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High resolution FTIR spectrum of the v1 band of bromochloromethane
SPECTROCHIMICA ACTA PART A
ELSEVIER
Spectrochimica Acta Part A 52 (1996) 337-342
High resolution FTIR spectrum of the Vl band of bromochloromethane W.F. Wang,
T.L. Tan*, B.L. Tan, P.P. Ong
National University o[' Singapore, Department o[ Physics, Faculty o[' Science, Lower Kent Ridge Road, Singapore 051 I, Singapore
First received 12 June 1995; in final form 31 July 1995
Abstract
The IR spectrum of the v1 band of natural bromochloromethane (CH2BrC1) has been recorded at a resolution of 0.008 cm -~ in the region 2940-3060 cm ~ on a BOMEM DA3.002 Fourier-transform spectrometer. The rovibrational analysis on the strong Q branch of this B-type band has been performed in the prolate symmetry top approximation due to the fairly small difference between constants B and C. A normal linear least-square fitting of the identified emQK(J) clusters provided the corresponding excited state molecular parameters for three of the four naturally existing isotopomers of bromochloromethane with a standard error of about 0.02 cm Keywords: Least-square fitting; Q branch
The interest of the spectroscopic study on bromochloromethane (CHzBrC1) is usually associated with both the application and pure academic considerations on the halogen-substituted methanes. Earlier low resolution work on this molecule was conducted mainly aimed to identify the fundamental modes [1]. Later works involved the C - H stretching absorption [2] and polarization IR transmission spectra [3], while recently the first comprehensive analysis on the gas phase IR vibrotational spectra was carried out by Giorgianni et al. [4]. A number of absorption bands were positively identified in their work at resolutions ranging from 0.3 to 0.7 cm ~. The current work, which is the first careful high resolution IR study on bromochloromethane, reports the interpreta*Corresponding author.
tion of its B-type vt band and the determination of the corresponding rovibrational parameters for its three isotopomers. Since the lines of the P and Q branches are very weak and the spectrum is heavily blended even at the resolution of 0.008 c m - i, the only strong K manifolds of the Q branch lines could be assigned. Commercial bromochloromethane was obtained from Aldrich with purity better than 99%. The sample consists of the following four species of natural abundance: CH279Br35CI (38.22%), CH2SIBr3SC1 (37.31%), CH279Br37C1 (12.38%), CHzS~Br37CI (12.09%). The absorption spectrum of vt band of bromochloromethane was recorded on the BOMEM DA3.002 interferometric Fourier-transform spectrometer [5] at National University of Singapore. A 20 cm long gas cell with ZnSe windows and a high-sensitive H g - C d
0584-8539/96/$15.00 (~ 1996 Elsevier Science B.V. All rights reserved SSDI 0584-8539(95)01539-6
W.F. Wang et al. / Spectrochimica dcta Part A 52 (1996) 3 3 7 - 3 4 2
338
.8iii t) Z
.65-
co Z
.5-
I,.-
.35-
.2
2940
i
i
i
p
i
I
2960
2980
3000
3020
3040
3060
WAVENUMBER(CM "1) Fig. 1. F T I R s p e c t r u m of the v t band of b r o m o c h l o r o m e t h a n e , r a n g i n g from 2940 3060 cm the a b s o r p t i v e length 20 cm.
