Energetics of methyl branches in hydrocarbon crystals

Semi-empirical energy calculations are reported for methyl branched hydrocarbon molecules in orthorhombic and monoclinic crystalline arrays of linear molecules. The 19 molecules in the array are flexible and twenty methylene units long. Two modes of packing the defect molecule in the crystals are found and for each the mechanical relaxation criterion of there being two or more sites available to the molecule is met. However, there is no significant relaxation strength because the differences in site energies are too large. This result holds for all cases because the unfavourable orientation of the molecular stem in one of the sites makes the energy much greater than in the other for which the stem has a favourable orientation. For the case with the lowest defect energy, the unit cell dimensions are comparable to those found experimentally for the same methyl group concentration. Drawings showing the molecular distortion are presented.