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Refined Molecular Parameters for the Carbon Monoxide Stretching Band of the CO–CO2van der Waals Complex
JOURNAL OF MOLECULAR SPECTROSCOPY ARTICLE NO .
179, 345–347 (1996)
0215
NOTE Refined Molecular Parameters for the Carbon Monoxide Stretching Band of the CO–CO2 van der Waals Complex Microwave (1) and infrared (1, 2) measurements have shown that the van der Waals complex formed from carbon dioxide and carbon monoxide has a reasonably rigid T-shaped structure. In this arrangement, the C atoms are neighbors, with the CO2 unit forming the head of the T, and the CO unit forming its leg. The resulting planar C2£ structure means that the complex is a near-prolate asymmetric rotor whose ground vibrational state has only even values of Ka due to nuclear spin statistics (for the normal isotope, 12 16 C O2 ). The previous infrared studies of CO–CO2 concerned the asymmetric CO2 stretching vibration (1) near 2349 cm01 , which gives rise to a perpendicular band ( DKa Å {1), and the CO stretching vibration (2) near 2148 cm01 , which gives rise to a parallel band ( DKa Å 0). In this note, we report a reinvestigation of this parallel CO stretching band. The new results were obtained using a continuous supersonic slit-jet source and a tunable infrared diode laser spectrometer. The cw slit expansion gave higher rotational temperatures than the earlier pulsed circular jet expansions (1, 2), so that the new data extended to considerably higher Jvalues (19 as compared to 12). Furthermore, levels with Ka Å 4 were observed for the first time. A total of 111 transitions were measured, as compared to 32 in Ref. (2). Analysis of these new data, together with the
existing microwave measurements of (1), enables us to determine more accurate values for the excited state rotational parameters, and to independently determine the quartic centrifugal distortion parameters for the ground and excited states. The apparatus used here was the same as that used in our recent studies of the Ar–CO (3, 4) and N2 –CO (5) complexes. It consisted of a 2.5-cm 1 50-mm slit nozzle in a chamber evacuated by an Edwards EH4200 Roots pump. The radiation from a liquid-nitrogen-cooled diode laser made 13 passes through the jet with the help of multitraversal mirrors. Absorption was detected by simple laser frequency modulation and 2 f lock-in detection. Wavenumber calibration was made by simultaneously recording e´talon and calibration gas (N2O) signals along with the CO–CO2 signal. The gas mixture was approximately 75% He, 25% CO, and a trace amount of CO2 , with a source pressure of 1100 Torr and a chamber pressure of 350 mTorr. The estimated rotational temperature in the jet was about 10 K. Separate spectra were also recorded without CO2 in order to determine which lines might be due to the CO dimer, (CO)2 . A section of the observed spectrum extending from P(2) to P(8) is shown in Fig. 1. In this figure, the Ka Å 0 transitions are denoted by the
FIG. 1. A portion of the infrared absorption spectrum of the CO–CO2 complex as recorded with the slit jet and diode laser. K Å 0 transitions are denoted by the downward-pointing arrows, the asymmetry-doubled K Å 2 transitions by open and closed circles, and the K Å 4 transitions by upwardpointing arrows. The lines have a second-derivative shape due to the use of laser frequency modulation followed by 2 f lock-in amplification.
