PNNL

Abstract

The heats of formation of benzene and six other small hydrocarbons (allyl, allene, cyclopropene, propyne, cyclopropane and propane) have been calculated at high levels of ab initio molecular orbital theory. Geometries and frequencies were determined, in general, with coupled cluster theory, including a perturbative treatment of the connected triple excitations and with basis sets up through augmented quadruple zeta in quality or, in some cases, augmented quintuple zeta. Subsequent extrapolation of the total energies to the complete 1-particle basis set limit was performed, in an effort to further reduce the basis set truncation error. Additional improvements in the atomization energy were achieved by applying corrections for core/valence correlation, scalar relativistic, atomic spin-orbit and higher order correlation effects. Zero point energies were based on an average of the vibrational energies obtained from the experimental fundamentals and theoretical harmonic frequencies. Using restricted open shell treatments for the atoms, we find the following heats of formation (kcal/mol) at 0K: DHf(allene) = 48.1 ? 0.5 (calc.) vs. 47.4 ? 0.3 (expt.); DHf(cyclopropene) = 70.5 ? 0.5 (calc.) vs. 68.3 ? 0.6 (expt.); DHf(propyne) = 46.5 ? 1.5 (calc.) vs. 46.0 ? 0.2 (expt.); DHf(cyclopropane) = 17.4 ? 1.5 (calc.) vs. 16.8 ? 0.1 (expt.); DHf(propane) = -20.0 ? 1.6 (calc.) vs. -19.6 ? 0.1 (expt.); DHf(allyl) = 42.7 ? 1.5 (calc.) vs. 42.7 ? 0.5 (expt.); and DHf(benzene) = 24.7 ? 3 (calc.) vs. 24.0 ? 0.2 (expt.).