Physical and Inorganic Chemistry Heats of Reaction from Self-Consistent Field Energies of Closed-Shell Molecules

Self-consistent field energies and wave functions have been computed for 26 closed-shell molecules including many medium-sized nonlinear organic molecules. A basic set of gaussian functions comparable to a best-atom double-ζ set of Slater orbitals has been used. Standard heats of reaction have been computed for a large number of hydrogenation and related reactions under the assumption that the sum of E_corr (the electronic correlation energy) and E_hf – E_dz (the difference of the electronic energy in the Hartree-Fock and our double-ζ basis) does not change in a reaction having closed-shell reactants and products. For 16 hydrogen-transfer reactions, the mean value of the theoretical minus the experimental heat is –0.6 kcal/mole, and the root mean square value of this difference is 6.5 kcal/mole. For molecules whose ground state is well represented by a single valence-bond structure, empirical heats of reaction estimated from bond energies agree as well with experiment as the theoretical values. For molecules expected to resonate between more than one valence-bond structure and for cyclic molecules expected to exhibit strain, the theoretical heats of reaction are in much better agreement with experiment. Although the change of correlation energy is apparently small in the reactions studied, an attempt is made to examine the structure of correlation energy change. Atomic orbital populations and pair correlation energies are employed to estimate the change of intraatomic correlation energy in these chemical reactions. After incorporation of this estimate of the intraatomic correlation energy change into the theoretical heats of reaction, the difference between the theoretical and experimental heats of complete hydrogenation is attributed mainly to changes of interatomic correlation energy and in lesser part to the decreasing adequacy of our double-ζ basis with increasing unsaturation. The difference is found to decrease rapidly with increasing length of the bond to which hydrogen is added. An empirical correction which we suggest relates interatomic correlation energy to bond length is given for bonded first-row atoms. This correction is combined with calculated values for the intraatomic correlation energy to give improved estimates of reaction heats.