345 0022-2852/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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JMS 7081
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6t0e$$$401
08-26-96 11:28:01
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AP: Mol Spec
346
NOTE
TABLE 1 Observed Transitions in the CO Stretching Band of CO–CO2
downward-pointing arrows, the asymmetry-doubled Ka Å 2 transitions by open and closed circles, and the Ka Å 4 transitions by upward-pointing arrows. Asymmetry doubling was not observed for Ka Å 4, nor would this be expected with the present spectral resolution. A comparison of Fig. 1 with the same range for the R-branch shown in Fig. 1 of Ref. (2) illustrates the narrower linewidth and higher rotational temperature given by the present slit-jet expansion. We fit the spectrum using an s-reduced asymmetric rotor Hamiltonian (6) so that the parameters would be directly comparable with those obtained in (2). The fit included 111 infrared transitions of CO–CO2 as listed in Table 1, together with the 10 microwave transition of the normal isotope
from (1). The microwave lines were given relative weights of 1 1 10 8 in the least-squares fit to reflect their higher accuracy. In six cases, infrared lines with different Ka-values in Table 1 were unresolved from each other, and these were given relative weights of 0.5 in the fit. In addition to the P- and R-branches listed here, the Q-branch was also clearly observed. However, since most Q-branch transitions were only partly resolved from each other, they were not used in the fit. The infrared and microwave data analyzed here are not sensitive to the absolute value of the A rotational constant, but only to its change between the ground and excited states. Therefore A 9 was held fixed in the fit at the known value ( 1) from the CO2 asymmetric stretching band, while A * was
Copyright q 1996 by Academic Press, Inc.
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08-26-96 11:28:01
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AP: Mol Spec
347
NOTE
TABLE 2 Molecular Parameters for CO–CO2
DK / DJK / DJ Å 0 (1/4) taaaa
[1]
D(CO2 ) Å 0 (1/4) taaaa ,
[2]
where D(CO2 ) is the distortion parameter (about 4 kHz) for the CO2 monomer and [2] holds in the present case, for C2£ symmetry, since only the O atoms of CO2 lie off the a-axis. Since both D(CO2 ) and DJ are much smaller than DJK , [1] and [2] mean that DK É 0DJK . This calculation explains our decision to fix the value of D K9 at a ‘‘best guess’’ value of 0350 kHz in the final fit. What about the Ka Å 6 levels? We carefully examined our spectra in the regions of predicted transitions involving Ka Å 6, and, in four or five cases, we observed weak lines located almost exactly at the expected positions. However, none of these lines were sufficiently strong and clean to include with confidence in the analysis. We conclude that it is likely that the Ka Å 6 transitions are located very close to the positions predicted by the parameters of Table 2. In conclusion, we have reexamined the parallel CO stretching band of the CO–CO2 van der Waals complex in the 2148 cm01 region, using a continuous slit-jet supersonic expansion and a tunable infrared diode laser spectrometer. In doing so, the coverage of J-levels has been extended from 12 to 19, and levels with Ka Å 4 have been observed for the first time. A refined set of molecular parameters, including quartic centrifugal distortion, has been obtained for this band. varied. The present data are similarly insensitive to the centrifugal distortion parameter DK , which, moreover, is not available from the perpendicular band since only K a9 Å 0 and 2 transitions were observed there (1). We chose to fix D K9 at an estimated value ( 0350 kHz, see below), while also slightly adjusting the fixed value of A 9 to compensate. Note that the present data are sensitive to the change in DK between the ground and excited states, and so D K* was allowed to vary in the fit. Parameters resulting from the fit are listed in Table 2, and the residuals (Obs 0 Calc) are listed in Table 1. The quality of the fit was very good, with an rms deviation of only 0.0003 cm01 for the observed infrared transitions. The agreement with the results of Randall et al. (2) is quite good. The present band origin is slightly (0.0003 cm01 ) higher in wavenumber, but this simply reflects the difficulty of obtaining very accurate absolute calibration in diode laser spectroscopy. The new rotational parameters are also close to those of (2). For the excited state, their uncertainties are reduced (by factors of from 2.4 to 5.5), whereas for the ground state their uncertainties are somewhat increased (by a factor of about 2.2), though still smaller than in (1). These slightly larger uncertainties for B 9 and C 9 occur because we independently varied ground and excited state centrifugal distortion parameters, as contrasted to Ref. (2), where the excited state centrifugal distortion parameters were fixed at their ground state values. It is perhaps not surprising that these previous centrifugal parameters (2) agree most closely with the present ground state values, since they were largely determined by the microwave data. At any rate, all of the present distortion parameters, except for d1 , show changes between the ground and excited states which are significant at the 1s level. Ground state parameters from a preliminary version of our fit with DK fixed to zero were used in a harmonic force field analysis. There are four van der Waals vibrational modes: one stretch and three bends. Since only four centrifugal distortion parameters were determined, it was necessary to constrain the three bending force constants to be equal. Due to this approximation, and to the fact that this sort of weakly bound complex is likely to be rather anharmonic, we feel that the resulting force field is of limited significance. However, one result of the calculation was a predicted value for DK of about 0350 kHz. Indeed, this value can also be understood from the relations (7)
ACKNOWLEDGMENT We are grateful to J. K. G. Watson for helpful discussions.
