Vibrational spectra, ab initio calculations, and assignments of the fundamentals of the C2v conformer of n-pentane

Ab initio calculations of the relative energies, optimized geometries and vibrational spectrum of the four conformers of n-pentane have been completed, and a normal coordinate calculation performed to establish the nature of the observed molecular vibrations. From the Raman spectrum of the annealed crystal, in which only aa conformer bands are observable, an assignment of the observed bands to the fundamental vibrations of the aa conformer of C2v symmetry is proposed. The relative energy in the liquid of the ag conformer to the aa conformer is measured in the CCC bending region near 400 cm−1 using both n-pentane and n-pentane-d12, and is found to be 164 ± 10 cm−1, corresponding to 460 ± 28 cal mol−1 in excellent agreement with the results of earlier workers obtained by other measurements, but roughly half the value calculated ab initio at the highest level of approximation available in this investigation.