Te detector were used in the experiments. The vapour pressure was about 1.4 mbar. The resolution was set to be 0.008 cm ~. The instrumental bandwidth was found to be 0.004 cm ~ using the H a m m i n g apodization. The ambient temperature was controlled at 296 K and thus render a small Doppler width (full width at half-maximum ( F W H M ) ) around 0.003 cm J at 3000cm ~. It is known that bromochloromethane belongs to the simple C~ point group which has only one symmetry plane containing the a and b axes. The v~ band, which represents the C - H symmetrical stretching, displays a prominent B-type contour. Though the A-type component may also be permitted, it is too weak to be observable in the spectrum. Thus the band is expected to be very close to a perpendicular spectrum of a prolate symmetry top molecule, noting that the molecule's asymmetry parameter t¢ ~ - 0 . 9 9 1 . In Fig. 1, we present a full view of the recorded spectrum. It is easy to see that the B-type band exhibits strong Q branch features throughout the whole spectrum with a deep gap around its band
~ with the gas pressure 1.4 m b a r a n d
center. Just like the perpendicular band, striking P'RQK clusters dominate the spectrum with a nearly homogeneous spacing 2 ( A - B) of about 1.8 cm ~. However, though other subbranches, i.e. "P(-1, -1), "P(+I,-T-1), "R(1,1) and "R(+ 1, T-1), are also allowed according to the selection rule AJ = 0 , _+1; Ak,,=_+l; Ak,= +-1, they are not strong enough to be accurately determined. Since the intensities of the suspected P and R branches are in the order of the noise level, these lines were not included in our fitting. In the survey of the Q branch (Figs 2 and 3), we notice that in spite of the well pronounced P.RQ~.(j) clusters, the individual J lines are severely blended and unfortunately only give us a general profile consisting of a number of blended single absorption lines. Thus it is apparently impossible for us to definitely distinguish the single transition and it will unavoidably limit the number of rovibrational parameters to be fitted. Nevertheless, it is noted that for each RQu(J) cluster, there is one striking peak with two satellite peaks lying on its either side, while for the eQK(J) subbranch, each dominating peak is accompanied
339
W.F. Wang et al. / Spectrochimica Acta Part A 52 (1996) 337 342
¢
z
9
F-0_ CX~ 0 O9 <
•
K=9
•
OD
)D
OD
OD
3019
,x
•
K=10
K=11
K=12
I
I
I
I
I
I
3020
3021
3022
3023
3024
3025
3026
W A V E N U M B E R ( C M 1) Fig. 2. A section of the ~QK subbranch of the vI band of bromochloromethane, where D,
CH2SIBr35CI; ©,
CHJgBr~SC1; i ,
CH281Br37C1; 0 , CH2VgBr~7C1.
z
0 FQ. O: 0 rn <~
\
2979.0
/
\Q
•
/
OD
OD
OD
on
K=13
K=12
K=I 1
K=10
I
I
I
I
2980.5
2982.0
2983.5
2985.0
2986.5
W A V E N U M B E R ( C M 1) Fig. 3. A section of the PQK subbranch of the v~ band of bromochloromethane, where D, CH28tBr~SCI; ©, CH2VgBr~SC1; i , CHz81Br37C1; O, CHJgBr37CI.
with a small side peak unanimously on its lower wavenumber. One would naturally suggest that this spectral structure could be caused by the band center shifts of isotopic species. The magni rude of isotopic shifts is not expected to be signifi-
cant because the C H stretching normal vibrational mode of the v~ band does not involve the motions of the heavier isotopic atoms Br and C1. Because the observed lines are strongly blended within each K cluster, it turned out to be ex-
W.F. Wang et al. / Spectrochimica Acta Part A 52 (19961 337 342
340