REFERENCES 1. A. C. Legon and A. P. Suckley, J. Chem. Phys. 91, 4440–4447 (1989). 2. R. W. Randall, J. P. L. Summersgill, and B. J. Howard, J. Chem. Soc. Faraday Trans. 86, 1943–1947 (1990). 3. Y. Xu, S. Civis, A. R. W. McKellar, S. Ko¨nig, M. Haverlag, G. Hilpert, and M. Havenith, Mol. Phys. 87, 1071–1082 (1996). 4. Y. Xu and A. R. W. McKellar, Mol. Phys. 88, 859–874 (1996). 5. Y. Xu and A. R. W. McKellar, J. Chem. Phys. 104, 2488–2496 (1996). 6. J. K. G. Watson, in ‘‘Vibrational Spectra and Structure’’ (J. R. Durig, Ed.), Vol. 6, pp. 1–89, Elsevier, Amsterdam, 1977. 7. For example, D. Kivelson and E. B. Wilson, J. Chem. Phys. 20, 1575– 1579 (1951). Yunjie Xu* ,1 A. R. W. McKellar* B. J. Howard† *Steacie Institute for Molecular Sciences National Research Council of Canada Ottawa, Ontario K1A 0R6, Canada †Physical Chemistry Laboratory Oxford University Oxford OX1 3QZ, United Kingdom Received April 19, 1996
1 Present address: Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada.
Copyright q 1996 by Academic Press, Inc.
AID
JMS 7081
/
6t0e$$$402
08-26-96 11:28:01
mspa
AP: Mol Spec
179, 345–347 (1996)
0215
NOTE Refined Molecular Parameters for the Carbon Monoxide Stretching Band of the CO–CO2 van der Waals Complex Microwave (1) and infrared (1, 2) measurements have shown that the van der Waals complex formed from carbon dioxide and carbon monoxide has a reasonably rigid T-shaped structure. In this arrangement, the C atoms are neighbors, with the CO2 unit forming the head of the T, and the CO unit forming its leg. The resulting planar C2£ structure means that the complex is a near-prolate asymmetric rotor whose ground vibrational state has only even values of Ka due to nuclear spin statistics (for the normal isotope, 12 16 C O2 ). The previous infrared studies of CO–CO2 concerned the asymmetric CO2 stretching vibration (1) near 2349 cm01 , which gives rise to a perpendicular band ( DKa Å {1), and the CO stretching vibration (2) near 2148 cm01 , which gives rise to a parallel band ( DKa Å 0). In this note, we report a reinvestigation of this parallel CO stretching band. The new results were obtained using a continuous supersonic slit-jet source and a tunable infrared diode laser spectrometer. The cw slit expansion gave higher rotational temperatures than the earlier pulsed circular jet expansions (1, 2), so that the new data extended to considerably higher Jvalues (19 as compared to 12). Furthermore, levels with Ka Å 4 were observed for the first time. A total of 111 transitions were measured, as compared to 32 in Ref. (2). Analysis of these new data, together with the
existing microwave measurements of (1), enables us to determine more accurate values for the excited state rotational parameters, and to independently determine the quartic centrifugal distortion parameters for the ground and excited states. The apparatus used here was the same as that used in our recent studies of the Ar–CO (3, 4) and N2 –CO (5) complexes. It consisted of a 2.5-cm 1 50-mm slit nozzle in a chamber evacuated by an Edwards EH4200 Roots pump. The radiation from a liquid-nitrogen-cooled diode laser made 13 passes through the jet with the help of multitraversal mirrors. Absorption was detected by simple laser frequency modulation and 2 f lock-in detection. Wavenumber calibration was made by simultaneously recording e´talon and calibration gas (N2O) signals along with the CO–CO2 signal. The gas mixture was approximately 75% He, 25% CO, and a trace amount of CO2 , with a source pressure of 1100 Torr and a chamber pressure of 350 mTorr. The estimated rotational temperature in the jet was about 10 K. Separate spectra were also recorded without CO2 in order to determine which lines might be due to the CO dimer, (CO)2 . A section of the observed spectrum extending from P(2) to P(8) is shown in Fig. 1. In this figure, the Ka Å 0 transitions are denoted by the
FIG. 1. A portion of the infrared absorption spectrum of the CO–CO2 complex as recorded with the slit jet and diode laser. K Å 0 transitions are denoted by the downward-pointing arrows, the asymmetry-doubled K Å 2 transitions by open and closed circles, and the K Å 4 transitions by upwardpointing arrows. The lines have a second-derivative shape due to the use of laser frequency modulation followed by 2 f lock-in amplification.
