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Microwave Rotational Spectrum ofgaucheEthyl Alcohol in Excited States of the –OH Torsion
JOURNAL OF MOLECULAR SPECTROSCOPY ARTICLE NO.
175, 390–394 (1996)
0045
Microwave Rotational Spectrum of gauche Ethyl Alcohol in Excited States of the –OH Torsion Chun Fu Su* and C. Richard Quade† *Department of Physics, Mississippi State University, Mississippi State, Mississippi 39762; and †Department of Physics, Texas Tech University, Lubbock, Texas 79409 Received October 30, 1995; in revised form November 27, 1995
The microwave rotational spectrum of gauche CH3CH2OH has been identified and assigned for the two substates of the first excited state of the –OH torsion. Since mb á 0 by accident, only a-dipole transitions have been assigned. It was not possible to observe the c-dipole transitions since the tunneling frequency between the two substates is expected to be on the order of 1.7 THz. The assigned lines do not give a rigid rotor fit. The spectrum has been calculated using the IAM program of Liu for asymmetric–asymmetric molecules, which was of little use in the analysis. However, it has been necessary to determine improved potential energy coefficients for the –OH torsion with the result V1 Å 45.3 cm01, V2 Å 13.0 cm01, and V3 Å 370.0 cm01. q 1996 Academic Press, Inc. INTRODUCTION
Many years ago, Culot (1) completed a definitive study of the microwave rotational spectra of several isotopic species of trans ethyl alcohol in the torsional groundstate. From the empirical rotational constants, he was able to determine a molecular structure for the molecule. From Stark effect measurements, he was able to determine the b-component of the electric dipole moment; mc is zero by symmetry and ma is accidentally zero. Several years later, Kakar et al. (2) published results of a study of the microwave torsional–rotational spectra for several isotopic species of gauche ethyl alcohol in the ground states. For this conformation, mb is accidentally zero, but aand c-dipole transitions are observed. The c-dipole transitions connect the two equivalent gauche configurations and provide for the measurement of the tunneling energy between the two levels. Using the relative intensities of the trans and gauche ground state transitions, the ground state gauche tunneling energy and the term values observed by infrared spectroscopy from the trans ground state to the trans first excited state, an empirical Fourier potential energy function was determined that gave good agreement with experimental data. However, these potential energy coefficients are considerably improved in the present work using a different basis for the calculation. Subsequently, Sasada (3) assigned spectra for the trans conformation in the first excited state of the –OH torsion, for another vibrational excited state, and for the first excited state of the –CH3 torsion. Very recently, Pearson et al. (4, 5) have assigned and analyzed many, many additional lines for CH3CH2OH in
both the trans and gauche torsional ground states. One important thing that they determined that is relevant to our calculations is an improved value for the trans–gauche energy difference. In this work, we have extended the study of the microwave rotational spectrum of CH3CH2OH to the first excited state of the –OH torsion for the gauche conformation. We were particularly interested to determine whether or not the behavior of the spectral lines would indicate sensitivity to vibration–rotation–internal-rotation interactions. Such has been found to be the case but no theoretical formulations are reported in this work. Since mb á 0, only a-dipole transitions have been observed for the gauche excited states since the tunneling frequency between the two substates is expected to be on the order of 1.7 THz. It has been found that the adipole gauche spectrum is far from rigid rotor. EXPERIMENTAL DETAILS
All measurements were made between 17.0 and 72.5 GHz on the Mississippi State H–P spectrometer in P, K, and R bands. Above 40 GHz, SpaceKom (Honeywell) doublers were incorporated into the H–P spectrometer. All measurements were made at room temperature. The CH3CH2OH sample was of reagent grade. The Stark effect, relative intensities, and rough predicted line position were the primary basis for the assignments. THE EXCITED STATE gauche SPECTRUM
The identification and assignment of the excited state gauche spectrum started with the 101 – 000 lines. These lines
390 0022-2852/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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AP: Mol Spec
MICROWAVE SPECTRUM CH3CH2OH EXCITED –OH
show a large shift to either side of the ground state lines and three lines were found, the third possibly being from one of the substates of the second excited state. Then R band was searched for the J Å 1 r 2 excited state lines and a complete set was found after a couple of false starts. The search was extended to the 50 – 55 GHz region for the J Å 2 r 3 lines after the previously unassigned ground state lines had been identified. The biggest problem of the search was the identification and assignment of the 312 – 211 line for the ( /) state. There was extremely weak power from the BWO in this area before doubling. By increasing the time constant and slowing the sweep, three lines were dug out of the noise in about the right region. One of these lines showed the proper Stark behavior for the assignment.