Table 1 O b s e r v e d a n d c a l c u l a t e d t r a n s i t i o n s (in c m ~) o f Q - b r a n c h clusters o f the v t b a n d o f b r o m o c h l o r o m e t h a n e . T h e d e v i a t i o n s A (rob ~ -- V~,,) a r e in 10 - c m . N o t e t h a t CH_~79Br~SCI a n d CH2S~Br35CI s h a r e the s a m e o b s e r v e d series o f Q b r a n c h d u e to t h e i r unresolved peaks K
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
79Br35C]
S l Br35C1
s l Br37CI
Obs
Cal
A
Obs
~
Obs
Cal
A
3009.160 3010.847 3012.677 3014.465 3016.250 3018.030 3019.788 3021.535 3023.273 3024.994 3026.710 3028.414 3030.110 3031.788 3033.457 3035. t 2 0 3036.772 3038.415 3040.047 3041.674 3043.281 2998.401 2996.586 2994.735 2992.899 2991.039 2989.202 2987.355 2985.510 2983.659 2981.807 2979.958 2978.107 2976.253 2974.398 2972.538 2970.672 2968.825 2966.978 2965.146 2963.320 2961.499 2959.659 2957.843 2955.994
3009.161 3010.945 3012.723 3014.494 3016.258 3018.015 3019.764 3021.506 3023.238 3024.963 3026.679 3028.386 3030.084 3031.772 3033.450 3035.117 3036.775 3038.421 3040.056 3041.680 3043.293 2998.337 2996,516 2994,691 2992.863 2991.031 2989.195 2987.357 2985.516 2983.673 2981.827 2979.980 2978.132 2976.283 2974.432 2972.581 2970.730 2968.879 2967.028 2965.177 2963.329 2961.481 2959.634 2957.789 2955.947
I - 98 -46 - 29 - 8 15 24 29 35 31 31 28 26 16 7 3 - 3 - 6 -9 -6 -12 64 70 44 36 8 7 -2 - 6 14 - 20 -22 - 25 - 30 - 34 -43 58 - 54 - 50 - 31 - 9 18 25 54 47
3009.160 3010.944 3012.722 3014.493 3016.257 3018.014 3019.763 3021.504 3023.237 3024.962 3026.678 3028.385 3030.083 3031.771 3033.449 3035.117 3036.774 31138.421 30411.057 3041.681 3043.294 2998.338 2996.517 2994.692 2992.863 2991.031 2989.196 2987.358 2985.517 2983.674 2981.829 2979.982 2978.133 2976.284 2974.433 2972.582 2970.731 2968.880 2967.029 2965.178 2963.329 2961.480 2959.634 2957.788 2955.945
0.1 97 45 -28 - 7 16 25 31 36 32 32 29 27 17 8 3 - 2 6 -10 -7 - 13 63 69 43 36 8 6 - 3 7 - 15 -22 - 24 26 - 31 - 35 44 - 59 - 55 - 51 - 32 - 9 19 25 55 49
3008.932 3010.709 3012.555 3014.280 3016.055 3017.804 3019.542 3021.308 3023.002 3024.732 3026.462 3028.170 3029.872 3031.552 3033.222 3034.924
3008.965 3010.746 3012.520 3014.288 3016.049 3017.804 3019.551 3021.290 3023.022 3024.746 3026.461 3028.168 3029.865 3031.554 3033.233 3034.901
-33 -37 35 -8 6 0.2 -9 18 - 20 -14
2998.235 2996.422 29941573 2992.716 2990.871 2989.034 2987.221 2985.373 2983.564 2981.711 2979.864 2978.023 2976.184 2974.355 2972.538 2970.672 2968.858 2967.020 2965.186 2963.346 2961.463 2959.649 2957.831
2998.170 2996.356 2994.537 2992.715 2990.890 2989.062 2987.231 2985.398 2983.562 2981.726 2979.887 2978.048 2976.208 2974.367 2972.526 2970.685 2968.844 2967.004 2965.164 2963.326 2961.489 2959.655 2957.822
65 66 36
1
2 7 -2 -ll 23
1
-19 -28 -I0 -25 2 -15 -23 -25 - 24 -12 12 -13 14 16 22 20 -26 -6 9
W.F. Wang et al. / Spectrochimica Acta Part A 52 (1996) 337 342
341
Table 2 Molecular spectroscopic parameters of the vt band of bromochloromethane derived from the fitting of its strong Q branches
v~ (cm i) A' - / J ' ( c m - i ) A" - / ~ " (cm i) (A' - / ~ ' ) - (A" - B") (cm-~) D~:-D~:(10 5cm-t) rms (cm i) Assignments Range (cm i)
79Br35C1
st Br35CI
81Br~7CI
3002.8705(86) " 0.902183(30) b 0.904803 ~ 0.002620(30) 1.221(21) 0.034 45
3002.8704(87) 0.902111(30) 0.904730 0.002619(30) 1.213(22) 0.035 45 2955 3044
3002.6903(65) 0.899740(33) 0.902201 0.002461(33) 1.210(26) 0.023 39
~' The uncertainty for the band centers does not include the absolute calibration uncertainty which is of the order of _+0.00032 cm ~. b Standard deviations of the last two digits are given in the parentheses as uncertainties, c G r o u n d state constants are taken from Ref. [7] and fixed in the fitting.
