On the evaluation of thermal corrections to gas phase ab initio relative energies: implications to the conformational analysis study of cyclooctane

In this paper we present an investigation of the influence of the thermal correction on the conformational population for the boat-chair (BC) and CROWN forms of the cyclooctane molecule, calculated using quantum mechanical ab initio Hartree-Fock (HF), MP2, MP4SDQ, CCSD and density functional methods (B3LYP, BLYP, BP86) in conjunction with various basis sets. A previous experimental gas phase electron diffraction study pointed out that the BC is either the exclusive or at least the strongly predominant form in gas phase at room temperature. We therefore analyzed the performance of various levels of calculation for the evaluation of the relative conformational population and also the role played by the thermal correction to gas phase calculated relative energies. It turns out that the thermal correction is very sensitive to the presence of low frequency modes that are indeed internal rotations and need to be treated separately, in what the cyclooctane molecule is concerned. Once internal rotations were considered, it can be seen that the HF level of calculation produces very satisfactory values for thermal correction, compared to MP2. Therefore, it can be used in single-point energy calculations employing a high correlated level of theory (MP4SDQ, CCSD), leading to a quite trustable Gibbs free energy difference data. When thermal energies are not corrected for low frequency internal rotation modes, a range of contrasting results is obtained by varying both the quantum mechanical approach and the basis set. (C) 2002 Elsevier Science B.V. All rights reserved.