IMOMO-G2MS approaches to accurate calculations of bond dissociation energies of large molecules

Accurate calculations were performed for single bond dissociation energies using the IMOMO (integrated MO + MO) method, a version of the ONIOM method, with a variety of molecular orbital (MO) combinations and were compared with the experimental values. The dissociation energies studied are for the C-H bond of benzene (with ethylene and butadiene as a model system), the C-F bond of fluorobenzene (model CH2=CHF), the C-CH3 bond of toluene (model CH2=CH-CH3), the Si-Ii bond of phenylsilane C6H5SiH2-H (models of CH2=CHSiH2-H and SiH3-H), the O-H bond of n-propanol, isopropanol, n-butanol, and t-butanol (model H2O), the C-S bond of PhCH2-SCH3 (model CH3-SH), and the O-O bond of SF5O-OSF3 (model MO-OH). The IMOMO(G2MS(R):ROMP2/6-31G(d)) calculation, which uses G2MS(R) for the model dissociation and ROMP2/6-31G(d) for the substituent effect, at the B3LYP/6-31G(d) (or sometimes /6-31G) optimized geometries (and zero-point corrections) using two non-hydrogen-atom model systems, A-B for the A-B bond or AB-H for the B-H bond, is found to consistently give an accurate bond dissociation energy within a few kcal/mol of the experimental value. This recommended scheme provides estimates of accurate bond energies for very large molecules, for which experimental values are rarely known, with a small additional cost beyond B3LYP/6-31G(d) geometry optimizations and MP2/6-31G(d) single-point energies.