tremely difficult for us to assign the individual lines using Watson's S-reduction Hamiltonian for asymmetric top molecules. Regarding its symmetry top like contour, we shifted to the use of the Hamiltonian applied to the perpendicular bands [6], which is F ( J , K ) = B J ( J + 1 ) + ( A - B ) K 2 -- O j j 2 ( j -- D j K J ( J + 1) K 2 - - D h . K 4
+ 1 )2 (1)
This Hamiltonian leads to the formula of the series of subband centers up to quartic centrifugal distortion terms, i.e. v v" ~ = v0 + (A' - / ~ ' ) _+ 2(A' - / ~ ' ) K + [(A' --/~') -- (A" --/~")]K 2 T 4(D~: -- D ~ ) K 3
(2)
where B = ( B + C ) / 2 and the upper sign applies to RQ while the lower sign to PQ. The contribution from Du to the lower order was not taken into account due to its neglectable effect compared to that of A and B. In Eq. (2), we also did not include the 2~A term (l doubling), for the v~ fundamental is nondegenerate. Besides, as we are unable to discern the J transitions, the quartic coefficients Dj and DjK have to be dropped off. Consequently, the final Hamiltonian becomes a simple polynomial with the order up to K 3. The Marquardt Levenberg algorithm was used to find the best upper state constants. The ground state constants supplied by Niide et al. [7] were fixed in the process to yield good statistical deviations and correlation coefficients.
The results are listed in Tables 1 and 2. Parts of the detailed assignments are also illustrated in Figs 2 and 3 for the RQK and PQ~. subbranches respectively. As shown (Fig. 2), for each K cluster in the RQK subbranch, the dominating peak belongs to the two abundant isotopomers CHJgBr35C1 and CH2~IBr35CI which have very close band centers, while the two side peaks should go to the other two isotopes of smaller compositions CH279Br37C1 and CH281Br37CI, though the latter was not inclued in the fitting due to its unresolvable eQA. clusters. This identification would be more understandable if one notices that the central peaks display rather smooth profiles indicating an extensive combination of a few single absorption lines. In comparison, the two satellite peaks feature small fluctuations which suggest that the manifolds are less overlapping. Mingled with the strong Q branch, the P and R branches are too weak to make a significant contribution to the whole spectrum, though some J clusters, standing as the small undulation in the background, can still be discerned to some extent. As for the eQ K subbranch (Fig. 3), the clusters appear even more simple. It is found that the side peaks should be attributed to CH28~Br37C1, while its dominant peaks, much wider than the former ones, are composed of lines from the other three isotopomers. A m o n g the contribution of these three isotopomers, clusters of CHzV9Br37CI, which is of the smaller composition, are intensely overlapped by those of the other two abundant isotopomers. As a result, we are deterred from fitting the isotopomer C H 2 7 9 B r 3 7 C 1 .
342
W.F. Wang et al. / Spectrochimica Acta Part A 52 (1996) 337-342
In s u m m a r y , we report the results of the polyn o m i a l least-squares fit of three isotopomers o f the B-type vj b a n d s o f b r o m o c h l o r o m e t h a n e at a resolution o f 0.008 cm -~. The r o v i b r a t i o n a l parameters can be used to calculate the spectrum that duplicates the observed spectrum with rms better t h a n 0.035 c m - J.
Acknowledgment The a u t h o r s t h a n k H.H. Teo for his technical assistance in the o p e r a t i o n of the spectrometer.
References [1] A. Weber, A.G. Meister and F.F. Clevel, Chem. Phys., 21 (1953) 930. [2] U. Kanbayashi, Bull. Chem. Soc. Jpn. 36 (1963) 1173. [3] M. Ovaska, A. Kivinen and M. Rasanen, J. Molec. Struct., 98 (1983) 19. [4] S. Giorgianni, R. Vsisnoni, A. Baldacci, A. Gambi and S. Ghersetti, Spectrochim. Acta, Part A, 44 (1988) 463. [5] T.L. Tan, E.C. Looi and K.T. Lua, Spectrochim. Acta, Part A, 48, 975 (1992). [6] H.C. Allen Jr. and P.C. Cross, Molecular rib-rotors (The theory and Interpretation of High Resolution Infrared Spectra), Wiley, New York, 1963. [7] Y. Niide and I. Ohkoshi, J. Mol. Spectrosc., 136 (1989) 17.
