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Dye laser photoacoustic overtone spectroscopy of CHBr3
Volume 144, number 1
DYE LASER PHOTOACOUSTIC
12 February 1988
CHEMICAL PHYSICS LETTERS
OVERTONE SPECTROSCOPY
OF CHBrj
Carlos MANZANARES I, N.L.S:YAMASAIU and Eric WEITZ Department of Chemistry, Northwestern University, Evanston, IL 60201, USA
Received 10 August 1987; in final form 30 November 1987
Spectra of the v= 5 and v= 6. levels of the C-H stretch in CHBr, have been recorded via in-cavity dye laser photoacoustic spectroscopy. Linewidths for these absorptions have been determined and are discussed in light of existing theory and results on the linewidths of the corresponding transitions in CHCI,. When data on the fundamental are combined with v= 5 and v= 6 data it is possible to determine values for the local mode harmonic frequency and anharmonicity. Absolute absorption cross sections for the v= 5 and a= 6 levels are reported.
1. Introduction
There has been considerable interest in the local mode behaviour that is evidenced by high lying overtones of various molecular vibrational modes [ 11. One area of investigation involves the mechanism that leads to broadening of the absorption lines of these high lying overtones. In particular, interest has centered on overtones of C-H stretches; in part due to the easier experimental accessibility of high lying overtones of these species versus lower frequency modes and in part due to the mass mismatch of the C and H atoms which accentuates local mode behaviour in these systems. A recently propounded theory dealing with the linewidths of these modes indicates that absorption lines will broaden due to the presence of resonances [ 21. In compounds with C-H stretching modes, a 2: 1 resonance will occur if the overtone of a lower lying mode is in resonance with the energy difference between the vth and (v- 1)th local mode. In hydrocarbons this resonance can occur when the overtone of the methyl rocking mode, typically near 2x 1450 cm-‘, is in resonance with the energy difference between two local mode levels. Of course resonances with other modes are also possible as are higher order resonances. This theory has been successful in explaining the trends in linewidth in a variety of hydrocarbon systems, particularly benzene and its deuterated derivatives [ 31. Quack and co-workers have put forth a theoretical treat-
ment, which has been applied to a variety of isolated C-H chromophores [ 41. This theory takes into account the coupling between the C-H stretching mode and various overtones of the C-H bending mode of the molecule (not just 2: 1 resonances) to explain observed positions and intensities of overtone absorptions. By fitting the spectrum, parameters for an . appropriate Hamiltonian have been determined which allows for the evaluation of the coupling between overtones of the stretching modes and overtones of the bending modes. In this treatment coupling between the stretching mode manifold and higher-order transitions involving the bending mode manifold have been shown to be important under a variety of circumstances. Recently there was the prediction [ 51, which was compatible with the observed broad overtone absorptions in the liquid [6], that CHC13 should possess broad gas phase overtone absorptions. In this molecule, which is a member of the CHX3 series, there are no fundamental absorptions in the region between 1150 and 3020 cm- ’ due to the large mass of the Cl atom [ 7 1. Thus, there are no modes for which the overtone is in near resonance with C-H local modes up to very high levels of excitation. Sub-. sequent to this prediction, a gas phase study of the CHC& system demonstrated that this compound had narrow overtone absorption lines at the v=6 level [8,9]. The observed spectra could be quantitatively simulated by a Hamiltonian which included cou-
0 009-26141881%03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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CHEMICAL PHYSICS LETTERS
pling between the bending and stretching modes. However, it was concluded that this coupling was sufficiently small so as not to be reflected in the overtone linewidth at this level of excitation [ 81. We have recently recorded overtone spectra of CHBr3. This compound is of interest since it is another member of the CHX, series; a series that has attracted previous experimental and theoretical interest [4-lo]. Thus the trends observed for the lighter members of this series can be extended, and the effect of a very large mass X on the behaviour of the isolated C-H chromophore can be observed. The purpose of this Letter is to report on our observations for this molecule and to compare and contrast these observations with the behaviour of the CHC& molecule.