Finally, K Å 0 and K Å 1 transitions were assigned for J Å 3 r 4. As part of the assignment, it was necessary to fix a particular series to the gauche(/) or gauche(0) substates. To accomplish this, relative intensity measurements were made of all pairs of lines for the J Å 1 r 2 sequence and selected pairs of the J Å 2 r 3 sequence. Since the gauche(0) substate is predicted to be 57 cm01 above the gauche(/) substate, it was expected that the gauche(0) lines would be of the order 80% intensity of the gauche(/) lines. This has been found to be the case which results in the (/) and (0) assignment of the spectrum given in Table 1. In the course of the work, two sets of excited state trans
TABLE 1 Microwave Rotational Spectrum of gauche CH3CH2OH in the First Excited State of the –OH Torsion (Units Are MHz)
Copyright q 1996 by Academic Press, Inc.
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391
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392
SU AND QUADE
TABLE 2 Rotational Constants for gauche CH3CH2OH Is the First Excited State of the –OH Torsion (Units Are MHz)
lines were observed. These turned out to be two of the three sets of the lines that Sasada previously assigned (3). One series is the first excited state of the – OH torsion. The second is assigned by Sasada to the first excited state of the CCO bend. We had thought that this series would be the second excited state of the – OH torsion for the trans, which was expected to have the intensity that we observed, but it does show an unusual shift in the rotational constants. We did not observe the lines for the first excited state of the – CH3 torsion. That could be due to the sensitivity of our spectrometer.
method three did not account for the representative trends of the data. Finally, an attempt was made to analyze the spectrum on the basis of Liu’s IAM program (6) for internal rotation in asymmetric – asymmetric molecules. It was expected that nonrigid rotor effects (7) would be large for the excited states (8), but it was hoped that these nonrigid rotor trends in the data would be accounted for with the single internal degree of freedom model. Such has not proved to be the case. For the (/) substate, none of the differences between (B / C)CalI and (B / C)CalII could be accounted for from the more rigorous calculation. However, the rigid rotor effects of nonrigidity were comparable to what would be expected from the results for CH2DOH and CHD2OH (8). For the ( 0) substate, the calculation gave term values substantially below the observed values. In this case, the calculation (I) requires very large nonrigidity effects and (II) gives a much smaller, by a factor of 2, B 0 C than has been observed. It was found that there is a very large pPz interaction from the torsion – rotation Hamiltonian that shifts the K Å 1 lines substantially and this could be detected if b-dipole transitions were allowed. In no sense did the rigorous calculation provide for the differences in B / C from Calculations I and II. TABLE 3 Seven Inertial Parameters for CH3CH2OH and CH3CH2OD (Units Are au A2)
ANALYSIS
The method of analysis of these a-dipole lines for the first excited gauche states, while straightforward, is not completely satisfying. It was expected to find a set of B and C that would work for all lines of a ( /) or (0) sequence. In first order we even expected that effects of nonrigidity would be ‘‘rigid rotor’’ in behavior. Such has not been the case. However, it is possible to find a B / C that will work moderately well for the K Å 0 lines of a given sequence and another B / C that will work for the K Å 1 lines. Three methods of analysis were tried to determine the B and C rotational constants. The first took B 0 C from the K Å 1, J Å 1 r 2 lines and B / C from the average of the appropriate K Å 0 lines. The second took B 0 C from the average of the K Å 1, J Å 1 r 2, and J Å 2 r 3 lines and B / C from the average of these lines. In Tables 1 and 2, these results are referred to as Calculations I and II. The third method took all of the lines of a sequence and found average B and C from which calculated term values were determined. In the end, it was felt that
Note. Torsional kinetic energy coefficients are in cm01.
Copyright q 1996 by Academic Press, Inc.
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JMS 6944
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6t08$$$302
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AP: Mol Spec
393
MICROWAVE SPECTRUM CH3CH2OH EXCITED –OH
TABLE 4 Calculated and Observed Torsional Energy Differences for CH3CH2OH and CH3CH2OD (Units Are cm01)
a
Reference Reference c Reference d Reference b
(5). (10). (11). (2).