ELSEVIER
Spectrochimica Acta Part A 52 (1996) 337-342
High resolution FTIR spectrum of the Vl band of bromochloromethane W.F. Wang,
T.L. Tan*, B.L. Tan, P.P. Ong
National University o[' Singapore, Department o[ Physics, Faculty o[' Science, Lower Kent Ridge Road, Singapore 051 I, Singapore
First received 12 June 1995; in final form 31 July 1995
Abstract
The IR spectrum of the v1 band of natural bromochloromethane (CH2BrC1) has been recorded at a resolution of 0.008 cm -~ in the region 2940-3060 cm ~ on a BOMEM DA3.002 Fourier-transform spectrometer. The rovibrational analysis on the strong Q branch of this B-type band has been performed in the prolate symmetry top approximation due to the fairly small difference between constants B and C. A normal linear least-square fitting of the identified emQK(J) clusters provided the corresponding excited state molecular parameters for three of the four naturally existing isotopomers of bromochloromethane with a standard error of about 0.02 cm Keywords: Least-square fitting; Q branch
The interest of the spectroscopic study on bromochloromethane (CHzBrC1) is usually associated with both the application and pure academic considerations on the halogen-substituted methanes. Earlier low resolution work on this molecule was conducted mainly aimed to identify the fundamental modes [1]. Later works involved the C - H stretching absorption [2] and polarization IR transmission spectra [3], while recently the first comprehensive analysis on the gas phase IR vibrotational spectra was carried out by Giorgianni et al. [4]. A number of absorption bands were positively identified in their work at resolutions ranging from 0.3 to 0.7 cm ~. The current work, which is the first careful high resolution IR study on bromochloromethane, reports the interpreta*Corresponding author.
tion of its B-type vt band and the determination of the corresponding rovibrational parameters for its three isotopomers. Since the lines of the P and Q branches are very weak and the spectrum is heavily blended even at the resolution of 0.008 c m - i, the only strong K manifolds of the Q branch lines could be assigned. Commercial bromochloromethane was obtained from Aldrich with purity better than 99%. The sample consists of the following four species of natural abundance: CH279Br35CI (38.22%), CH2SIBr3SC1 (37.31%), CH279Br37C1 (12.38%), CHzS~Br37CI (12.09%). The absorption spectrum of vt band of bromochloromethane was recorded on the BOMEM DA3.002 interferometric Fourier-transform spectrometer [5] at National University of Singapore. A 20 cm long gas cell with ZnSe windows and a high-sensitive H g - C d
0584-8539/96/$15.00 (~ 1996 Elsevier Science B.V. All rights reserved SSDI 0584-8539(95)01539-6
W.F. Wang et al. / Spectrochimica dcta Part A 52 (1996) 3 3 7 - 3 4 2
338
.8iii t) Z
.65-
co Z
.5-
I,.-
.35-
.2
2940
i
i
i
p
i
I
2960
2980
3000
3020
3040
3060
WAVENUMBER(CM "1) Fig. 1. F T I R s p e c t r u m of the v t band of b r o m o c h l o r o m e t h a n e , r a n g i n g from 2940 3060 cm the a b s o r p t i v e length 20 cm.
