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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 7, Number 5
May 20, 1986 

Geometries and Energies of SN2 Transition States
Ikchoon Lee*, Chan Kyung Kim, Chang Hyun Song
MNDO calculations were carried out to determine reactant complexes and transition states of the SN2 reactions of CH3X + Y- → CH3Y + X- where X = F, Cl, CN and Y = CN, OH, F, Cl. The leaving group ability was found to vary inversely with the activation barrier, which in turn was mainly ascribable to the deformation energies accompanied with bond stretching of C-X bond and inversion of CH3 group. The nucleophilicity was shown to be in the order Cl- > F- > OH- > CN- but the effect on the activation barrier was relatively small compared with that of the leaving group. The bond breaking and bond formation indices and energy decomposition analysis showed that the TS for the reaction of CH3Cl occurs in the early stage of the reaction coordinate relative to that of CH3F. It has been shown that the potential energy surface (PES) diagrams approach can only accommodate thermodynamic effects but fails to correlate intrinsic kinetic effects on the TS structure.
391 - 395
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