In this paper, we report the geometries and properties of the structural isomers obtained from a random walk of the potential energy surface (PES) of the methanol tetramer. Thirty-three structures were obtained after B3LYP/6-31+g* optimization of 94 candidate structures generated from a stochastic search of the PM3 conformational space. The random search was carried out using a recently proposed modified Metropolis acceptance test in the simulated annealing (SA) procedure. Corrections for the basis set superposition error (BSSE) show improvements on the binding energies of the clusters in an average of approximately 2.0 kcal/mol, while geometries are predicted to be less sensitive to BSSE corrections. MP2/aug-cc-pvdz calculations on representative structures did not change the geometries but predicted better binding energies. Highly correlated CCSD(T) energies were calculated on the B3LYP and MP2 stationary points and used to establish relative stabilities. We report several new conformations and group the structures into six distinct geometrical motifs. Only the cyclic tetramers with four primary hydrogen bonds in the same plane are predicted to have significant populations. Secondary hydrogen bonds, those for which the donated proton comes from an alkyl group, lead to a rich conformational space.

In this paper we report the geometries and properties of 38 distinct geometrical motifs located on the B3LYP/6-31+G(d), MP2/6-311++G(d, p) potential energy surfaces of the water heptamer. Binding energies of up to 45 kcal/mol are calculated. All motifs fall within 10 kcal/mol of the most stable conformation, with at least 13 structural patterns located no more than 3 kcal/mol above, leading to a very complex potential energy surface, populated by a multitude of motifs each one allowing large numbers of conformations. Cluster stability does not seem to be correlated with the number of hydrogen bonds. Compact structures are energetically favored by electronic energies with zero-point energy corrections, while more open structures are preferred when temperature and entropy are accounted for. The molecular interactions holding the clusters as discrete units lead to large binding energies but are not strong enough to cause significant changes in the geometries of the interacting monomers. Our results indicate that bonding in the water heptamers can be considered as largely non-shared interactions with contributions from intermediate character of increasing covalency.

Two competitive reaction mechanisms for the gas-phase chemical transformation of singlet chlorocarbene into chloromethanol in the presence of one and two water molecules are examined in detail. An analysis of bond orders and bond order derivatives as well as of properties of bond critical points in the electron densities along the intrinsic reaction coordinates (IRCs for intermediates → transition state (TS) → products) suggests that, from the perspective of bond breaking/formation, both reactions should be considered to be highly nonsynchronous, concerted processes. Both transition states are early, resembling the intermediates, yielding rate constants whose magnitudes are mostly influenced by structural changes and to a lesser degree by bond breaking/formation. For the case of one water molecule, most of the energy in the reactants region of the IRC is used for structural changes, while the transition state region encompasses the majority of electron activity, except for the formation of the C-O bond, which extends well into the products region. In the case of two water molecules, very little electron flux and comparatively less work required for structural changes is noticed in the reactants region, leading to an earlier transition state and therefore to a smaller activation energy and to a larger rate constant. This, together with evidence gathered from other sources, allows us to provide plausible explanations for the observed difference in rate constants.

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