Quantum chemistry study of conformational energies and rotational energy barriers in n-alkanes

The gauche energy, the t-g barrier, and the cis barrier in n-butane and n-hexane have been investigated using high-level ab initio electronic structure calculations. CCSD(T)/cc-pVTZ//MP2/6-311G(2df(3)p) calculations yield a gauche energy in n-butane of 0.59 kcal/mol, which after correction for zero-point and thermal vibrational effects is within 0.05 kcal/mol of the values obtained from recent experiments. Calculations at the same level yield t-g and cis barrier energies in n-butane of 3.31 and 5.48 kcal/mol, respectively, supporting the traditional view that the cis barrier in n-butane is much higher than the t-g barrier. We found that torsional potential functions previously parametrized to reproduce the measured low-lying torsional vibrational transitions in the trans and gauche conformers of n-butane and which yield only a small energy difference between the barriers can be modified to reproduce both the spectroscopic data and the large quantum chemistry energy difference between barriers. For the central bond in n-hexane, the gauche energy is about 0.07 kcal/mol lower than the gauche energy in n-butane at the same level of theory. The chain length effect is greater on the barriers, with the t-g and cis barriers being about 0.5 and 0.3 kcal/mol lower, respectively, for the central bond in n-hexane compared to n-butane at the same level of theory.