2. Experimental Details of the experimental procedure have been presented previously [ 111. Briefly, the fundamental spectrum of CHB, was obtained using a Nicolet 7 199 FTIR spectrometer. Spectra of the U= 5 and v=:6 levels of CHBr, were obtained using a photoacoustic cell placed in the cavity of a Coherent 599 dye laser pumped by a CR 3000K krypton ion laser. For the purpose of providing a direct comparison on the same apparatus, a fundamental spectrum and spectra of the v=5 and v=6 levels of CHCl, were also recorded. LD 700 was pumped by the red lines of the Kr+ laser for the u= 5 spectrum while rhodamine 6G pumped by the blue green lines of the Kr+ laser was used to obtain the v=6 spectrum. Signals were ratioed against the power spectrum of the dye laser before being plotted. The dye laser linewidth was determined to be approximately 0.75 cm-‘. The method used for the determination of absolute absorption cross sections has also been described previously [ 111. C(CHJ)4 was used as a reference for the V= 5 level and CzH4 was used as a reference for the u=6 level. All spectra were obtained with the sample at its vapor pressure of approximately 4.2 Torr at 2 1“C. Due to the relatively low vapor pressure of CHBr, it was not possible to obtain spectra of the U= 2, 3 or 4 overtones above U= 6. CHBr3 was obtained from Aldrich Chemical and specified as > 99% pure. The sample was degassed and ethanol 44
12 February 1988
was removed by multiple freeze-pump-thaw cycles prior to use. CHC& was obtained from Aldrich with a stated purity 99 t O/o(gold label). It was also degassed by multiple freeze-pump-thaw cycles prior to use.
3. Results and discussion Figs. 1 and 2 depict the overtone spectrum of CHBr3 for the v=5 and v=6 levels respectively. These absorptions are centered at 13963 and 16393 cm-r, in comparison to their reported positions of 13840 and 16240 cm-’ in neat liquid [ 61. This corresponds to a shift in the gas phase versus liquid of approximately 120 to 150 cm- ’to higher frequency, which is not unexpected. The v=5 region was scanned in an effort to find other possible absorptions which could result from interaction of the C-H stretch with higher overtones of the C-H bending mode [ 121. No absorptions were found which could be deflnitively assigned to these types of interactions. Our signal/noise level let us set an upper limit
I
14000
/
13960 Wavenumbers
13920 (cm..’
)
Fig. 1. Photoacoustic absorption spectrum of the u= 5 level of the C-H stretch of CHBr,. The sample pressure was 4.2 Torr.
12 February 1988
CHEMICAL PHYSICS LETTERS
Volume 144, number 1
Table 1 Fundamental vibrational frequencies for CHCl, and CHBr, Mode
Symmetry
CHCl,
CHBr3
VI V4 VS V2 V3 V6
a, e e aI al e
3033 a’ 1205 a) 760 a’ 667 .’ 364 =’ 260 =’
3045 “), 3020 ‘), 3023 d,c) 1149 a), 1145 C),1142 d, 656 a) 655 d’ 539 %d.C.O 222 C.0) 152”
a) Ref. [7]. b, Ref. [ 131. ‘Ref. [ 141. d)Ref. [15]. “Ref. [16].
l.
16450
16400
Wavenumbers
16j50 (cm’-’
16300
)
Fig. 2. Photoacoustic absorption spectrum of the v= 6 level of the C-H stretch of CHBr,. The sample pressure was 4.2 Torr.
