Thermodynamic Properties of the C₅, C₆, and C₈ n-Alkanes from ab Initio Electronic Structure Theory

The heats of formation for the n-alkanes C(n)H(n+2) for n = 5, 6, and 8 have been calculated using ab initio molecular orbital theory. Coupled-cluster calculations with perturbative triples (CCSD(T)) were employed for the total valence electronic energies. Correlation-consistent basis sets were used, up through the augmented quadruple zeta, to extrapolate to the complete basis set limit. Geometries were optimized at the B3LYP/TZVP and MP2/aug-cc-pVTZ levels. The MP2 geometries were used in the CCSD(T) calculations. Frequencies were determined at the density functional level (B3LYP/TZVP), and scaled zero point energies were calculated from the B3LYP frequencies. Core/valence, scalar relativistic, and spin-orbit corrections were included in an additive fashion to predict the atomization energies. The core/valence corrections are not small, (approximately 1.1 kcal/mol per carbon unit) and cannot be neglected for chemical accuracy. The calculated deltaH(298)f values are -35.0, -40.2, and -50.2 kcal/mol for C5H12, C6H14, and C8H18, respectively, in excellent agreement with the respective experimental values of -35.11 +/- 0.19, -39.89 +/- 0.19, and -49.90 +/- 0.31 kcal/mol. Isodesmic reaction energies are presented for some simple reactions involving C8H18 and are shown not to be strongly method dependent.