Abstract: Ab initio and semi-empirical NO AM1 methods have been employed to study theelectronic structures of a series of nucleophiles and methyl nitrate, and the gas-phase S_N2reactions between them. The regularity of changes of structure, energy and charge during thereaction are investigated. According to the calculated activation energies of the reactions, themagnitude of the nucleophilicity of eight nucleophilies is given asOH^-＞F^-＞I^-＞NO₃^-＞CN^-＞Br^-＞Cl^-＞N₃^-.

Key words: Methyl nitrate, Nucleophile, Nucleophilicity, S_N2 reaction, MO AM1 method