on the size of any such absorptions as no more than f the magnitude of the v= 5 absorption shown in fig. 1. The position of the fundamental absorption along with the v=5 and v=6 absorptions can be used to construct a Birge-Sponer plot. Though this plot consists of only three points, the points do correlate well with a straight line. Thus this plot can be used to obtain an approximate value of 3166 cm-’ for w,, the local mode harmonic frequency, and an approximate value of -62 cm-’ for 0,x,, the local mode anharmonicity. The differences between these values and those determined for CHCl, are statistically insignificant. Absolute absorption cross sections have been determined as (4.7f0.6) x 1O-24 and (5.2+0.8)X10-26cm2 for the v=5 and 6 levels of CHBr, respectively. These values also agree quite well with the values we have obtained for the corresponding cross sections in CHCL These are (2.5 kO.3) x 1O-24 and (5.5kO.5) x 1O-26cm2 respectively. As previously stated, the very narrow lines observed in CHC& (approximately 6 cm-’ fwhm for the Q branch of v= 5 and 4 cm- ’ fwhm for the Q branch of ~6) are compatible with the lack of significant coupling, at this level of excitation, between the C-H overtones and the bending modes. Consulting table 1, which contains the frequencies of the fundamentals of both CHBr, and CHC&, it is apparent that, as previously stated, CHC& would not
be expected to have a 2 : 1 resonance which would be operative for either the u= 5 or II= 6 levels of the C-H stretch. Thus within the context of a model that attributes broadening to the presence of 2: 1 resonances, narrow absorption lines would be expected and are ,observed in this molecule [ 81. When CHC& is treated in the context of a theory that allows for coupling of the C-H stretching modes to higher overtones of the bending modes it is still the case that little broadening of the C-H overtones at the u=6 level is predicted [S] _ Some degree of mixing between the higher overtones of the C-H stretch and the bending mode manifold is expected; however, it has been stated that the degree of mixing is insufficient to result in significant spectroscopically observable broadening of the stretching overtones under study. The degree of mixing in this treatment will also depend on the energy difference between the states involved. A comparison can now be made with the CHBr, molecule. Based on the frequencies of the various fundamentals and overtones (see table I), CHBr3 also would not be expected to have a 2 : 1 resonance at either the v= 5 or v= 6 level. Higher-order interactions would be necessary before some combination of stretching and bending modes would be near resonant at the level of the V= 5 or v= 6 C-H stretching mode. Thus extensive mixing between the C-H stretching overtones and the bending modes would not be expected, which is compatible with the observed narrow linewidths. However, for CHBr3 there. is a possible 3 : 1 resonance at the U= 6 level, 2v,f v4_ Because of the effect of the mass of the halogen atom on the normal mode frequencies in CHX, molecules, CHCl, does not possess 3: 1 interactions that are as close to resonance as in the CHBr, case. The role of 45
Volume 144, number 1
CHEMICAL PHYSICS LETTERS
3: 1 resonances for overtone states has not been treated in as much detail as 2: 1 resonances. However, it can be said with reasonable certainty that the same general characteristics that govern 2: 1 resonances will govern 3: 1 resonances. Of course it would be expected that the matrix element coupling the initial and final states in the 3 : 1 resonance case will be smaller than in the 2 : 1 case. Thus if resonances occur and there is the possibility of both 2; 1 and 3; 1 types of interactions, it would be expected that 2: 1 interactions will involve a stronger coupling and thus the linewidth will be dominated by these interactions for similar energy gaps. However, in the absence of 2 : 1 interactions (or if the energy gap is much larger than in the 3: 1 case), 3: 1 interactions could be expected to result in some broadening of overtone lines and could be the dominant factor in this broadening. Interestingly while the U= 5 level in CHBr3, with a fwhm of the Q branch of %7 cm-‘, is of similar width to the V= 5 or tr= 6 level in CHC&, the v= 6 level in CHBr3, where there is an expected 3: 1 resonance, is broader, with a fwhm of z 22 cm-‘, than either the V= 5 level of this molecule or the v= 6 level of CHC&. It is premature to say that this broadening is definitely due to the existence of a 3 : 1 resonance since other resonant interactions are possible and the observed change in width could be due to a change in rotational congestion. Additionally, the width of the v=6 level in CHBr3 is only slightly larger than the widths of the V= 5 level in CHBr, and the v= 5 and v=6 levels in CHC&. However, a 3: 1 interaction is an appealing explanation and it is certainly warranted to point out that the v=6 level in CHBrS is broader than either the V= 5 level in CHBr3 or the V= 5 or 6 levels in CHC&. In these latter cases, neither 2 : 1 nor 3 : 1 interactions would be expected to be significant.
