Density functional study of SN2 substitution reactions for CH3Cl+CX1X2•- (X1X2=HH, HF, HCl, HBr, HI, FF, ClCl, BrBr, and II)

A systematic investigation on the SN2 displacement reactions of nine carbene radical anions toward the substrate CH3Cl has been theoretically carried out using the popular density functional theory functional BHandHLYP level with different basis sets 6-31+G (d, p)/relativistic effective core potential (RECP), 6-311++G (d, p)/RECP, and aug-cc-pVTZ/RECP. The studied models are CX1X2- + CH3Cl ? X2X1CH3C + Cl-, with CX1X2- = CH2-, CHF-, CHCl-, CHBr-, CHI-, CF2-, CCl2-, CBr2-, and CI2-. The main results are proposed as follows: (a) Based on natural bond orbital (NBO), proton affinity (PA), and ionization energy (IE) analysis, reactant CH2- should be a strongest base among the anion-containing species (CX1X2-) and so more favorable nucleophile. (b) Regardless of frontside attacking pathway or backside one, the SN2 reaction starts at an identical precomplex whose formation with no barrier. (c) The back-SN2 pathway is much more preferred than the front-SN2 one in terms of the energy gaps [?E?cent?(front)-?E?cent?(back)], steric demand, NBO population analysis. Thus, the back-SN2 reaction was discussed in detail. On the one hand, based on the energy barriers (?E?cent? and ?E?ovr?) analysis, we have strongly affirmed that the stabilization of back attacking transition states (b-TSs) presents increase in the order: b-TS-CI2 < b-TS-CBr2 < b-TS-CCl2 < b-TS-CHI < b-TS-CHBr < b-TS-CHCl < b-TS-CF2 < b-TS-CHF < b-TS-CH2. On the other hand, depended on discussions of the correlations of ?E?ovr? with influence factors (PA, IE, bond order, and ?E?def?), we have explored how and to what extent they affect the reactions. Moreover, we have predicted that the less size of substitution (a-atom) required for the gas-phase reaction with a-nucleophile is related to the a-effect and estimated that the reaction with the stronger PA nucleophile, holding the lighter substituted atom, corresponds to the greater exothermicity given out from reactants to products. (C) 2012 Wiley Periodicals, Inc. J Comput Chem, 2012