PNNL

Abstract

The heats of formation for the n-alkanes CnHn+2 for n=5, 6, 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, (~1.1 kcal/mol per carbon unit) and cannot be neglected for chemical accuracy. The calculated ?Hf298 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.