Y. Morino, Bull. Chem. SOC. Jpn., 1965,38,805. C. Yamada, Y. Endo and E. Hirota, J. Mol. Spectrosc., 1986, 115, 105. T. J. Lee, J. Chem. Phys., 1993,99,9783. D. Christen, J. Mol. Struct., 1978,48, 101. J. M. L. Martin, J. P. Franqois and R. Gijbels, J . Phys. Chem., 1994,98,11394. C. Degli Esposti, G. Cazzoli and P. G. Favero, J. Mol. Spec- trosc., 1985, 109, 229. C. Degli Esposti, F. Tamassia, G. Cazzoli and Z. Kisiel, J. Mol. Spectrosc., 1995, 170, 582. G. Cazzoli, P. G. Favero and A. Del Borgo, J. Mol. Spectrosc., 1974,50,82. W. Sawodny, H. Hartner, R. Minkwitz and D. Bernstein, J. Mol. Struct., 1989, 213, 145. H. S. P. Muller and H. Willner, J. Phys. Chem., 1993, 97, 10589 and references therein. J. K. McDonald, J. A. Merritt, V. F. Kalasinsky, H. L. Heusel and J. R. Durig, J. Mol. Spectrosc., 1986, 117, 69. W. Gordy and R. L. Cook, Microwave Molecular Spectra, Wiley, New York, 3rd edn., 1984. M. Fujitake and E. Hirota, J. Chem. Phys., 1989,91,3426. C. Degli Esposti, F. Tamassia and G. Cazzoli, J. Mol. Spectrosc., 1994,163,313. W. H. Flygare, J . Chem. Phys., 1964,41,793. W. H. Flygare and J. Goodisman, J. Chem. Phys., 1962,49,3122. J. Mason, Faraday Trans., 1976,72,2064. C. Styger, B. Gatehouse, N. Heineking, W. Jager and M. C. L. Gerry, J. Chem. SOC., Faraday Trans., 1993,89, 1899. D. J. Millen and D. Mitra, Trans. Faraday SOC., 1970,66,2414. J. J. Gallagher and C. M. Johnson, Phys. Rev., 1956,103,1727.

Paper 5/02818C; Received 2nd May, 1995

Publ

ishe

d on

01

Janu

ary

1995

. Dow

nloa

ded

by U

nive

rsity

of

Cal

ifor

nia

- Sa

nta

Cru

z on

27/

10/2

014

02:2

9:57


http://dx.doi.org/10.1039/ft9959103347

structure, harmonic force field and hyperfine coupling constants of nitrosyl chloride

Documents