Ab initio studies of structural features not easily amenable to experiment: Part 16. Some characteristic structural aspects of non-cyclic hydrocarbons

The molecular structures of n-propane, n-butane, iso-butane and n-pentane have been determined by geometrically unconstrained ab initio force relaxations on the 4—21G level. Subtle but consistent trends suggest that the local symmetry of methyl groups in the stretched form of hydrocarbons is twofold rather than threefold due to differences in the planar and non-planar CH bonds and in the HCH angles. CH bond distances in the methylene groups are slightly longer than in methyl and increase with the number of adjacent CC bonds that are staggering the CH₂-group. Inner CC bonds in n-butane and n-pentane are slightly longer than those at the end of a chain. Even though most of these features have not yet been observed experimentally they can be expected to describe reasonable trends for the systems studied.