In the course of the analysis, it was necessary to determine improved potential energy coefficients for the –OH torsion. The earlier calculations of Kakar and Quade (2) used a harmonic oscillator basis for the torsion, which it turns out reproduced the vibrational energies accurately but did not work well for the tunneling energy of the ground state due to the asymptotic nature of the wavefunctions. The seven inertial parameters (6), calculated using Culot’s molecular structure with a CO tilt of 3.57, are reported in Table 3 along with the torsional coefficients calculated from the parameters. The improved potential energy coefficients are from Liu’s theory for the torsional problem (6): This work
Ref. (2)
V1 Å 45.3 cm01 V2 Å 13.0 cm01 V3 Å 370.0 cm01
57.0 cm01 0.8 cm01 395.0 cm01
[1]
The calculated rotational frequencies of the lines assigned in this work were relatively insensitive to the potential energy coefficients. The calculated and observed term values from the microwave and infrared data are given in Table 4 for CH3CH2OH and CH3CH2OD.
DISCUSSION
The assignment and observed behavior of the spectral lines for gauche CH3CH2OH in the excited states of the –OH torsion show interesting features. The non-rigid-rotor behavior of the a-dipole lines for excited states of CH2DOH and CHD2OH has been observed previously (9). In the case of these molecules, the lines were much stronger, but so many excited states were observed that it was not possible to assign the lines to a particular substate. However, for CH3CH2OH, things were more favorable since there are fewer lines because (i) ma á 0 for the trans conformation and (ii) the intensity is down for the heavier molecule; therefore, it was possible to make the assignments. Even so, the identifications and assignments for CH3CH2OH were not easy and were time-consuming. In the course of the work, we did assign a few additional ground state lines for both the trans and gauche conformations. However, when we were essentially through with our work, Pearson et al. made their results (4, 5) available to us, and this was a big help in firming things up. A couple of our assignments, it turned out, needed to be reinvestigated. At the present time there is not a suitable and/or useful
Copyright q 1996 by Academic Press, Inc.
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JMS 6944
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6t08$$$302
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AP: Mol Spec
394
SU AND QUADE
theory for analyzing the features of a spectrum for the excited internal rotation states in the asymmetric–asymmetric molecules when there are substantial non-rigid-rotor effects that arise from the interactions of internal rotation and rotation with other vibrations. In summary, the spectral data for CH3CH2OH in the excited states of the –OH torsion should provide a useful check on the theory for rotation–internalrotation–vibration interactions when it is available.
ACKNOWLEDGMENT The authors are indebted to Dr. J. C. Pearson for making his measurements and calculations on trans and gauche CH3CH2OH in the torsional groundstates available to them. Further, in comparing some of our measurements with his, Dr. Pearson noted that we had a serious problem with our spectrometer’s reference oscillator, which we fortunately were able to correct.
REFERENCES 1. (a) J. P. Culot, Fourth Austin Symposium on Gas Phase Molecular Structure, Austin, Texas, 1972, Paper T8; (b) J. P. Culot, Ph.D. thesis, University of Louvain, Belgium, July 1971. 2. (a) R. K. Kakar and C. R. Quade, J. Chem. Phys. 72, 4300–4307 (1980); (b) R. K. Kakar and P. J. Seibt, J. Chem. Phys. 57, 4060 (1972). 3. Y. Sasada, J. Mol. Struct. 190, 93–97 (1988). 4. J. C. Pearson, K. V. L. N. Sastry, M. Winnewisser, E. Herbst, and F. C. DeLucia, J. Chem. Phys. Ref. Data 24, 1–32 (1995). 5. J. C. Pearson, K. V. L. N. Sastry, E. Herbst, and F. C. DeLucia, ‘‘The Millimeter and Submillimeter Wave Spectrum of gauche Ethyl Alcohol,’’ preprint. 6. M. Liu and C. R. Quade, J. Mol. Spectrosc. 146, 238–251 (1991). 7. M. Liu and C. R. Quade, J. Mol. Spectrosc. 146, 252–263 (1991). 8. C. F. Su, M. Liu, and C. R. Quade, J. Mol. Spectrosc. 149, 557–558 (1991). 9. (a) R. D. Suenram and C. R. Quade, unpublished; (b) H. R. Test and C. R. Quade, unpublished. 10. J. H. S. Green, private communication as quoted in A. J. Barnes and H. E. Hallam, Trans. Faraday Soc. 66, 1932–1940 (1970). 11. J. R. Durig, W. E. Bucy, C. J. Wurrey, and L. A. Carreira, J. Phys. Chem. 79, 988–993 (1975).
Copyright q 1996 by Academic Press, Inc.
