Gauging the applicability of ONIOM (MO/MO) methods to weak chemical interactions in large systems: hydrogen bonding in alcohol dimers

Abstract

The applicability of two-layer ONIOM methods to hydrogen bonding has been systematically investigated. The structures of 12 hydrogen bonded dimers composed of water, methanol, ethanol and n-propanol have been optimized using second-order Møller–Plesset perturbation theory (MP2) in conjunction with a double- ζ basis set with polarization and diffuse functions on all atoms, denoted DZP++. The dissociation energies (both counterpoise corrected and uncorrected) of these dimers were computed at the coupled-cluster level that includes all single and double excitations as well as a perturbative estimate of connected triple excitations [CCSD(T)] with the DZP++ basis set. These target values were used to gauge the performance of various ONIOM schemes. The methods, basis sets, and partitioning schemes were all systematically varied to determine which combinations, if any, produces substituent values ( S values) for the dissociation energy that mimic those at the target level (i.e. CCSD(T) with the DZP++ basis). All possible combinations of four popular methods [SCF, B3LYP, MP2, CCSD(T)] and seven common basis sets (3-21G, 6-31G, 6-31++G, 6-31+G( d), 6-31++G, 6-31G( d, p), 6-31++G( d, p), DZP++) were investigated. Only the MP2 and DZP++ combination gives satisfactory agreement for all 47 model systems. The proton acceptor is very sensitive to the size of the model system. As such, the minimal model recommended for these primary, aliphatic alcohols should include zero C atoms on the donor and one on the acceptor (Model-0,1). With larger models (Model-1,1 or Model-1,2) less expensive methods and smaller basis sets begin to reproduce the target S values.