Ab Initio Study of Hydrogen-Bonded Complexes of Small Organic Molecules with Water

Hydrogen bonding between water and a series of small organic molecules was examined via electronic structure calculations. Several computational methods were examined, including both a hybrid density functional procedure (Becke3LYP) and second-order Møller−Plesset theory (MP2) coupled with a double-ζ basis set augmented by diffuse polarization functions on heteroatoms. The agreement between Becke3LYP and MP2 energies was generally good, as was the agreement with energies obtained using more sophisticated and costly methods. The energies and structures of 53 hydrogen-bonded complexes of water with various small organic molecules, including alcohols, thiols, ethers, thioethers, carboxylic acids, esters, amines, amides, nitriles, and nitro compounds, were then examined systematically using the Becke3LYP and MP2 procedures. The hydrogen bond geometries were generally linear, and acceptor sites corresponded closely to the positions of lone pairs as predicted by simple hybridization arguments. Structures with sulfur and chlorine atoms showed some deviation from these simple expectations and seemed to be largely determined by molecular dipole−dipole interactions. Categorization of the hydrogen bonds involved in the various complexes led to an ordering of hydrogen bond donor and acceptor abilities for some common functional groups. The strength of association was found to correlate moderately well with experimental gas-phase basicity in those cases where water acted unambiguously as the hydrogen bond donor at a single site. Interestingly, sulfur was found to be close to oxygen in hydrogen bond acceptor strength, and the surprisingly strong acceptor ability of sulfur could not be explained in terms of its enhanced polarizability relative to oxygen. Calculations were also carried out on the AT and GC base pairs and yielded results in very close agreement with the highest levels of calculation previously reported.