Conformation of alkanes in the gas phase and pure liquids.

Monte Carlo (MC) statistical mechanics simulations have been carried out for the homologous alkane series of n-butane through n-dodecane in the gas phase and for the pure liquids at 298 K and 1 atm using the OPLS-AA force field. The study addresses potential cumulative deviations of computed properties and potential conformational differences between the gas phase and pure liquids, for example, from self-solvation in the gas phase. The average errors in comparison with experimental data for the computed densities and heats of vaporization are modest at 0.7% and 6.9%, respectively. Also, the invariant gas and liquid-phase results for average end-to-end distances and percentages of trans conformations for each nonterminal C-C bond assert that the conformer populations are not altered upon transfer from the gas phase to the pure liquid for the n-alkanes in this size range. Average end-to-end distances were also computed from the results of conformational searches and corroborated the MC findings. Quantitatively, the OPLS-AA result for the trans population of the C3-C4 bond in n-undecane is in close agreement with the findings from (13)C NMR experiments. Finally, previous work on determining the shortest n-alkane that does not have an all-trans global energy minimum has been extended. The smallest n-alkane with a hairpin geometry that is lower in energy than the all-trans conformer occurs for C(22)H(46) with OPLS-AA, though with a correction for GG sequences, the true turning point is likely in the C(16)-C(18) range.