title: Density functional study of S(N) 2 substitution reactions for CH(3) Cl + CX(1) X(2•-)  (X(1) X(2)  = HH, HF, HCl, HBr, HI, FF, ClCl, BrBr, and II).
abstract: 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(2•-) + CH(3) Cl → X(2) X(1) CH(3) C(•) + Cl(-) , with CX(1) X(2•-) = CH(2) (•-) , CHF(•-) , CHCl(•-) , CHBr(•-) , CHI(•-) , CF(2) (•-) , CCl(2) (•-) , CBr(2) (•-) , and CI(2) (•-) . 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) (•-) should be a strongest base among the anion-containing species (CX(1) X(2•-) ) 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.
authors.affiliation: College of Chemical Engineering, Northwest University for Nationalities, Lanzhou, Gansu 730030, PR China.
authors.forename: Jun-Xi
authors.initials: JX
authors.lastname: Liang
authors.forename: Zhi-Yuan
authors.initials: ZY
authors.lastname: Geng
authors.forename: Yong-Cheng
authors.initials: YC
authors.lastname: Wang
mesh_terms: D001966:Bromine / Q000737:chemistry*; D002713:Chlorine / Q000737:chemistry*; D005740:Gases; D006851:Hydrochloric Acid / Q000737:chemistry*; D006858:Hydrofluoric Acid / Q000737:chemistry*; D007454:Iodides / Q000737:chemistry*; D007455:Iodine / Q000737:chemistry*; D008737:Methyl Chloride / Q000737:chemistry*; D011789:Quantum Theory
chemical_list: D005740:Gases; D007454:Iodides; D002713:Chlorine; D007455:Iodine; D008737:Methyl Chloride; D006851:Hydrochloric Acid; D006858:Hydrofluoric Acid; D001966:Bromine