The structure of n-alkanes: High precision a b i n i t i o calculation and relation to vibrational spectra

Detailed geometries and deuterium-isolated CH stretching frequencies are calculated for several different conformers of small n-alkanes at a number of levels of ab initio theory. The results of the calculations are carefully compared to experiment. The calculated frequencies fit a linear correlation to the calculated bond lengths with a mean absolute error of about 3 cm−1. The experimental frequencies fit this correlation more closely. Notable deviations from the correlation occur for the crowded C–H bonds that exist in gauche conformers. The changes in geometry on rotating a methyl group in propane are calculated and compared with experimental results on frequency shifts for n-alkane methyl groups.