4. Conclusions CHBr3 has been studied to provide information on the overtones of the C-H stretches of another member of the CHX3 series. Absolute absorption coefficients for both the v=5 and v=6 levels have been measured and are similar to measurements for the corresponding absorption coefficients in the 46
I2 February 1988
CHC& molecule. Approximate values for the local harmonic frequency, o,, and anharmonicity, 0,x,, have been determined. The values obtained are very similar to those obtained for CHCl+ The linewidths of the u=5 and v=6 levels of the C-H stretch are quite narrow relative to linewidths of typical hydrocarbons. The possible reasons for these narrow linewidths and a potential explanation for somewhat broader v=6 level in CHBr3 are discussed in terms of the absence or presence of resonant interactions.
Acknowledgement
We would like to thank the National Science Foundation for support of this work under grant CHE85-0657. We thank Lewis Baylor for his help with some of the experiments on the v= 5 overtones.
References [ 1] B.R. Henry, Accounts Chem. Res. 10 (1977) 207; J.S. Wang and C.B. Moore, in: Lasers and applications, eds. W.O.M. Guimaraes, C.-T. Lin and A. Mooradian (Springer, Berlin, 1981). [2] E.L. Sibert, W.P. Reinhardt and J.T. Hynes, J. Chem. Phys. 81 (1984) 1115. [ 31 K.V. Reddy, D.F. Heller and M.J. Berry, J. Chem. Phys. 76 (1982) 2814. [ 41 H.R. Dilbal and M. Quack, J. Chem. Phys. 81 (1984) 3779; J.E. Baggott, MC. Chuang, R.N. Zare, H.R. Diibal and M. Quack, J. Chem. Phys. 82 (1985) 1186. [5] S. Kato, J. Chem. Phys. 83 (1985) 1085. [6] H.L. Fang and R.L. Swofford, J. Chem. Phys. 72 (1980) 6382. [ 71 G. Herzberg, Infrared and Raman spectra (Van Nostrand, Princeton, 1945). [S] J.E. Baggott, H.J. Clase and LM. Mills, J. Chem. Phys. 84 (1986) 4193. [ 91 J.S. Wang and C.B. Moore, Proceedings of the 28th International Union of Pure and Applied Chemistry Congress, ed. K.J. Laidler (1981) p. 353. [ IO] G.A. Voth, R.A. Marcus and A.H. Zewail, J. Chem. Phys. 81 (1984) 5494; J.W. Perry; D.J. Moll, A. Kuppermann and A.H. Zewail, J. Chem. Phys. 82 (1985) 1195; A. Compargue and F. Stoeckel, J. Chem. Phys. 85 (1986) 1220. [ 111 C. Manzanares, N.L.S. Yamasaki, E. Weitz and J.T. Knudtson, Chem. Phys. Letters 117 (1985) 477. [ 121 M. Lewerenzand M. Quack, Chem. Phys. Letters 123 (1986) 197.
Volume 144, number I
CHEMICAL PHYSICS LETTERS
[ 131 A. Hacura and T.W. Zerda, J. Mol. Spectry. 60 (1980) 277; N.A. Borisevich, G.A. Zalesskaya, V.A. Lastocbkina and T. Shukurov, Spectry. Letters 18 ( 1982) 99 1. [ 141 E. Burgos, E. Halac and H. Bonadeo, J. Chem. Phys. 74 (1981) 1546.
12 February 1988
[ 151 Y. Isbikawa, S. Arai and R. Nakone, J. Nucl. Sci. Technol. 17 (1980) 275. [16] J.C. Decius, J. Chem.Phys. 16 (1948) 214.
