Hydration of Formaldehyde in Water: Insight from ONIOM Study

Abstract

The theoretical representation of solvent effects in studying reaction mechanisms and rates in condensed phases is an important but a difficult problem in chemical physics. The neutral hydration of carbonyl group is commonly thought to proceed via a stepwise pathway with charged intermediates in water solvent. However, this mechanism is disfavored due to continuous solvation and desolvation of reaction species with respect to a cyclic, cooperative mechanism (Scheme 1), as originally suggested by Eigen and by Long. The neutral hydration of a carbonyl group by a cooperative mechanism involving several water molecules implies the formation of a cyclic n-membered reaction complex, and protons moving, more or less synchronously, in the transition state. This suggestion was supported and taken up theoretically and experimentally. With an assumption that the water clusters (H2O)n are in rapid equilibrium with each other and with the reactant complexes RCn, the neutral hydration of formaldehyde was theoretically found to involve four water molecules in the gas phase and in water. But the experimental works gave inconsistent results for the mechanism three water molecules involving, or four water molecules involving cooperative mechanism without catalyst. One possible reason for the discrepancy between the theory and experiment might be that both works were unable to consider the water-assisted effect in the gas phase and the solvent effect accompanied by the water-assisted effect in water, and it is obvious that the water-assisted effect is more important in water than in the gas phase since there are lots of available water molecules surrounding the reaction system. In this work, hydration of formaldehyde is reexamined in detail by micro-solvation which divides the role of water molecules into three parts directly involved in the reaction (called active water molecules), in the first and second solvation shells. Especially, the model system selected in this work is the hydration of formaldehyde by two water molecules, 1 (n = 2 in Scheme 1). In considering the first solvation shell, solvent water molecules could be positioned around the three regions as shown in Scheme 2. Region 1 relates to the interaction of ancillary water molecules with two hydrogens of formaldehyde, region 2 relates to the interaction with carbonyl oxygen, and region 3 relates to the interaction with active water molecules. Final model was generated after adding additional water molecules in the second solvation shell under the constraint of water density of 1.0 g/cm, which was calculated from the Connolly volume surface by using MS Modeling software. The calculations started by gradually adding a series of water, nH2O (n = 0, 2, 3, 5, 6, 20) around the reaction system. Energies for the reactant complexes (RCs), transition states (TSs), product complexes (PCs), activation energies and reaction energies are summarized in Table 1. As the number of water molecules in the first solvation increased, activation and reaction energies showed similar concave pattern activation and reaction energies show minima when n = 3 and 5, respectively. Here we chose n = 5 as the first solvation shell model, which was employed to generate the final model. This was because the reaction energy showed minimum value at n = 5 and activation energy could be improved by considering the second solvation. In this model, two orientations of five ancillary water molecules were found to be possible as shown in Figures 1 and 2. (1) Model 1 (labeled M1): four of them were located near the active water molecules and the remaining one near to the carbonyl oxygen, (2) Model 2 (labeled M2): three ancillary water molecules were linked to the active water molecules and two others to the carbonyl oxygen and hydrogen, respectively. These models were further developed to construct the