Microsolvation effects on the reactivity of oxy-nucleophiles: the case of gas-phase SN2 reactions of YO−(CH3OH) n=1,2 towards CH3Cl
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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 SN2 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.
KeywordsMethanol-microsolvation SN2 reactions Oxy-nucleophiles Reactivity Modified G4(MP2) method
This work was supported by National Natural Science Foundation of China (No.J1310008), Strategic Research Grants from City University of Hong Kong (No. 7004403), the Open Research Fund of State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen University (No. 201511), and the Open Research Fund of Key Laboratory of Advanced Scientific Computation, Xihua University (No. szjj2013-024).
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