47
DYE LASER PHOTOACOUSTIC
12 February 1988
CHEMICAL PHYSICS LETTERS
OVERTONE SPECTROSCOPY
OF CHBrj
Carlos MANZANARES I, N.L.S:YAMASAIU and Eric WEITZ Department of Chemistry, Northwestern University, Evanston, IL 60201, USA
Received 10 August 1987; in final form 30 November 1987
Spectra of the v= 5 and v= 6. levels of the C-H stretch in CHBr, have been recorded via in-cavity dye laser photoacoustic spectroscopy. Linewidths for these absorptions have been determined and are discussed in light of existing theory and results on the linewidths of the corresponding transitions in CHCI,. When data on the fundamental are combined with v= 5 and v= 6 data it is possible to determine values for the local mode harmonic frequency and anharmonicity. Absolute absorption cross sections for the v= 5 and a= 6 levels are reported.
1. Introduction
There has been considerable interest in the local mode behaviour that is evidenced by high lying overtones of various molecular vibrational modes [ 11. One area of investigation involves the mechanism that leads to broadening of the absorption lines of these high lying overtones. In particular, interest has centered on overtones of C-H stretches; in part due to the easier experimental accessibility of high lying overtones of these species versus lower frequency modes and in part due to the mass mismatch of the C and H atoms which accentuates local mode behaviour in these systems. A recently propounded theory dealing with the linewidths of these modes indicates that absorption lines will broaden due to the presence of resonances [ 21. In compounds with C-H stretching modes, a 2: 1 resonance will occur if the overtone of a lower lying mode is in resonance with the energy difference between the vth and (v- 1)th local mode. In hydrocarbons this resonance can occur when the overtone of the methyl rocking mode, typically near 2x 1450 cm-‘, is in resonance with the energy difference between two local mode levels. Of course resonances with other modes are also possible as are higher order resonances. This theory has been successful in explaining the trends in linewidth in a variety of hydrocarbon systems, particularly benzene and its deuterated derivatives [ 31. Quack and co-workers have put forth a theoretical treat-
ment, which has been applied to a variety of isolated C-H chromophores [ 41. This theory takes into account the coupling between the C-H stretching mode and various overtones of the C-H bending mode of the molecule (not just 2: 1 resonances) to explain observed positions and intensities of overtone absorptions. By fitting the spectrum, parameters for an . appropriate Hamiltonian have been determined which allows for the evaluation of the coupling between overtones of the stretching modes and overtones of the bending modes. In this treatment coupling between the stretching mode manifold and higher-order transitions involving the bending mode manifold have been shown to be important under a variety of circumstances. Recently there was the prediction [ 51, which was compatible with the observed broad overtone absorptions in the liquid [6], that CHC13 should possess broad gas phase overtone absorptions. In this molecule, which is a member of the CHX3 series, there are no fundamental absorptions in the region between 1150 and 3020 cm- ’ due to the large mass of the Cl atom [ 7 1. Thus, there are no modes for which the overtone is in near resonance with C-H local modes up to very high levels of excitation. Sub-. sequent to this prediction, a gas phase study of the CHC& system demonstrated that this compound had narrow overtone absorption lines at the v=6 level [8,9]. The observed spectra could be quantitatively simulated by a Hamiltonian which included cou-
0 009-26141881%03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
43
Volume 144, number 1
CHEMICAL PHYSICS LETTERS
pling between the bending and stretching modes. However, it was concluded that this coupling was sufficiently small so as not to be reflected in the overtone linewidth at this level of excitation [ 81. We have recently recorded overtone spectra of CHBr3. This compound is of interest since it is another member of the CHX, series; a series that has attracted previous experimental and theoretical interest [4-lo]. Thus the trends observed for the lighter members of this series can be extended, and the effect of a very large mass X on the behaviour of the isolated C-H chromophore can be observed. The purpose of this Letter is to report on our observations for this molecule and to compare and contrast these observations with the behaviour of the CHC& molecule.
