On the relative intensities of the Raman active fundamentals, r0 structural parameters, and pathway of chair-boat interconversion of cyclohexane and cyclohexane-d12

Raman spectra of liquid cyclohexane, C6H12, and deuterated cyclohexane, C6D12, were recorded with both parallel and perpendicular polarizations. The observed vibrational wavenumbers, depolarization ratios, and their intensities were measured and compared with the corresponding predicted values as well as the experimental values previously reported. The conformational energetics were obtained with the Moller-Plesset perturbation method to the second order [MP2(full)] as well as with density functional theory by the B3LYP method utilizing a variety of basis sets. The average ab initio predicted difference in energy between the more stable chair form (D-3d) and the less stable twisted-boat form (D-2) is 2213 cm(-1) (26.47 kJ/mol), with a similar value of 2223 cm(-1) (26.59 kJ/mol) from the density function theory calculations. By using two dihedral angles as variables, we calculated the chair-boat interconversion pathway for cyclohexane at the MP2(full)/6-31G(d) level. The harmonic force constants, Raman intensities, depolarization values, and the potential energy distribution were predicted from both MP2(full) and B3LYP calculations with the 6-31G(d) basis set and compared with the experimental values for the chair form when available. The 'adjusted' r(0) structural parameters were obtained from MP2/6-311+G(d,p) calculations and previously reported microwave rotational constants of five isotopomers of cyclohexane: i.e. 1,1-d(2), C-13-1,1-d(2), 1,1,2,2,3,3-d(6), and d(1) (equatorial and axial). The determined distances in angstrom are: r(CC) = 1.536(3), r(CH)(ax) = 1.098(1); r(CH)(eq) = 1.095(1); and the angles in degrees: angle CCHax = 108.8(3); angle CCHeq = 110.2(3); angle CCC = 111.1(3); and angle HCH = 107.6(3) with dihedral angle angle CCCC = 55.7(3). These values are compared with those previously reported and it is found that the difference in the r(0) distances (0.003 angstrom) between the two CH values is much smaller than the difference (0.008 angstrom) previously reported for the r, values. Copyright (C) 2008 John Wiley & Sons, Ltd.