In a kinetic experiment on the SN2 reaction of sodium p-nitrophenoxide with iodomethane in acetone–water mixed solvent, Humeres et al. (J. Org. Chem. 2001, 66, 1163) found that the reaction depends strongly on the medium, and the fastest rate constant was observed in pure acetone. The present work tries to explore why acetone can enhance the reactivity of the title reactions. Accordingly, we make a mechanistic study on the reactions of sodium p-nitrophenoxide with halomethanes (CH3X, X = Cl, Br, I) in acetone by using a supramolecular/continuum model at the PCM-MP2/6-311+G(d,p)//B3LYP/6-311+G(d,p) level, in which the ion pair nucleophile is microsolvated by one to three acetone molecules. We compared the reactivity of the microsolvated ion pair nucleophiles with solvent-free ion pair and anionic ones. Our results clearly reveal that the microsolvated ion pair nucleophile is favorable for the SN2 reactions; meanwhile, the origin of the enhanced reactivity induced by microsolvation of the nucleophile is discussed in terms of the geometries of transition state (TS) structures and activation strain model, suggesting that lower deformation energies and stronger interaction energies between the deformed reactants in the TS lead to the lower overall reaction barriers for the SN2 reaction of microsolvated sodium p-nitrophenoxide toward halomethanes in acetone.

We have made an extensive theoretical exploration of gas-phase N-alkylamino cation affinities (NAAMCA), including amino cation affinities (AMCA) and N-dimethylamino cation affinities (NDMAMCA), of neutral main-group element hydrides of groups 15–17 and periods 2–4 in the periodic table by using the G2(+)M method. Some similarities and differences are found between NAAMCA and the corresponding alkyl cation affinities (ACA) of HnX. Our calculations show that the AMCA and NDMAMCA are systematically lower than the corresponding proton affinities (PA) for HnX. In general, there is no linear correlation between NAAMCA and PA of HnX. Instead, the correlations exist only within the central elements X in period 2, or periods 3–4, which is significantly different from the reasonable correlations between ACA and PA for all HnX. NAAMCA (HnX) are weaker than NAAMCA (Hn–1X–) by more than 700 kJ/mol and generally stronger than ACA (HnX), with three exceptions: H2ONR2+(R = H, Me) and HFNH2+. These new findings can be rationalized by the negative hyperconjugation and Pauli repulsion.

The water-mediated neutral hydrolysis mechanism of carbonyl sulfide (OCS) has been re-examined using the hybrid supramolecule/continuum models with n = 2–8 explicit water cluster at the level of MP2(fc)(CPCM)/6-311++G(d,p)//MP2(fc)(CPCM) /6-31+G(d). Present calculations indicate that the potential energy surface in water solution is different from the one in the gas-phase, and only stepwise mechanism is observed in aqueous solution, i.e., monothiocarbonic acid (H2CO2S) is formed via monothiocarbonate (OCSOH–, MTC) and its counterion, protonated water cluster, (H2O)nH3O+. The predicted rate-determining step (RDS) barrier for the stepwise mechanism in water solution, about 90 kJ/mol, shows good agreement with the experimental values, 83.7–96.2 kJ/mol using six- or eight-water model including two cooperative water molecules. Moreover, two reaction pathways, the nucleophilic addition of water molecule across the C═O or the C═S bond of OCS are competitive.