The water hexamer has been studied with a classical water-water interaction potential and by quantum calculation at both RHF and MP2 levels. The influence of a virtual metal surface on (H 2 O) 6 has been modeled through geometry constraints on the cluster. Additional data on (H 2 O) 2 and (H 2 O) 3 are presented to assist the interpretation of the results obtained for the hexamer. These calculations suggest that water molecules in the first layer with their hydrogens pointing away from the surface ('flip up') only occur for a small range of values of surface lattice constants. In all other cases, the dipole moment of the water molecules is found to lie nearly parallel to the metal surface.

The theorem of matching proton affinities (PA) has been widely used in the analysis of hydrogen bonds. However, most experimental and theoretical investigations have to cope with the problem that the variation of the PA of one partner in the hydrogen bond severely affects the properties of the interface between both molecules. The B3LYP/d95+(d,p) analysis of two hydrogen bonds coupled by a 5-methyl-1H-imidazole molecule showed that it is possible to change the PA of one partner of the hydrogen bond while maintaining the properties of the interface. This technique allowed us to correlate various properties of the hydrogen bond directly with the difference in the PAs between both partners: it is possible to tune the potential energy surface of the bonding hydrogen atom from that of an ordinary hydrogen bond (localized hydrogen atom) to that of a low barrier hydrogen bond (LBHB, delocalized hydrogen atom) just by varying the proton affinity of one partner. This correlation shows clearly that matching PAs are of lesser importance for the formation of a LBHB than the relative energy difference between the two tautomers of the hydrogen bond.

Allenoates and enones form cyclopentenes via a phosphine-catalyzed [3 + 2] cycloaddition while the amine-catalyzed [2 + 4] cycloaddition yields dihydropyrans or pyrans. The difference between these catalysts is studied with M06-2X/6-31+G* calculations. The addition of the catalyst to the allenoate is the first step in both pathways followed by the reaction with the enone. The formation of the [3 + 2] phosphorus-ylide is exergonic, and hence, the [3 + 2] cycloaddition is kinetically favored over the [2 + 4] addition. Amines do not stabilize [3 + 2] ammonium-ylides. However, electron-withdrawing groups on the enone enable [2 + 4] cycloadditions. The strength of the electron-withdrawing group further controls the α/γ regioselectivity of the [2 + 4] cycloaddition, and the analysis of the HOMO-LUMO interactions explains why only E-dihydropyrans from the direct γ-[2 + 4] cycloaddition have been observed in experiments. The quantum calculations further reveal a new path to the α-[2 + 4] product starting with an intermediate Rauhut-Currier reaction. This new path is kinetically favored over the direct amine-catalyzed α-[2 + 4] cycloaddition.

The influence of the methane-water interaction potential on the value of the Henry constant obtained from molecular dynamics simulations was investigated. The SPC, SPC/E, MSPC/E, and TIP3P potentials were used to describe water and the OPLS-UA and TraPPE potentials for methane. Nonbonding interactions between unlike atoms were calculated both with one of four mixing rules and with our new methane-water interaction potential. The Henry constants obtained from simulations using any of the mixing rules differed significantly from the experimental ones. Good agreement between simulation and experiment was achieved with the new potential over the whole temperature range.

Many-body effects in a water hexamer attached to a metal surface have been studied by quantum calculations. The metal component of the interface has been replaced by a set of geometrical constraints (virtual surface) [J. Phys. Chem. A 2000, 105, 4084-4095] which permits the analysis of the properties of the interface independently of the precise electronic structure of the metal, as a function of the surface lattice constant. Our calculations show that cooperative forces have a significant influence on the energy and geometry of the metal-water interface. The energy decomposition of the energy of formation of the water hexamer demonstrates how strong cooperative effects favor the growth of the water bilayer in the experimentally observed range of surface lattice constants, while a model based solely on two-center energies predicts the formation of the water bilayer at unphysically large values of surface lattice constant.