Influence of the number of water molecules on the mechanism of N-sulfinylaniline hydrolysis

The mechanism Of the uncatalyzed hydrolysis of N-sulfinylaniline (Ph-N=S=O) has been Studied with B3LYP/6-31+G(2d,2p) in the gas phase, with explicit treatment of water molecules. Hydrolysis involves water attack on Sulfur, with a close to perpendicular alignment of a water molecule and the NSO plane in both prereaction complexes and transition states for the rate-determining step. Consequently, the distance of the weak S-... O interaction, together with the efficiency of protonation of either nitrogen (attack across the N=S bond) or oxygen (attack across the S=O bond) atoms of the NSO group, determines the height of the activation barrier for hydrolysis. While the reaction with one water molecule is characterized by an unreasonably high enthalpy of activation, a cooperative effect from the weak interactions appears With the inclusion of a second water molecule, where both participate in the reaction, and the activation enthalpy drops significantly. The preference for attack across the S=O bond that is found in the reaction with one water molecule switches to a dominance of attack across the N=S bond in the reaction with three water molecules.