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 CH 2 CHF), the C−CH 3 bond of toluene (model CH 2 CH−CH 3 ), the Si−H bond of phenylsilane C 6 H 5 SiH 2 −H (models of CH 2 CHSiH 2 −H and SiH 3 −H), the O−H bond of n -propanol, isopropanol, n -butanol, and t -butanol (model H 2 O), the C−S bond of PhCH 2 −SCH 3 (model CH 3 −SH), and the O−O bond of SF 5 O−OSF 3 (model HO−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.