Reassessment of methyl rotation barriers and conformations by correlated quantum chemistry methods

Internal rotations of the methyl group in ortho‐substituted and 2,6‐disubstituted toluenes in their ground state have been investigated by means of various ab initio quantum chemistry methods. Computed barriers at the Hartree‐Fock (HF) level using medium sized basis sets agreed reasonably with experimental results in the case of the studied ortho‐substituted toluenes. However, this agreement worsens when using very large basis sets. Furthermore, the determination of the conformation and barriers of more weakly hindered methyl groups, that is, for 2,6‐dihalogenotoluenes or toluene itself, necessitates high level correlated computations, because of a possible failure of HF calculations in this case. Density functional theory (DFT) techniques required, in several cases, much more extended basis sets than the post‐HF Møller‐Plesset perturbation (MP2, MP4) ones, to insure the convergence of the computed barriers. Non‐negligible variations of the computed barriers when using different DFT functionals are observed for some systems. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 2093–2100, 2003