title: Reassessment of methyl rotation barriers and conformations by correlated quantum chemistry methods.
abstract: 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.
authors.affiliation: LCSIM, UMR 6511 CNRS-Université de Rennes 1, Institut de Chimie de Rennes, Campus de Beaulieu, 35042 Rennes Cedex, France.
authors.forename: Alberto
authors.initials: A
authors.lastname: Del Rio
authors.forename: Abdou
authors.initials: A
authors.lastname: Boucekkine
authors.forename: Jean
authors.initials: J
authors.lastname: Meinnel