PNNL

Abstract

Coupled cluster theory through quasiperturbative, connected triple excitations (CCSD(T)) was used to obtain optimized structures, harmonic vibrational frequencies and heats of formation for seven related small molecules. For the three systems possessing reliable experimental heats of formation, the level of agreement between theory and experiment was excellent. In order to achieve this level of agreement and to simultaneously minimize the theoretical uncertainty it was necessary to apply large correlation consistent basis sets (through septuple zeta in some cases) followed by a number of small, but nonnegligible, energetic corrections. For CO, DHf0(0K) = -27.0 !A 0.2 (theory) vs ?C27.20 !A 0.04 kcal/mol (expt.). For CO2, DHf0(0K) = -93.7 !A 0.2 (theory) vs ?C93.97 !A 0.01 kcal/mol (expt.). For HC(O)OH (formic acid), DHf0(0K) = -88.9 !A 0.4 (theory) vs -88.7 !A 0.1 kcal/mol (expt.). For HCO, the experimental and theoretical values are in near perfect agreement, with DHf0(0K) = 10.4 !A 0.2 (theory) vs 10.3 !A 2 kcal/mol (expt.), although this may be somewhat fortuitous since the experimental value is not very reliable. For trans-HOCO, we predict a value of DHf0(0K) = -43.9 !A 0.5, compared to the revised photoionization value of !Y ?C45.8 !A 0.7 kcal/mol. Theory, however, is in good agreement with the possible experimental value of ?C42.7 !A 0.9 kcal/mol suggested in the same photoionization experimental analysis. For HCO2 and HC(O)OOH, where no experimental data exists, DHf0(0K) = -27.6 !A 0.4 and ?C65.6 !A 0.6 kcal/mol respectively. trans-HOCO is only slightly bound (1.1 kcal/mol) with respect to the H + CO2 asymptote. HCO2 is 15.7 kcal/mol higher in energy than trans-HOCO and lies above the H + CO2 asymptote by 14.6 kcal/mol. It is only bound with respect to the OH + CO asymptote by 9.0 kcal/mol. Three parameterized methods (G2, G3 and CBS-Q) were compared to the best coupled cluster heats of formation and found to differ by up to 3.2 kcal/mol.