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 Møller–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 (D₂) 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’ r₀ 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‐d₂, ¹³C‐1,1‐d₂, 1,1,2,2,3,3‐d₆, and d₁ (equatorial and axial). The determined distances in Å are: r(CC) = 1.536(3), r(CH)_ax = 1.098(1); r(CH)_eq = 1.095(1); and the angles in degrees: ∠CCH_ax = 108.8(3); ∠CCH_eq = 110.2(3); ∠CCC = 111.1(3); and ∠HCH = 107.6(3) with dihedral angle ∠CCCC = 55.7(3). These values are compared with those previously reported and it is found that the difference in the r₀ distances (0.003 Å) between the two CH values is much smaller than the difference (0.008 Å) previously reported for the r_s values. Copyright © 2008 John Wiley & Sons, Ltd.