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Conformational stability from Raman spectra, r 0 structural parameters, and vibrational assignment of methylcyclohexane
Author(s) -
Durig James R.,
Ward Rachel M.,
Guirgis Gamil A.,
Gounev Todor K.
Publication year - 2009
Publication title -
journal of raman spectroscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.748
H-Index - 110
eISSN - 1097-4555
pISSN - 0377-0486
DOI - 10.1002/jrs.2341
Subject(s) - conformational isomerism , methylcyclohexane , raman spectroscopy , chemistry , ab initio , ab initio quantum chemistry methods , enthalpy , infrared , infrared spectroscopy , computational chemistry , molecular physics , molecule , thermodynamics , physics , optics , biochemistry , organic chemistry , catalysis
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 r 0 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.