RESUMO
A systematic investigation on the S(N) 2 displacement reactions of nine carbene radical anions toward the substrate CH(3) Cl 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 CX(1) X(2â¢-) + CH(3) Cl â X(2) X(1) CH(3) C(â¢) + Cl(-) , with CX(1) X(2â¢-) = CH(2) (â¢-) , CHF(â¢-) , CHCl(â¢-) , CHBr(â¢-) , CHI(â¢-) , CF(2) (â¢-) , CCl(2) (â¢-) , CBr(2) (â¢-) , and CI(2) (â¢-) . The main results are proposed as follows: (a) Based on natural bond orbital (NBO), proton affinity (PA), and ionization energy (IE) analysis, reactant CH(2) (â¢-) should be a strongest base among the anion-containing species (CX(1) X(2â¢-) ) and so more favorable nucleophile. (b) Regardless of frontside attacking pathway or backside one, the S(N) 2 reaction starts at an identical precomplex whose formation with no barrier. (c) The back-S(N) 2 pathway is much more preferred than the front-S(N) 2 one in terms of the energy gaps [ΔE cent≠(front)-ΔE cent≠(back)], steric demand, NBO population analysis. Thus, the back-S(N) 2 reaction was discussed in detail. On the one hand, based on the energy barriers (ΔE cent≠ and ΔE ovr≠) analysis, we have strongly affirmed that the stabilization of back attacking transition states (b-TSs) presents increase in the order: b-TS-CI(2) < b-TS-CBr(2) < b-TS-CCl(2) < b-TS-CHI < b-TS-CHBr < b-TS-CHCl < b-TS-CF(2) < b-TS-CHF < b-TS-CH(2) . On the other hand, depended on discussions of the correlations of ΔE ovr≠ with influence factors (PA, IE, bond order, and ΔE def≠), we have explored how and to what extent they affect the reactions. Moreover, we have predicted that the less size of substitution (α-atom) required for the gas-phase reaction with α-nucleophile is related to the α-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.