Intermediate complexes and transition structure for the reactions methyl + hydrogen halide .fwdarw. methane + halogen: application of G1 theory

Abstract

Parts of the potential energy surface for the reactions CH3 + HX -*• CH4 + X (X = Cl, Br) were evaluated at the Gaussian-l (Gl) level of theory. Both reactions proceed via a loosely bonded complex (la, lb) which is formed without activation energy.For each reaction, the complex is hydrogen bridged, has C3l) symmetry, and is lower in energy at Gl level than reactants by 0.67 and 0.28 kcal/mol for CH3 + HC1 and CH3 + HBr systems, respectively. The transition state structures (2a, 2b) for decomposition of the complexes to products were located. The transition state (2a) for the CH3 + HC1 reaction has energy 2.53 kcal/mol relative to the reactants, while for the CH3 + HBr system the transition state (2b) is calculated to lie only 0.67 kcal/mol above reactants at Gl level. Heats of reaction for the two reactions compared to experimental values at the same temperature reaffirm error limits for Gl theory, 1.3 kcal/mol (X = Cl) and 2.2 kcal/mol (X = Br).

Theoretical study of the interaction mechanism of single-electron halogen bond complexes H3C⋯Br-Y (Y = H, CN, NC, CCH, C2H3)

“…However, previous studies of unusual lithium and halogen bonds, especially the latter, have been very limited [10]. Most recently, the concept of single-electron weak interactions has been introduced to characterize several special weak bonds, including single-electron hydrogen bonds [11][12][13][14][15], single-electron lithium bonds [16], single-electron sodium bond [17] and singleelectron halogen bonds [18,19].…”

“…Single-electron halogen bonds are very rare and have been much less studied than single-electron hydrogen bonds such as CH 3 ⋅⋅⋅HF [11], CH 3 ⋅⋅⋅HX (X = Cl, Br) [12], CH 3 ⋅⋅⋅H 2 O [13], CH 3 ⋅⋅⋅ CH 4 [14], CH 3 ⋅⋅⋅HCN and CH 3 ⋅⋅⋅HNC [15]. The few theoretical studies of single-electron halogen bonds that have been reported to date have concentrated on the structure-activity relationships between the strength of interaction and the nature of the substituents.…”

Computational study of the rate constants and kinetic isotope effects for the CH3+HBr→CH4+Br reaction

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] Not only they are important for the fundamental chemical kinetics, but also the measured rate constants provide a major source for determination the heats of formation of the radicals and the C-H bond energies. 5,6 The most early measured results revealed that these reactions of small hydrocarbon free radicals ͑such as CH 3 , C 2 H 5 , i-C 3 H 7 , t-C 4 H 9 ) with HBr were activated processes characterized by small positive activation energy.…”

An ab initio study of the structures, barriers for internal rotation, vibrational frequencies, and thermodynamic functions of the hydrochlorofluorocarbon CH3CF2Cl and the corresponding radical CH2CF2Cl

“…However, the literature does not contain any experimentally measured or theoretically estimated heat of formation of the radical CH2CF2C1. In previous publications (19,(28)(29)(30) we demonstrated the calculation and the reliability of "theoretical" radical heats of formation by coupling ab initio calculated energy changes for homodesmic reactions with the known heats of formation of other reaction components. For the purpose of determining the heat of formation of the CH2CF2Cl radical, the following homodesmic reaction was chosen:…”