Te detector were used in the experiments. The vapour pressure was about 1.4 mbar. The resolution was set to be 0.008 cm ~. The instrumental bandwidth was found to be 0.004 cm ~ using the H a m m i n g apodization. The ambient temperature was controlled at 296 K and thus render a small Doppler width (full width at half-maximum ( F W H M ) ) around 0.003 cm J at 3000cm ~. It is known that bromochloromethane belongs to the simple C~ point group which has only one symmetry plane containing the a and b axes. The v~ band, which represents the C - H symmetrical stretching, displays a prominent B-type contour. Though the A-type component may also be permitted, it is too weak to be observable in the spectrum. Thus the band is expected to be very close to a perpendicular spectrum of a prolate symmetry top molecule, noting that the molecule's asymmetry parameter t¢ ~ - 0 . 9 9 1 . In Fig. 1, we present a full view of the recorded spectrum. It is easy to see that the B-type band exhibits strong Q branch features throughout the whole spectrum with a deep gap around its band
~ with the gas pressure 1.4 m b a r a n d
center. Just like the perpendicular band, striking P'RQK clusters dominate the spectrum with a nearly homogeneous spacing 2 ( A - B) of about 1.8 cm ~. However, though other subbranches, i.e. "P(-1, -1), "P(+I,-T-1), "R(1,1) and "R(+ 1, T-1), are also allowed according to the selection rule AJ = 0 , _+1; Ak,,=_+l; Ak,= +-1, they are not strong enough to be accurately determined. Since the intensities of the suspected P and R branches are in the order of the noise level, these lines were not included in our fitting. In the survey of the Q branch (Figs 2 and 3), we notice that in spite of the well pronounced P.RQ~.(j) clusters, the individual J lines are severely blended and unfortunately only give us a general profile consisting of a number of blended single absorption lines. Thus it is apparently impossible for us to definitely distinguish the single transition and it will unavoidably limit the number of rovibrational parameters to be fitted. Nevertheless, it is noted that for each RQu(J) cluster, there is one striking peak with two satellite peaks lying on its either side, while for the eQK(J) subbranch, each dominating peak is accompanied
339
W.F. Wang et al. / Spectrochimica Acta Part A 52 (1996) 337 342
¢
z
9
F-0_ CX~ 0 O9 <
•
K=9
•
OD
)D
OD
OD
3019
,x
•
K=10
K=11
K=12
I
I
I
I
I
I
3020
3021
3022
3023
3024
3025
3026
W A V E N U M B E R ( C M 1) Fig. 2. A section of the ~QK subbranch of the vI band of bromochloromethane, where D,
CH2SIBr35CI; ©,
CHJgBr~SC1; i ,
CH281Br37C1; 0 , CH2VgBr~7C1.
z
0 FQ. O: 0 rn <~
\
2979.0
/
\Q
•
/
OD
OD
OD
on
K=13
K=12
K=I 1
K=10
I
I
I
I
2980.5
2982.0
2983.5
2985.0
2986.5
W A V E N U M B E R ( C M 1) Fig. 3. A section of the PQK subbranch of the v~ band of bromochloromethane, where D, CH28tBr~SCI; ©, CH2VgBr~SC1; i , CHz81Br37C1; O, CHJgBr37CI.
with a small side peak unanimously on its lower wavenumber. One would naturally suggest that this spectral structure could be caused by the band center shifts of isotopic species. The magni rude of isotopic shifts is not expected to be signifi-
cant because the C H stretching normal vibrational mode of the v~ band does not involve the motions of the heavier isotopic atoms Br and C1. Because the observed lines are strongly blended within each K cluster, it turned out to be ex-
W.F. Wang et al. / Spectrochimica Acta Part A 52 (19961 337 342
340