AID
JMS 6944
/
6t08$$$304
02-05-96 20:22:41
mspas
AP: Mol Spec
175, 390–394 (1996)
0045
Microwave Rotational Spectrum of gauche Ethyl Alcohol in Excited States of the –OH Torsion Chun Fu Su* and C. Richard Quade† *Department of Physics, Mississippi State University, Mississippi State, Mississippi 39762; and †Department of Physics, Texas Tech University, Lubbock, Texas 79409 Received October 30, 1995; in revised form November 27, 1995
The microwave rotational spectrum of gauche CH3CH2OH has been identified and assigned for the two substates of the first excited state of the –OH torsion. Since mb á 0 by accident, only a-dipole transitions have been assigned. It was not possible to observe the c-dipole transitions since the tunneling frequency between the two substates is expected to be on the order of 1.7 THz. The assigned lines do not give a rigid rotor fit. The spectrum has been calculated using the IAM program of Liu for asymmetric–asymmetric molecules, which was of little use in the analysis. However, it has been necessary to determine improved potential energy coefficients for the –OH torsion with the result V1 Å 45.3 cm01, V2 Å 13.0 cm01, and V3 Å 370.0 cm01. q 1996 Academic Press, Inc. INTRODUCTION
Many years ago, Culot (1) completed a definitive study of the microwave rotational spectra of several isotopic species of trans ethyl alcohol in the torsional groundstate. From the empirical rotational constants, he was able to determine a molecular structure for the molecule. From Stark effect measurements, he was able to determine the b-component of the electric dipole moment; mc is zero by symmetry and ma is accidentally zero. Several years later, Kakar et al. (2) published results of a study of the microwave torsional–rotational spectra for several isotopic species of gauche ethyl alcohol in the ground states. For this conformation, mb is accidentally zero, but aand c-dipole transitions are observed. The c-dipole transitions connect the two equivalent gauche configurations and provide for the measurement of the tunneling energy between the two levels. Using the relative intensities of the trans and gauche ground state transitions, the ground state gauche tunneling energy and the term values observed by infrared spectroscopy from the trans ground state to the trans first excited state, an empirical Fourier potential energy function was determined that gave good agreement with experimental data. However, these potential energy coefficients are considerably improved in the present work using a different basis for the calculation. Subsequently, Sasada (3) assigned spectra for the trans conformation in the first excited state of the –OH torsion, for another vibrational excited state, and for the first excited state of the –CH3 torsion. Very recently, Pearson et al. (4, 5) have assigned and analyzed many, many additional lines for CH3CH2OH in
both the trans and gauche torsional ground states. One important thing that they determined that is relevant to our calculations is an improved value for the trans–gauche energy difference. In this work, we have extended the study of the microwave rotational spectrum of CH3CH2OH to the first excited state of the –OH torsion for the gauche conformation. We were particularly interested to determine whether or not the behavior of the spectral lines would indicate sensitivity to vibration–rotation–internal-rotation interactions. Such has been found to be the case but no theoretical formulations are reported in this work. Since mb á 0, only a-dipole transitions have been observed for the gauche excited states since the tunneling frequency between the two substates is expected to be on the order of 1.7 THz. It has been found that the adipole gauche spectrum is far from rigid rotor. EXPERIMENTAL DETAILS
All measurements were made between 17.0 and 72.5 GHz on the Mississippi State H–P spectrometer in P, K, and R bands. Above 40 GHz, SpaceKom (Honeywell) doublers were incorporated into the H–P spectrometer. All measurements were made at room temperature. The CH3CH2OH sample was of reagent grade. The Stark effect, relative intensities, and rough predicted line position were the primary basis for the assignments. THE EXCITED STATE gauche SPECTRUM
The identification and assignment of the excited state gauche spectrum started with the 101 – 000 lines. These lines
390 0022-2852/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
AID
JMS 6944
/
6t08$$$301
02-05-96 20:22:41
mspas
AP: Mol Spec
MICROWAVE SPECTRUM CH3CH2OH EXCITED –OH
show a large shift to either side of the ground state lines and three lines were found, the third possibly being from one of the substates of the second excited state. Then R band was searched for the J Å 1 r 2 excited state lines and a complete set was found after a couple of false starts. The search was extended to the 50 – 55 GHz region for the J Å 2 r 3 lines after the previously unassigned ground state lines had been identified. The biggest problem of the search was the identification and assignment of the 312 – 211 line for the ( /) state. There was extremely weak power from the BWO in this area before doubling. By increasing the time constant and slowing the sweep, three lines were dug out of the noise in about the right region. One of these lines showed the proper Stark behavior for the assignment.
