Microsolvation effects on the reactivity of oxy-nucleophiles: the case of gas-phase S N 2 reactions of YO - (CH 3 OH) n=1,2 towards CH 3 Cl.

The modified G4(MP2) method was applied to explore microsolvation effects on the reactivity of four solvated normal oxy-nucleophiles YO- (CH3 OH)n=1,2 (Y = CH3 , C2 H5 , FC2 H4 , ClC2 H4 ), and five α-oxy-nucleophiles YO- (CH3 OH)n=1,2 (Y = HO, CH3 O, F, Cl, Br), in gas-phase SN 2 reactions towards the substrate CH3 Cl. 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- (CH3 OH)2 becomes weaker for Y = HO and CH3 O, whereas it becomes stronger for Y = F, Cl, Br than that of YO- (H2 O)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. Graphical abstract Variation of alpha-effect in the gas-phase SN 2 reaction with the microsolvation.