Table 1 O b s e r v e d a n d c a l c u l a t e d t r a n s i t i o n s (in c m ~) o f Q - b r a n c h clusters o f the v t b a n d o f b r o m o c h l o r o m e t h a n e . T h e d e v i a t i o n s A (rob ~ -- V~,,) a r e in 10 - c m . N o t e t h a t CH_~79Br~SCI a n d CH2S~Br35CI s h a r e the s a m e o b s e r v e d series o f Q b r a n c h d u e to t h e i r unresolved peaks K
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
79Br35C]
S l Br35C1
s l Br37CI
Obs
Cal
A
Obs
~
Obs
Cal
A
3009.160 3010.847 3012.677 3014.465 3016.250 3018.030 3019.788 3021.535 3023.273 3024.994 3026.710 3028.414 3030.110 3031.788 3033.457 3035. t 2 0 3036.772 3038.415 3040.047 3041.674 3043.281 2998.401 2996.586 2994.735 2992.899 2991.039 2989.202 2987.355 2985.510 2983.659 2981.807 2979.958 2978.107 2976.253 2974.398 2972.538 2970.672 2968.825 2966.978 2965.146 2963.320 2961.499 2959.659 2957.843 2955.994
3009.161 3010.945 3012.723 3014.494 3016.258 3018.015 3019.764 3021.506 3023.238 3024.963 3026.679 3028.386 3030.084 3031.772 3033.450 3035.117 3036.775 3038.421 3040.056 3041.680 3043.293 2998.337 2996,516 2994,691 2992.863 2991.031 2989.195 2987.357 2985.516 2983.673 2981.827 2979.980 2978.132 2976.283 2974.432 2972.581 2970.730 2968.879 2967.028 2965.177 2963.329 2961.481 2959.634 2957.789 2955.947
I - 98 -46 - 29 - 8 15 24 29 35 31 31 28 26 16 7 3 - 3 - 6 -9 -6 -12 64 70 44 36 8 7 -2 - 6 14 - 20 -22 - 25 - 30 - 34 -43 58 - 54 - 50 - 31 - 9 18 25 54 47
3009.160 3010.944 3012.722 3014.493 3016.257 3018.014 3019.763 3021.504 3023.237 3024.962 3026.678 3028.385 3030.083 3031.771 3033.449 3035.117 3036.774 31138.421 30411.057 3041.681 3043.294 2998.338 2996.517 2994.692 2992.863 2991.031 2989.196 2987.358 2985.517 2983.674 2981.829 2979.982 2978.133 2976.284 2974.433 2972.582 2970.731 2968.880 2967.029 2965.178 2963.329 2961.480 2959.634 2957.788 2955.945
0.1 97 45 -28 - 7 16 25 31 36 32 32 29 27 17 8 3 - 2 6 -10 -7 - 13 63 69 43 36 8 6 - 3 7 - 15 -22 - 24 26 - 31 - 35 44 - 59 - 55 - 51 - 32 - 9 19 25 55 49
3008.932 3010.709 3012.555 3014.280 3016.055 3017.804 3019.542 3021.308 3023.002 3024.732 3026.462 3028.170 3029.872 3031.552 3033.222 3034.924
3008.965 3010.746 3012.520 3014.288 3016.049 3017.804 3019.551 3021.290 3023.022 3024.746 3026.461 3028.168 3029.865 3031.554 3033.233 3034.901
-33 -37 35 -8 6 0.2 -9 18 - 20 -14
2998.235 2996.422 29941573 2992.716 2990.871 2989.034 2987.221 2985.373 2983.564 2981.711 2979.864 2978.023 2976.184 2974.355 2972.538 2970.672 2968.858 2967.020 2965.186 2963.346 2961.463 2959.649 2957.831
2998.170 2996.356 2994.537 2992.715 2990.890 2989.062 2987.231 2985.398 2983.562 2981.726 2979.887 2978.048 2976.208 2974.367 2972.526 2970.685 2968.844 2967.004 2965.164 2963.326 2961.489 2959.655 2957.822
65 66 36
1
2 7 -2 -ll 23
1
-19 -28 -I0 -25 2 -15 -23 -25 - 24 -12 12 -13 14 16 22 20 -26 -6 9
W.F. Wang et al. / Spectrochimica Acta Part A 52 (1996) 337 342
341
Table 2 Molecular spectroscopic parameters of the vt band of bromochloromethane derived from the fitting of its strong Q branches
v~ (cm i) A' - / J ' ( c m - i ) A" - / ~ " (cm i) (A' - / ~ ' ) - (A" - B") (cm-~) D~:-D~:(10 5cm-t) rms (cm i) Assignments Range (cm i)
79Br35C1
st Br35CI
81Br~7CI
3002.8705(86) " 0.902183(30) b 0.904803 ~ 0.002620(30) 1.221(21) 0.034 45
3002.8704(87) 0.902111(30) 0.904730 0.002619(30) 1.213(22) 0.035 45 2955 3044
3002.6903(65) 0.899740(33) 0.902201 0.002461(33) 1.210(26) 0.023 39
~' The uncertainty for the band centers does not include the absolute calibration uncertainty which is of the order of _+0.00032 cm ~. b Standard deviations of the last two digits are given in the parentheses as uncertainties, c G r o u n d state constants are taken from Ref. [7] and fixed in the fitting.
