Conformational stability from Raman spectra, r0 structural parameters, and vibrational assignment of methylcyclohexane

The Raman spectra (3500–50 cm−1) of the liquid and solid methylcyclohexane and the infrared spectra of the gas and solid methylcyclohexane have been recorded. The Raman band at 754 cm−1 in the liquid has been confidently assigned to the less stable axial conformer and its intensity was recorded as a...

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Veröffentlicht in:Journal of Raman spectroscopy 2009-12, Vol.40 (12), p.1919-1930
Hauptverfasser: Durig, James R., Ward, Rachel M., Guirgis, Gamil A., Gounev, Todor K.
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Sprache:eng
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Zusammenfassung:The Raman spectra (3500–50 cm−1) of the liquid and solid methylcyclohexane and the infrared spectra of the gas and solid methylcyclohexane have been recorded. The Raman band at 754 cm−1 in the liquid has been confidently assigned to the less stable axial conformer and its intensity was recorded as a function of temperature from 25 to −95 °C. By the utilization of 15 different temperatures, the enthalpy difference between the more stable chair‐equatorial conformer and the chair‐axial form was determined to be 712 ± 71 cm−1 (8.50 ± 0.84 kJ/mol). The ab initio predicted value of 710 cm−1 (8.50 kJ/mol) from the MP2(full)/6‐311G(2d,2p) calculations with and without diffuse functions is in excellent agreement. The harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational wavenumbers have been obtained for both conformers from MP2(full)/6‐31G(d) ab initio calculations. With two scaling factors of 0.88 for the C‐H stretches and 0.9 for the remaining ones, the fundamental wavenumbers have been predicted and along with the depolarization values and infrared band contours (B‐type for A″ modes) a complete vibrational assignment has been made for the chair‐equatorial conformer. Predicted r0 structural parameters have been provided from adjusted parameters from ab initio MP2(full)/6‐311+G(d,p) calculations. The results are discussed and compared with the corresponding properties of some similar molecules. Copyright © 2009 John Wiley & Sons, Ltd. Raman spectra of the liquid and solid methylcyclohexane and the infrared spectra of the gas and solid methylcyclohexane have been recorded, and the observed vibrational wavenumbers, depolarization ratios, and their intensities have been measured and compared with the corresponding predicted values and a full vibrational assignment has been made. The average ab initio predicted differences in energy between the more stable equatorial form and the less stable axial conformer are compared with a temperature‐dependent study of the Raman liquid.
ISSN:0377-0486
1097-4555
DOI:10.1002/jrs.2341