2. Experimental Details of the experimental procedure have been presented previously [ 111. Briefly, the fundamental spectrum of CHB, was obtained using a Nicolet 7 199 FTIR spectrometer. Spectra of the U= 5 and v=:6 levels of CHBr, were obtained using a photoacoustic cell placed in the cavity of a Coherent 599 dye laser pumped by a CR 3000K krypton ion laser. For the purpose of providing a direct comparison on the same apparatus, a fundamental spectrum and spectra of the v=5 and v=6 levels of CHCl, were also recorded. LD 700 was pumped by the red lines of the Kr+ laser for the u= 5 spectrum while rhodamine 6G pumped by the blue green lines of the Kr+ laser was used to obtain the v=6 spectrum. Signals were ratioed against the power spectrum of the dye laser before being plotted. The dye laser linewidth was determined to be approximately 0.75 cm-‘. The method used for the determination of absolute absorption cross sections has also been described previously [ 111. C(CHJ)4 was used as a reference for the V= 5 level and CzH4 was used as a reference for the u=6 level. All spectra were obtained with the sample at its vapor pressure of approximately 4.2 Torr at 2 1“C. Due to the relatively low vapor pressure of CHBr, it was not possible to obtain spectra of the U= 2, 3 or 4 overtones above U= 6. CHBr3 was obtained from Aldrich Chemical and specified as > 99% pure. The sample was degassed and ethanol 44
12 February 1988
was removed by multiple freeze-pump-thaw cycles prior to use. CHC& was obtained from Aldrich with a stated purity 99 t O/o(gold label). It was also degassed by multiple freeze-pump-thaw cycles prior to use.
3. Results and discussion Figs. 1 and 2 depict the overtone spectrum of CHBr3 for the v=5 and v=6 levels respectively. These absorptions are centered at 13963 and 16393 cm-r, in comparison to their reported positions of 13840 and 16240 cm-’ in neat liquid [ 61. This corresponds to a shift in the gas phase versus liquid of approximately 120 to 150 cm- ’to higher frequency, which is not unexpected. The v=5 region was scanned in an effort to find other possible absorptions which could result from interaction of the C-H stretch with higher overtones of the C-H bending mode [ 121. No absorptions were found which could be deflnitively assigned to these types of interactions. Our signal/noise level let us set an upper limit
I
14000
/
13960 Wavenumbers
13920 (cm..’
)
Fig. 1. Photoacoustic absorption spectrum of the u= 5 level of the C-H stretch of CHBr,. The sample pressure was 4.2 Torr.
12 February 1988
CHEMICAL PHYSICS LETTERS
Volume 144, number 1
Table 1 Fundamental vibrational frequencies for CHCl, and CHBr, Mode
Symmetry
CHCl,
CHBr3
VI V4 VS V2 V3 V6
a, e e aI al e
3033 a’ 1205 a) 760 a’ 667 .’ 364 =’ 260 =’
3045 “), 3020 ‘), 3023 d,c) 1149 a), 1145 C),1142 d, 656 a) 655 d’ 539 %d.C.O 222 C.0) 152”
a) Ref. [7]. b, Ref. [ 131. ‘Ref. [ 141. d)Ref. [15]. “Ref. [16].
l.
16450
16400
Wavenumbers
16j50 (cm’-’
16300
)
Fig. 2. Photoacoustic absorption spectrum of the v= 6 level of the C-H stretch of CHBr,. The sample pressure was 4.2 Torr.