tremely difficult for us to assign the individual lines using Watson's S-reduction Hamiltonian for asymmetric top molecules. Regarding its symmetry top like contour, we shifted to the use of the Hamiltonian applied to the perpendicular bands [6], which is F ( J , K ) = B J ( J + 1 ) + ( A - B ) K 2 -- O j j 2 ( j -- D j K J ( J + 1) K 2 - - D h . K 4
+ 1 )2 (1)
This Hamiltonian leads to the formula of the series of subband centers up to quartic centrifugal distortion terms, i.e. v v" ~ = v0 + (A' - / ~ ' ) _+ 2(A' - / ~ ' ) K + [(A' --/~') -- (A" --/~")]K 2 T 4(D~: -- D ~ ) K 3
(2)
where B = ( B + C ) / 2 and the upper sign applies to RQ while the lower sign to PQ. The contribution from Du to the lower order was not taken into account due to its neglectable effect compared to that of A and B. In Eq. (2), we also did not include the 2~A term (l doubling), for the v~ fundamental is nondegenerate. Besides, as we are unable to discern the J transitions, the quartic coefficients Dj and DjK have to be dropped off. Consequently, the final Hamiltonian becomes a simple polynomial with the order up to K 3. The Marquardt Levenberg algorithm was used to find the best upper state constants. The ground state constants supplied by Niide et al. [7] were fixed in the process to yield good statistical deviations and correlation coefficients.
The results are listed in Tables 1 and 2. Parts of the detailed assignments are also illustrated in Figs 2 and 3 for the RQK and PQ~. subbranches respectively. As shown (Fig. 2), for each K cluster in the RQK subbranch, the dominating peak belongs to the two abundant isotopomers CHJgBr35C1 and CH2~IBr35CI which have very close band centers, while the two side peaks should go to the other two isotopes of smaller compositions CH279Br37C1 and CH281Br37CI, though the latter was not inclued in the fitting due to its unresolvable eQA. clusters. This identification would be more understandable if one notices that the central peaks display rather smooth profiles indicating an extensive combination of a few single absorption lines. In comparison, the two satellite peaks feature small fluctuations which suggest that the manifolds are less overlapping. Mingled with the strong Q branch, the P and R branches are too weak to make a significant contribution to the whole spectrum, though some J clusters, standing as the small undulation in the background, can still be discerned to some extent. As for the eQ K subbranch (Fig. 3), the clusters appear even more simple. It is found that the side peaks should be attributed to CH28~Br37C1, while its dominant peaks, much wider than the former ones, are composed of lines from the other three isotopomers. A m o n g the contribution of these three isotopomers, clusters of CHzV9Br37CI, which is of the smaller composition, are intensely overlapped by those of the other two abundant isotopomers. As a result, we are deterred from fitting the isotopomer C H 2 7 9 B r 3 7 C 1 .
342
W.F. Wang et al. / Spectrochimica Acta Part A 52 (1996) 337-342
In s u m m a r y , we report the results of the polyn o m i a l least-squares fit of three isotopomers o f the B-type vj b a n d s o f b r o m o c h l o r o m e t h a n e at a resolution o f 0.008 cm -~. The r o v i b r a t i o n a l parameters can be used to calculate the spectrum that duplicates the observed spectrum with rms better t h a n 0.035 c m - J.
Acknowledgment The a u t h o r s t h a n k H.H. Teo for his technical assistance in the o p e r a t i o n of the spectrometer.
References [1] A. Weber, A.G. Meister and F.F. Clevel, Chem. Phys., 21 (1953) 930. [2] U. Kanbayashi, Bull. Chem. Soc. Jpn. 36 (1963) 1173. [3] M. Ovaska, A. Kivinen and M. Rasanen, J. Molec. Struct., 98 (1983) 19. [4] S. Giorgianni, R. Vsisnoni, A. Baldacci, A. Gambi and S. Ghersetti, Spectrochim. Acta, Part A, 44 (1988) 463. [5] T.L. Tan, E.C. Looi and K.T. Lua, Spectrochim. Acta, Part A, 48, 975 (1992). [6] H.C. Allen Jr. and P.C. Cross, Molecular rib-rotors (The theory and Interpretation of High Resolution Infrared Spectra), Wiley, New York, 1963. [7] Y. Niide and I. Ohkoshi, J. Mol. Spectrosc., 136 (1989) 17.