Finally, K Å 0 and K Å 1 transitions were assigned for J Å 3 r 4. As part of the assignment, it was necessary to fix a particular series to the gauche(/) or gauche(0) substates. To accomplish this, relative intensity measurements were made of all pairs of lines for the J Å 1 r 2 sequence and selected pairs of the J Å 2 r 3 sequence. Since the gauche(0) substate is predicted to be 57 cm01 above the gauche(/) substate, it was expected that the gauche(0) lines would be of the order 80% intensity of the gauche(/) lines. This has been found to be the case which results in the (/) and (0) assignment of the spectrum given in Table 1. In the course of the work, two sets of excited state trans
TABLE 1 Microwave Rotational Spectrum of gauche CH3CH2OH in the First Excited State of the –OH Torsion (Units Are MHz)
Copyright q 1996 by Academic Press, Inc.
AID
JMS 6944
/
6t08$$$302
02-05-96 20:22:41
391
mspas
AP: Mol Spec
392
SU AND QUADE
TABLE 2 Rotational Constants for gauche CH3CH2OH Is the First Excited State of the –OH Torsion (Units Are MHz)
lines were observed. These turned out to be two of the three sets of the lines that Sasada previously assigned (3). One series is the first excited state of the – OH torsion. The second is assigned by Sasada to the first excited state of the CCO bend. We had thought that this series would be the second excited state of the – OH torsion for the trans, which was expected to have the intensity that we observed, but it does show an unusual shift in the rotational constants. We did not observe the lines for the first excited state of the – CH3 torsion. That could be due to the sensitivity of our spectrometer.
method three did not account for the representative trends of the data. Finally, an attempt was made to analyze the spectrum on the basis of Liu’s IAM program (6) for internal rotation in asymmetric – asymmetric molecules. It was expected that nonrigid rotor effects (7) would be large for the excited states (8), but it was hoped that these nonrigid rotor trends in the data would be accounted for with the single internal degree of freedom model. Such has not proved to be the case. For the (/) substate, none of the differences between (B / C)CalI and (B / C)CalII could be accounted for from the more rigorous calculation. However, the rigid rotor effects of nonrigidity were comparable to what would be expected from the results for CH2DOH and CHD2OH (8). For the ( 0) substate, the calculation gave term values substantially below the observed values. In this case, the calculation (I) requires very large nonrigidity effects and (II) gives a much smaller, by a factor of 2, B 0 C than has been observed. It was found that there is a very large pPz interaction from the torsion – rotation Hamiltonian that shifts the K Å 1 lines substantially and this could be detected if b-dipole transitions were allowed. In no sense did the rigorous calculation provide for the differences in B / C from Calculations I and II. TABLE 3 Seven Inertial Parameters for CH3CH2OH and CH3CH2OD (Units Are au A2)
ANALYSIS
The method of analysis of these a-dipole lines for the first excited gauche states, while straightforward, is not completely satisfying. It was expected to find a set of B and C that would work for all lines of a ( /) or (0) sequence. In first order we even expected that effects of nonrigidity would be ‘‘rigid rotor’’ in behavior. Such has not been the case. However, it is possible to find a B / C that will work moderately well for the K Å 0 lines of a given sequence and another B / C that will work for the K Å 1 lines. Three methods of analysis were tried to determine the B and C rotational constants. The first took B 0 C from the K Å 1, J Å 1 r 2 lines and B / C from the average of the appropriate K Å 0 lines. The second took B 0 C from the average of the K Å 1, J Å 1 r 2, and J Å 2 r 3 lines and B / C from the average of these lines. In Tables 1 and 2, these results are referred to as Calculations I and II. The third method took all of the lines of a sequence and found average B and C from which calculated term values were determined. In the end, it was felt that
Note. Torsional kinetic energy coefficients are in cm01.
Copyright q 1996 by Academic Press, Inc.
AID
JMS 6944
/
6t08$$$302
02-05-96 20:22:41
mspas
AP: Mol Spec
393
MICROWAVE SPECTRUM CH3CH2OH EXCITED –OH
TABLE 4 Calculated and Observed Torsional Energy Differences for CH3CH2OH and CH3CH2OD (Units Are cm01)
a
Reference Reference c Reference d Reference b
(5). (10). (11). (2).
