Ab Initio Calculation of Entropy and Heat Capacity of Gas-Phase n-Alkanes Using Internal Rotations

P. Vansteenkiste, V. Van Speybroeck, G.B. Marin, M. Waroquier
Journal of Physical Chemistry A
107(17), 3139-3145


This work attempts to improve the theoretical reproduction of thermodynamic properties, such as entropies and heat capacities of gas-phase n-alkanes, by using a more precise quantum-mechanical treatment of the internal rotations. Present ab initio methods all handle the internal modes in the harmonic oscillator approach. It has already been noted that this approach underestimates the microscopic partition functions (Van Speybroeck et al., J. Phys. Chem. A 2000, 104, 10939). In this work, an uncoupled scheme for internal rotations is applied to a large number of n-alkanes within the DFT formalism at the B3LYP/6-311g** level. The method being examined in this paper drastically improves the agreement between theoretical and experimental thermodynamic properties. Moreover, the method has been shown to be efficient and to be easily implemented in each ab initio software package.