A valence bond method study for S(N)2 reaction X-r(-) (X-1 = Xr = F, Cl, Br, I)
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Barriers and VB quantities for S(N)2 reaction X-1(-)+CH3Xr-->X1CH3+X-r(-)(X-1=X-r=F, Cl, Br, I) are calculated by valence bond configuration interaction (VBCI) method. The computational results show that the VB results obtained by using CI method are in good agreement with those obtained by using the molecular orbital based coupled cluster CCSD method. The reaction parameters for the S(N)2 identity reaction, including promotion gap G, height of crossing point, resonance energy B, and curvature factor f, are discussed. They show the same trend as that predicted by semiempirical VB theory, and provide quantitative data for understanding the reaction mechanism.