The detailed reaction pathways for the hydration of carbon dioxide by water and water clusters containing two, three, and four water molecules (CO2 + nH2O → H2CO3 + (n − 1)H2O, n = 1−4) have been investigated in both gas phase and aqueous solution using ab initio molecular orbital (MO) theory up to the quadratic configuration interaction QCISD(T)/6-31G(d,p)//MP2/6-31G(d,p) level, both SCRF and PCM models of continuum theory, and a mixed approach based on MO calculations in conjunction with Monte Carlo and reaction field simulations. It is confirmed that the CO2 hydration constitutes a case of active solvent catalysis where solvent molecules actively participate as a catalyst in the chemical process. In aqueous solution the hydration mechanism is multimolecular, where geometric parameters of the solvent fully intervene in the reaction coordinate. The hydration reaction was found to proceed through an attack of a water oxygen to the CO2 carbon in concert with a proton transfer to a CO2 oxygen. The proton transfer is assisted by a chain of water molecules, which is necessary for a proton relay between different oxygens. Owing to a significantly larger charge separation in the transition structures, nonspecific electrostatic interactions between solute and solvent continuum also play a more important stabilizing role. Regarding the answer to the title question, our calculations suggest that although a water tetramer (n = 4) seems to be necessary for CO2 hydration in the gaseous phase, a reaction channel involving formation of a bridge containing three water molecules (n = 3) is likely to be actively involved in the neutral hydration of CO2 in aqueous solution.

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A kinetic and mechanistic investigation of the catalyzed alcoholysis of isocyanates was undertaken. Both experimental and theoretical results showed that the alcoholysis should be understood by a multimolecular intervention of the alcohols. The alcoholysis of isocyanate was examined experimentally for 2-propanol and cyclohexanol in low and high concentrations. It is suggested that either two or three molecules of the alcohol are implicated from the kinetic study, while the reaction with trimers becomes dominant at high alcohol concentrations. In accordance with these results, theoretical study suggests an active participation of at least three alcohol molecules in a reacting supersystem, giving rise to a genuine effect. The detailed reaction mechanism for the alcoholysis reaction by methanol and methanol clusters (HN=C=O + n(CH(3)OH), n = 1-3) was modeled by ab initio methods, both in the gas phase and in solution. The nucleophilic addition occurs in a concerted way across the N=C bond of the isocyanate rather than across the C=O, similar to the isocyanate hydrolysis. The bulk solvent effect, which is treated by a polarizable continuum model (PCM), does not affect the preference of the alcohol to attack across the N=C bond as pointed out by the gas-phase values.

Substituent and solvent effects on the reaction pathway of the Beckmann rearrangement were studied. Energy surfaces of the isolated gas phase systems were mapped out using ab initio MO calculations at the MPn and QCISD(T) levels with basis sets ranging from 6-31G(d,p) to 6-311++G(2df,2p). In the simplest gas phase system, the most favored path is as follows:  protonation of formaldehyde oxime → N-protonated species → O-protonated species → fragmentation products, in which the 1,2-H-shift connecting both protonated isomers is rate-determining. While both methyl and formyl substituents on C and O of the oxime have only a small effect on the rate-controlling energy barrier, they significantly modify the barrier to fragmentation. The bulk solvent effect which is treated by a polarizable continuum model does affect only marginally the activation parameters with respect to the gas phase values. A combination of both quantum and statistical mechanics was also used to probe the solvent effect. In order to investigate the active role of the solvent, ab initio calculations were carried out within a supermolecule approach. An active participation of one solvent molecule in a reacting supersystem gives rise to a genuine effect. As simple solvent molecules, H2O, H2CO, and HCOOH were studied, the latter being a model for the Beckmann solution (HCl + acetic acid + acetic anhydride). While their involvement as a coreactant considerably reduces the barrier of the 1,2-H-shift by about 50% and hence approaches the experimental results, the effect of the bulk solvent on the reacting supersystem remains small. The calculated results suggest that the Beckmann rearrangement represents a strong case of active solvent participation, which consists in assisting the rate-determining 1,2-H-shift by catching the migrating hydrogen of the substrate and putting it back at the other end; the migration is thereby considerably accelerated.

A kinetic and mechanistic investigation of the catalyzed hydration of isocyanates was undertaken. Both experimental and theoretical results showed that the hydrolysis reaction involves a chain of water molecules. The detailed hydration mechanism by water and water clusters (H-N=C=O + n(H(2)O) --> H(2)NCOOH + (n - 1)H(2)O, n = 1-3) has been modeled by ab initio methods, both in the gas phase and in aqueous solution. While two water molecules in the form of a dimer seem to play the key role in hydrating the isocyanate, a third water molecule may be needed to bridge the gap from the point of attack on the isocyanate to the water dimer and to facilitate further the hydration. In accordance with these facts, experimental results imply a second-order dependence on water during its nucleophilic addition to phenyl isocyanate, over a wide concentration range. In this specific case, water oligomers higher than the dimer seem to make no appreciable contribution to the rate of the hydrolysis reaction. The nucleophilic addition occurs in a concerted way across the N=C bond of the isocyanate rather than across the C=O bond. This preferential reaction mechanism could be rationalized in terms of Fukui functions for both nucleophilic and electrophilic attacks. Although a charge separation occurs in the transition state, electrostatic solvent effects are not quite important in reducing only marginally the energy barriers.