on the size of any such absorptions as no more than f the magnitude of the v= 5 absorption shown in fig. 1. The position of the fundamental absorption along with the v=5 and v=6 absorptions can be used to construct a Birge-Sponer plot. Though this plot consists of only three points, the points do correlate well with a straight line. Thus this plot can be used to obtain an approximate value of 3166 cm-’ for w,, the local mode harmonic frequency, and an approximate value of -62 cm-’ for 0,x,, the local mode anharmonicity. The differences between these values and those determined for CHCl, are statistically insignificant. Absolute absorption cross sections have been determined as (4.7f0.6) x 1O-24 and (5.2+0.8)X10-26cm2 for the v=5 and 6 levels of CHBr, respectively. These values also agree quite well with the values we have obtained for the corresponding cross sections in CHCL These are (2.5 kO.3) x 1O-24 and (5.5kO.5) x 1O-26cm2 respectively. As previously stated, the very narrow lines observed in CHC& (approximately 6 cm-’ fwhm for the Q branch of v= 5 and 4 cm- ’ fwhm for the Q branch of ~6) are compatible with the lack of significant coupling, at this level of excitation, between the C-H overtones and the bending modes. Consulting table 1, which contains the frequencies of the fundamentals of both CHBr, and CHC&, it is apparent that, as previously stated, CHC& would not
be expected to have a 2 : 1 resonance which would be operative for either the u= 5 or II= 6 levels of the C-H stretch. Thus within the context of a model that attributes broadening to the presence of 2: 1 resonances, narrow absorption lines would be expected and are ,observed in this molecule [ 81. When CHC& is treated in the context of a theory that allows for coupling of the C-H stretching modes to higher overtones of the bending modes it is still the case that little broadening of the C-H overtones at the u=6 level is predicted [S] _ Some degree of mixing between the higher overtones of the C-H stretch and the bending mode manifold is expected; however, it has been stated that the degree of mixing is insufficient to result in significant spectroscopically observable broadening of the stretching overtones under study. The degree of mixing in this treatment will also depend on the energy difference between the states involved. A comparison can now be made with the CHBr, molecule. Based on the frequencies of the various fundamentals and overtones (see table I), CHBr3 also would not be expected to have a 2 : 1 resonance at either the v= 5 or v= 6 level. Higher-order interactions would be necessary before some combination of stretching and bending modes would be near resonant at the level of the V= 5 or v= 6 C-H stretching mode. Thus extensive mixing between the C-H stretching overtones and the bending modes would not be expected, which is compatible with the observed narrow linewidths. However, for CHBr3 there. is a possible 3 : 1 resonance at the U= 6 level, 2v,f v4_ Because of the effect of the mass of the halogen atom on the normal mode frequencies in CHX, molecules, CHCl, does not possess 3: 1 interactions that are as close to resonance as in the CHBr, case. The role of 45
Volume 144, number 1
CHEMICAL PHYSICS LETTERS
3: 1 resonances for overtone states has not been treated in as much detail as 2: 1 resonances. However, it can be said with reasonable certainty that the same general characteristics that govern 2: 1 resonances will govern 3: 1 resonances. Of course it would be expected that the matrix element coupling the initial and final states in the 3 : 1 resonance case will be smaller than in the 2 : 1 case. Thus if resonances occur and there is the possibility of both 2; 1 and 3; 1 types of interactions, it would be expected that 2: 1 interactions will involve a stronger coupling and thus the linewidth will be dominated by these interactions for similar energy gaps. However, in the absence of 2 : 1 interactions (or if the energy gap is much larger than in the 3: 1 case), 3: 1 interactions could be expected to result in some broadening of overtone lines and could be the dominant factor in this broadening. Interestingly while the U= 5 level in CHBr3, with a fwhm of the Q branch of %7 cm-‘, is of similar width to the V= 5 or tr= 6 level in CHC&, the v= 6 level in CHBr3, where there is an expected 3: 1 resonance, is broader, with a fwhm of z 22 cm-‘, than either the V= 5 level of this molecule or the v= 6 level of CHC&. It is premature to say that this broadening is definitely due to the existence of a 3 : 1 resonance since other resonant interactions are possible and the observed change in width could be due to a change in rotational congestion. Additionally, the width of the v=6 level in CHBr3 is only slightly larger than the widths of the V= 5 level in CHBr, and the v= 5 and v=6 levels in CHC&. However, a 3: 1 interaction is an appealing explanation and it is certainly warranted to point out that the v=6 level in CHBrS is broader than either the V= 5 level in CHBr3 or the V= 5 or 6 levels in CHC&. In these latter cases, neither 2 : 1 nor 3 : 1 interactions would be expected to be significant.
