Microsolvation effects on the reactivity of oxy-nucleophiles: the case of gas-phase S_N2 reactions of YO⁻(CH₃OH)_n=1,2 towards CH₃Cl

The modified G4(MP2) method was applied to explore microsolvation effects on the reactivity of four solvated normal oxy-nucleophiles YO⁻(CH₃OH)_n=1,2 (Y = CH₃, C₂H₅, FC₂H₄, ClC₂H₄), and five α-oxy-nucleophiles YO⁻(CH₃OH)_n=1,2 (Y = HO, CH₃O, F, Cl, Br), in gas-phase S_N2 reactions towards the substrate CH₃Cl. Based on a Brønsted-type plot, our calculations reveal that the overall activation barriers of five microsolvated α-oxy-nucleophiles are obviously smaller than the prediction from the correlation line constructed by four normal microsolvated ones to different degrees, and clearly demonstrate the existence of an α-effect in the presence of one or two methanol molecule(s). Moreover, it was found that the α-effect of the mono-methanol microsolvated α-nucleophile is stronger than that of the monohydrated α-nucleophile. However, the α-effect of YO⁻(CH₃OH)₂ becomes weaker for Y = HO and CH₃O, whereas it becomes stronger for Y = F, Cl, Br than that of YO⁻(H₂O)₂, which can be explained by analyses of the activation strain model in the two cases. It was also found that the rationale about the low ionization energy of α-nucleophile inducing the α-effect was not widely significant. [Figure not available: see fulltext.].