345 0022-2852/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
AID
JMS 7081
/
6t0e$$$401
08-26-96 11:28:01
mspa
AP: Mol Spec
346
NOTE
TABLE 1 Observed Transitions in the CO Stretching Band of CO–CO2
downward-pointing arrows, the asymmetry-doubled Ka Å 2 transitions by open and closed circles, and the Ka Å 4 transitions by upward-pointing arrows. Asymmetry doubling was not observed for Ka Å 4, nor would this be expected with the present spectral resolution. A comparison of Fig. 1 with the same range for the R-branch shown in Fig. 1 of Ref. (2) illustrates the narrower linewidth and higher rotational temperature given by the present slit-jet expansion. We fit the spectrum using an s-reduced asymmetric rotor Hamiltonian (6) so that the parameters would be directly comparable with those obtained in (2). The fit included 111 infrared transitions of CO–CO2 as listed in Table 1, together with the 10 microwave transition of the normal isotope
from (1). The microwave lines were given relative weights of 1 1 10 8 in the least-squares fit to reflect their higher accuracy. In six cases, infrared lines with different Ka-values in Table 1 were unresolved from each other, and these were given relative weights of 0.5 in the fit. In addition to the P- and R-branches listed here, the Q-branch was also clearly observed. However, since most Q-branch transitions were only partly resolved from each other, they were not used in the fit. The infrared and microwave data analyzed here are not sensitive to the absolute value of the A rotational constant, but only to its change between the ground and excited states. Therefore A 9 was held fixed in the fit at the known value ( 1) from the CO2 asymmetric stretching band, while A * was
Copyright q 1996 by Academic Press, Inc.
AID
JMS 7081
/
6t0e$$$402
08-26-96 11:28:01
mspa
AP: Mol Spec
347
NOTE
TABLE 2 Molecular Parameters for CO–CO2
DK / DJK / DJ Å 0 (1/4) taaaa
[1]
D(CO2 ) Å 0 (1/4) taaaa ,
[2]
where D(CO2 ) is the distortion parameter (about 4 kHz) for the CO2 monomer and [2] holds in the present case, for C2£ symmetry, since only the O atoms of CO2 lie off the a-axis. Since both D(CO2 ) and DJ are much smaller than DJK , [1] and [2] mean that DK É 0DJK . This calculation explains our decision to fix the value of D K9 at a ‘‘best guess’’ value of 0350 kHz in the final fit. What about the Ka Å 6 levels? We carefully examined our spectra in the regions of predicted transitions involving Ka Å 6, and, in four or five cases, we observed weak lines located almost exactly at the expected positions. However, none of these lines were sufficiently strong and clean to include with confidence in the analysis. We conclude that it is likely that the Ka Å 6 transitions are located very close to the positions predicted by the parameters of Table 2. In conclusion, we have reexamined the parallel CO stretching band of the CO–CO2 van der Waals complex in the 2148 cm01 region, using a continuous slit-jet supersonic expansion and a tunable infrared diode laser spectrometer. In doing so, the coverage of J-levels has been extended from 12 to 19, and levels with Ka Å 4 have been observed for the first time. A refined set of molecular parameters, including quartic centrifugal distortion, has been obtained for this band. varied. The present data are similarly insensitive to the centrifugal distortion parameter DK , which, moreover, is not available from the perpendicular band since only K a9 Å 0 and 2 transitions were observed there (1). We chose to fix