Structural and vibrational analysis of thymoquinone.

The molecular structure of 2-isopropyl-5-methyl-1,4-benzoquinone, C(6)O(2)H(2) (CH(3))(3)CH, has been optimized using methods based on density functional theory (DFT) and Moller-Plesset second-order perturbation theory (MP2). As regards C(6)O(2)H(2) (CH(3))(3)CH, two populated conformations with C(1) (trans) and C(s) (cis) symmetries are obtained, the former being more stable than the latter. The theoretical data indicate that although both anti and cis conformers are possible by rotation about the C-C bond, the preferred conformation is trans. The effects governing the torsion barriers and preferred conformations were analyzed at B3LYP/6-311++G** level. The atoms in molecules (AIM) theory and natural bond orbital (NBO) analysis was applied to the cis and trans conformers in order to detect intramolecular contacts. Furthermore, the infrared spectra for the gas and solid phases and the Raman spectrum for the solid one, were recorded and the observed bands assigned to the vibrational modes. Copyright 2010 Elsevier B.V. All rights reserved.