Conformation of Alkanes in the Gas Phase and Pure Liquids

Abstract

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.

A molecular dynamics investigation into the adsorption behavior inside {001} kaolinite and {1014} calcite nano-scale channels: the case with confined hydrocarbon liquid, acid gases, and water

“…The results are in good agreement to previous reports on pure hydrocarbons (modeled using United-Atom or All-Atom force fields) (Feng et al 2013;Ferguson et al 2009;Thomas et al 2006). Comparison against the bulk-configuration results shows reduction in these properties, which is more conspicuous for longer chain hydrocarbons.…”

Gas-liquid transfer data used to analyze hydrophobic hydration and find the nature of the Kauzmann-Tanford hydrophobic factor

“…Lee used the heat of vaporization to calculate E a for liquid alkanes and he used the Monte Carlo simulations of Jorgensen, et al (21,22) to find E a for the aqueous solutions. The simulation results for liquid hydrocarbons were tested against experiment (21,23) by calculating the heats of vaporization and densities of the liquid hydrocarbons; the estimated and experimental values agreed within 2% for alkanes with six or fewer carbon atoms (23). Lee's goal (14) in 1991 was to test how well scaled particle theory calculates the cavity work, as his 1985 model for the hydrophobicity of water [based on the small size of the water molecule (13)] relied on calculations made with scaled particle theory, as did much of the earlier work on the hydrophobicity of water (24,25).…”

Understanding the factors affecting the activation of alkane by Cp ′ Rh(CO) 2 (Cp ′ = Cp or Cp*)

“…For linear alkanes experimental gauche-trans energy differences were reported ranging from approximately 0.5 to 1.0 kcal mol −1 (21-23) in the gas phase and are in the range of 0.5-0.6 kcal mol −1 in the liquid (22). An electron diffraction study of the alkanes butane through heptane revealed that the gas-phase conformations often contain gauche dihedral angles (24), and earlier molecular dynamics studies predicted that gauche population should be enhanced upon transfer from the gas phase to the liquid (25) Fig.…”