In the course of the analysis, it was necessary to determine improved potential energy coefficients for the –OH torsion. The earlier calculations of Kakar and Quade (2) used a harmonic oscillator basis for the torsion, which it turns out reproduced the vibrational energies accurately but did not work well for the tunneling energy of the ground state due to the asymptotic nature of the wavefunctions. The seven inertial parameters (6), calculated using Culot’s molecular structure with a CO tilt of 3.57, are reported in Table 3 along with the torsional coefficients calculated from the parameters. The improved potential energy coefficients are from Liu’s theory for the torsional problem (6): This work
Ref. (2)
V1 Å 45.3 cm01 V2 Å 13.0 cm01 V3 Å 370.0 cm01
57.0 cm01 0.8 cm01 395.0 cm01
[1]
The calculated rotational frequencies of the lines assigned in this work were relatively insensitive to the potential energy coefficients. The calculated and observed term values from the microwave and infrared data are given in Table 4 for CH3CH2OH and CH3CH2OD.
DISCUSSION
The assignment and observed behavior of the spectral lines for gauche CH3CH2OH in the excited states of the –OH torsion show interesting features. The non-rigid-rotor behavior of the a-dipole lines for excited states of CH2DOH and CHD2OH has been observed previously (9). In the case of these molecules, the lines were much stronger, but so many excited states were observed that it was not possible to assign the lines to a particular substate. However, for CH3CH2OH, things were more favorable since there are fewer lines because (i) ma á 0 for the trans conformation and (ii) the intensity is down for the heavier molecule; therefore, it was possible to make the assignments. Even so, the identifications and assignments for CH3CH2OH were not easy and were time-consuming. In the course of the work, we did assign a few additional ground state lines for both the trans and gauche conformations. However, when we were essentially through with our work, Pearson et al. made their results (4, 5) available to us, and this was a big help in firming things up. A couple of our assignments, it turned out, needed to be reinvestigated. At the present time there is not a suitable and/or useful
Copyright q 1996 by Academic Press, Inc.
AID
JMS 6944
/
6t08$$$302
02-05-96 20:22:41
mspas
AP: Mol Spec
394
SU AND QUADE
theory for analyzing the features of a spectrum for the excited internal rotation states in the asymmetric–asymmetric molecules when there are substantial non-rigid-rotor effects that arise from the interactions of internal rotation and rotation with other vibrations. In summary, the spectral data for CH3CH2OH in the excited states of the –OH torsion should provide a useful check on the theory for rotation–internalrotation–vibration interactions when it is available.
ACKNOWLEDGMENT The authors are indebted to Dr. J. C. Pearson for making his measurements and calculations on trans and gauche CH3CH2OH in the torsional groundstates available to them. Further, in comparing some of our measurements with his, Dr. Pearson noted that we had a serious problem with our spectrometer’s reference oscillator, which we fortunately were able to correct.
REFERENCES 1. (a) J. P. Culot, Fourth Austin Symposium on Gas Phase Molecular Structure, Austin, Texas, 1972, Paper T8; (b) J. P. Culot, Ph.D. thesis, University of Louvain, Belgium, July 1971. 2. (a) R. K. Kakar and C. R. Quade, J. Chem. Phys. 72, 4300–4307 (1980); (b) R. K. Kakar and P. J. Seibt, J. Chem. Phys. 57, 4060 (1972). 3. Y. Sasada, J. Mol. Struct. 190, 93–97 (1988). 4. J. C. Pearson, K. V. L. N. Sastry, M. Winnewisser, E. Herbst, and F. C. DeLucia, J. Chem. Phys. Ref. Data 24, 1–32 (1995). 5. J. C. Pearson, K. V. L. N. Sastry, E. Herbst, and F. C. DeLucia, ‘‘The Millimeter and Submillimeter Wave Spectrum of gauche Ethyl Alcohol,’’ preprint. 6. M. Liu and C. R. Quade, J. Mol. Spectrosc. 146, 238–251 (1991). 7. M. Liu and C. R. Quade, J. Mol. Spectrosc. 146, 252–263 (1991). 8. C. F. Su, M. Liu, and C. R. Quade, J. Mol. Spectrosc. 149, 557–558 (1991). 9. (a) R. D. Suenram and C. R. Quade, unpublished; (b) H. R. Test and C. R. Quade, unpublished. 10. J. H. S. Green, private communication as quoted in A. J. Barnes and H. E. Hallam, Trans. Faraday Soc. 66, 1932–1940 (1970). 11. J. R. Durig, W. E. Bucy, C. J. Wurrey, and L. A. Carreira, J. Phys. Chem. 79, 988–993 (1975).
Copyright q 1996 by Academic Press, Inc.
AID
JMS 6944
/
6t08$$$304
02-05-96 20:22:41
mspas
AP: Mol Spec