CONFORMER DISTRIBUTION AND THE KINETICS OF TRANS-GAUCHE ISOMERIZATIONIN A MODEL OF LIQUID N-OCTANE

Molecular-dynamics simulations have been used to study the nature of t he conformational disorder in liquid n-octane. The potential model use d is one in which all atomic sites are treated as centres of force, wi th intramolecular terms to describe the C-C-C bending and C-C-C-C tors ional degrees of freedom and the rotation of the methyl groups. The do minant classes of conformer are those containing not more than one gau che bond, but there are measurable contributions from a wide range of structures and the distribution of end-to-end lengths extends from abo ut 3.5 to 9.5 angstrom. The rate of trans-gauche isomerization is trea ted by a first-order kinetics approach. The calculated rate constant i s smallest for the torsional bond at the centre of the molecule, and i ts variation with position along the chain is associated with a signif icant rise in the frequency of barrier recrossing as the distance from the chain ends increases. Comparison is made with molecular-dynamics results previously obtained for liquid n-butane.