Conformational dependence of vibrational spectra in some branched octanes: 3,3-and 2,2-dimethylhexanes

As a preliminary study before vibrational analysis, conformational analyses of 3,3- and 2,2-dimethylhexane (3,3-DMH and 2,2-DMH)were done using HF, post-HF, and DFT methods applied to various basis sets. Moreover, computations on a possible rotational barrier from the most stable conformer to the lowest energy secondary conformers were done. The ab initio Cartesian force field has been converted to the force field in terms of symmetry coordinates. Normal coordinate calculations were performed using a scaled quantum mechanical (SQM) force field. The scale factors have been optimized in order to reproduce the observed frequencies. Variable temperature study of Raman spectrum of 3,3-DMH in liquid phase was done to confirm the vibrational assignment and to obtain the enthalpy of conformational equilibrium. In addition, the temperature dependence of Raman and IR spectra of 2,2-DMH, made in our previous work, was confronted in this paper with SQM vibrational analysis.