D K9 at an estimated value ( 0350 kHz, see below), while also slightly adjusting the fixed value of A 9 to compensate. Note that the present data are sensitive to the change in DK between the ground and excited states, and so D K* was allowed to vary in the fit. Parameters resulting from the fit are listed in Table 2, and the residuals (Obs 0 Calc) are listed in Table 1. The quality of the fit was very good, with an rms deviation of only 0.0003 cm01 for the observed infrared transitions. The agreement with the results of Randall et al. (2) is quite good. The present band origin is slightly (0.0003 cm01 ) higher in wavenumber, but this simply reflects the difficulty of obtaining very accurate absolute calibration in diode laser spectroscopy. The new rotational parameters are also close to those of (2). For the excited state, their uncertainties are reduced (by factors of from 2.4 to 5.5), whereas for the ground state their uncertainties are somewhat increased (by a factor of about 2.2), though still smaller than in (1). These slightly larger uncertainties for B 9 and C 9 occur because we independently varied ground and excited state centrifugal distortion parameters, as contrasted to Ref. (2), where the excited state centrifugal distortion parameters were fixed at their ground state values. It is perhaps not surprising that these previous centrifugal parameters (2) agree most closely with the present ground state values, since they were largely determined by the microwave data. At any rate, all of the present distortion parameters, except for d1 , show changes between the ground and excited states which are significant at the 1s level. Ground state parameters from a preliminary version of our fit with DK fixed to zero were used in a harmonic force field analysis. There are four van der Waals vibrational modes: one stretch and three bends. Since only four centrifugal distortion parameters were determined, it was necessary to constrain the three bending force constants to be equal. Due to this approximation, and to the fact that this sort of weakly bound complex is likely to be rather anharmonic, we feel that the resulting force field is of limited significance. However, one result of the calculation was a predicted value for DK of about 0350 kHz. Indeed, this value can also be understood from the relations (7)
ACKNOWLEDGMENT We are grateful to J. K. G. Watson for helpful discussions.
REFERENCES 1. A. C. Legon and A. P. Suckley, J. Chem. Phys. 91, 4440–4447 (1989). 2. R. W. Randall, J. P. L. Summersgill, and B. J. Howard, J. Chem. Soc. Faraday Trans. 86, 1943–1947 (1990). 3. Y. Xu, S. Civis, A. R. W. McKellar, S. Ko¨nig, M. Haverlag, G. Hilpert, and M. Havenith, Mol. Phys. 87, 1071–1082 (1996). 4. Y. Xu and A. R. W. McKellar, Mol. Phys. 88, 859–874 (1996). 5. Y. Xu and A. R. W. McKellar, J. Chem. Phys. 104, 2488–2496 (1996). 6. J. K. G. Watson, in ‘‘Vibrational Spectra and Structure’’ (J. R. Durig, Ed.), Vol. 6, pp. 1–89, Elsevier, Amsterdam, 1977. 7. For example, D. Kivelson and E. B. Wilson, J. Chem. Phys. 20, 1575– 1579 (1951). Yunjie Xu* ,1 A. R. W. McKellar* B. J. Howard† *Steacie Institute for Molecular Sciences National Research Council of Canada Ottawa, Ontario K1A 0R6, Canada †Physical Chemistry Laboratory Oxford University Oxford OX1 3QZ, United Kingdom Received April 19, 1996
1 Present address: Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada.
Copyright q 1996 by Academic Press, Inc.
AID
JMS 7081
/
6t0e$$$402
08-26-96 11:28:01
mspa
AP: Mol Spec