Model Identity S_N2 Reactions CH₃X + X^- (X = F, Cl, CN, OH, SH, NH₂, PH₂):  Marcus Theory Analyzed

Abstract

The structures of seven gas phase identity S(N)2 reactions of the form CH(3)X + X(-) have been characterized with seven distinct theoretical methods: RHF, B3LYP, BLYP, BP86, MP2, CCSD, and CCSD(T), in conjunction with basis sets of double and triple zeta quality. Additionally, the energetics of said reactions have been definitively computed using focal point analyses utilizing extrapolation to the one-particle limit for the Hartree-Fock and MP2 energies using basis sets of up to aug-cc-pV5Z quality, inclusion of higher order correlation effects [CCSD and CCSD(T)] with basis sets of aug-cc-pVTZ quality, and additional auxiliary terms for core correlation and scalar relativistic effects. Final net activation barriers for the reactions are E(b)(F,F)= -0.8, E(b)(Cl,Cl)= 1.6, E(b)(CN,CN)= 28.7, E(b)(OH,OH)= 14.3, E(b)(SH,SH)= 13.8, E(b)(NH2,NH2)= 28.6, and E(b)(PH2,PH2)= 25.7 kcal mol(-1). General trends in the energetics, specifically the performance of the density functionals, and the component energies of the focal point analyses are discussed. The utility of classic Marcus theory as a technique for barrier predictions has been carefully analyzed. The standard Marcus theory results show disparities of up to 9 kcal mol(-1) with respect to explicitly computed results. However, when alternative approaches to Marcus theory, independent of the well-depths, are considered, excellent performance is achieved, with the largest deviations being under 3 kcal mol(-1).

Quantum Chemical Calculations of the Cl− + CH3I → CH3Cl + I− Potential Energy Surface

“…59 DFT models have been assessed for the CH 3 X + F -(X ) F, Cl, CN, OH, SH, NH 2 , PH 2 ) reactions 60 and a range of ab initio and DFT methods have been used to analyze the X -+ CH 3 X (X ) F, Cl, CN, OH, SH, NH 2 , PH 2 ) S N 2 reactions. 61 Recently, Bickelhaupt and co-workers carried out a detailed evaluation of the performance of many DFT functionals for describing PES's of a wide range of S N 2 reactions. 62 Analytics PES's and ab initio PES's for direct dynamics have been used in studies of the Cl -+ CH 3 Cl, 35,42,[63][64][65] Cl -+ CH 3 Br, 38,66 F -+ CH 3 Cl, 41,67 F -+ CH 3 OH, 43 and Cl -+ CH 3 I 31 S N 2 reactions.…”

“…High-level CCSD(T) ab initio calculations have provided properties for the F -+ CH 3 Cl, 67,68 F -+ CH 3 F, 69 Cl -+ CH 3 Cl, 70 59 CH 3 X + F -(X ) F, Cl, CN, OH, SH, NH 2 , PH 2 ), 60 OH -+ CH (4-n) Cl n (n ) 1-4), 71 and CH 3 X + X -(X ) F, Cl, CN, OH, SH, NH 2 , PH 2 ) S N 2 reactions. 61 In recent work a direct chemical dynamics simulation, 31 at the MP2(fc)/ECP/aug-cc-pVDZ level of theory, was used to study the dynamics of the Cl -+ CH 3 I S N 2 reaction and compare with results of ion imaging experiments. To investigate the adequacy of these MP2 direct dynamics, it is of interest to compare properties of the Cl -+ CH 3 I f ClCH 3 + I -PES, given by the above MP2 method and other electronic structure theoretical methods.…”

Direct ab initio molecular dynamics study on a microsolvated SN2 reaction of OH−(H2O) with CH3Cl

“…[1][2][3] From the theoretical points of view, ab initio, classical, and quantum chemical dynamics calculations [4][5][6][7][8][9][10][11][12] were obtained by the ab initio method. The reaction rates for several systems were also investigated on the basis of transition state theory using the ab initio data.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Nevertheless, fewer studies have addressed the role of solvation in these reactions. 16,17 From an experimental point of view, O'Hair et al investigated solvent effects on the S N 2 reaction of fluoride ion with methyl chloride and measured the product ions formed by the reaction F − ͑H 2 O͒ +CH 3 Cl.…”

Atmospheric reactivity of CH3I and CH2I2 with OH radicals: A comparative study of the H- versus I-abstraction

“…Similar to analogous gas-phase S N 2 reactions with anionic nucleophiles and alkyl halides [55][56][57][58][59], the electron correlation from CC calculations significantly decreases the barrier height, with respect to the reactants or products. In the forward H-or I-abstractions, this 3 .…”