Potential functions of internal rotation in n-alkanes and its contribution to thermodynamic properties

Abstract

The potential functions of braked internal rotation V(ϕ) in n-alkanes (ethane, propane, butane, n-pentane, n-hexane, n-heptane) were calculated by ab initio and DFT methods with the 6-311++G(3df,3pd) basis set. The functions were approximated as a series of six cosines. The dependences of V(ϕ) on the length of the hydrocarbon chain in n-alkanes were analyzed. The heights of the trans-cis and trans-gauche barriers and the differences between the energies of the trans and gauche conformers were calculated and compared with the experimental data. From the calculated geometric parameters and V(ϕ), the contributions of the braked internal rotation to the enthalpy, entropy, heat capacity, and Gibbs free energy at 298 K were determined. The contributions of internal rotations are transferable within the framework of additive approaches. The generalized function Vav(ϕ) for n-alkanes and averaged contributions of internal rotation of the C-C bonds and CH3- and -CH2- tops to the thermodynamic properties were suggested.