4. Conclusions CHBr3 has been studied to provide information on the overtones of the C-H stretches of another member of the CHX3 series. Absolute absorption coefficients for both the v=5 and v=6 levels have been measured and are similar to measurements for the corresponding absorption coefficients in the 46
I2 February 1988
CHC& molecule. Approximate values for the local harmonic frequency, o,, and anharmonicity, 0,x,, have been determined. The values obtained are very similar to those obtained for CHCl+ The linewidths of the u=5 and v=6 levels of the C-H stretch are quite narrow relative to linewidths of typical hydrocarbons. The possible reasons for these narrow linewidths and a potential explanation for somewhat broader v=6 level in CHBr3 are discussed in terms of the absence or presence of resonant interactions.
Acknowledgement
We would like to thank the National Science Foundation for support of this work under grant CHE85-0657. We thank Lewis Baylor for his help with some of the experiments on the v= 5 overtones.
References [ 1] B.R. Henry, Accounts Chem. Res. 10 (1977) 207; J.S. Wang and C.B. Moore, in: Lasers and applications, eds. W.O.M. Guimaraes, C.-T. Lin and A. Mooradian (Springer, Berlin, 1981). [2] E.L. Sibert, W.P. Reinhardt and J.T. Hynes, J. Chem. Phys. 81 (1984) 1115. [ 31 K.V. Reddy, D.F. Heller and M.J. Berry, J. Chem. Phys. 76 (1982) 2814. [ 41 H.R. Dilbal and M. Quack, J. Chem. Phys. 81 (1984) 3779; J.E. Baggott, MC. Chuang, R.N. Zare, H.R. Diibal and M. Quack, J. Chem. Phys. 82 (1985) 1186. [5] S. Kato, J. Chem. Phys. 83 (1985) 1085. [6] H.L. Fang and R.L. Swofford, J. Chem. Phys. 72 (1980) 6382. [ 71 G. Herzberg, Infrared and Raman spectra (Van Nostrand, Princeton, 1945). [S] J.E. Baggott, H.J. Clase and LM. Mills, J. Chem. Phys. 84 (1986) 4193. [ 91 J.S. Wang and C.B. Moore, Proceedings of the 28th International Union of Pure and Applied Chemistry Congress, ed. K.J. Laidler (1981) p. 353. [ IO] G.A. Voth, R.A. Marcus and A.H. Zewail, J. Chem. Phys. 81 (1984) 5494; J.W. Perry; D.J. Moll, A. Kuppermann and A.H. Zewail, J. Chem. Phys. 82 (1985) 1195; A. Compargue and F. Stoeckel, J. Chem. Phys. 85 (1986) 1220. [ 111 C. Manzanares, N.L.S. Yamasaki, E. Weitz and J.T. Knudtson, Chem. Phys. Letters 117 (1985) 477. [ 121 M. Lewerenzand M. Quack, Chem. Phys. Letters 123 (1986) 197.
Volume 144, number I
CHEMICAL PHYSICS LETTERS
[ 131 A. Hacura and T.W. Zerda, J. Mol. Spectry. 60 (1980) 277; N.A. Borisevich, G.A. Zalesskaya, V.A. Lastocbkina and T. Shukurov, Spectry. Letters 18 ( 1982) 99 1. [ 141 E. Burgos, E. Halac and H. Bonadeo, J. Chem. Phys. 74 (1981) 1546.
12 February 1988
[ 151 Y. Isbikawa, S. Arai and R. Nakone, J. Nucl. Sci. Technol. 17 (1980) 275. [16] J.C. Decius, J. Chem.Phys. 16 (1948) 214.
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