The intrinsic reaction coordination (IRC) for the dehydrogenation reaction of vinyl radicals was traced by means of MCSCF/6-31G (210 configurations). The activation barrier of this reaction is 40.0kcal mol-1. Using the zero-point energy correction, the activation energy is 33.5kcal mol-1, which is in much better agreement with the experimental value (31.5kcalmol-1). The rate constants at five temperatures from 800 to 1200K were calculated by CVT, ICVT and μ VT. The reverse process is also discussed.

CAS(14,12)/cc-pvdz calculations are reported for the reaction of 3CH2+3O2fproducts. On the singlet potential energy surface, a transition state has been located with an energy barrier of 1.65kcal/mol, which is in good agreement with the experimental estimation of 1.0-1.5kcal/mol. The rearrangement and metathesis of the singlet intermediates have been also investigated at the same level of theory. For the triplet case, the formation of CH2OO has an energy barrier of 5.79kcal/mol, and the formed triplet CH2OO could be further decomposed into CH2O+O(3P) with an energy barrier of 2.92kcal/mol. The geometries of some key points have been relocated at the CAS (8,6)+1+2/cc-pvdz level of theory for comparison. The present theoretical results for the total reaction rates, at the CAS (8, 6)+1+2/cc-pvdz level, can be expressed by the three-parameter expression: k(T)) 4.273 10-18T2.245exp (-185/T) within (5% error at the temperature range 295-2600K.

Ab initio calculations have been used to study the mechanism of the cycloaddition reaction between ketene imine and formaldehyde. In the gas phase, the reaction proceeds via a concerted but asynchronous way; while in dichloromethane solvent, the reaction becomes a two-step zwitterionic one, with the C-C bond formed firstly. The BH3-catalyzed reaction is predicted to be a concerted process, in which BH 3 is bonded to formaldehyde. The energy barrier of this process is 12.22kcal/mol (in the gas reaction) or 13.81kcal/mol (in the dichloromethane solven0 respectively. They are much lower than that of the non-catalyzed reactions.

Ab initio calculations (HF/3-ZIG) have been used to study the mechanism of peptide biosynthesis (R1COOR2 + R3NH2-R1CONHR3 + R2OH). Two or four water molecules are included to represent the primary solvent shell. The studies show that the reaction proceeds via a gauche or truns transition state if it starts from the reactant's gauche or trans complex. The energy barrier of the reaction with two water molecules is calculated to be 27.96 (gauche) or 26.85kcal mol-1 (trans), while that of the reaction with four water molecules is only 14.82 (gauche) or 13.21kcal mol-1 (trans).

A combination of high-level quantum-chemical simulations and sophisticated transition state theory analyses is employed in a study of the temperature dependence of the N2H+OH+HNNOH recombination reaction. The implications for the branching between and in the reaction are also explored. The transition N2H+OH N2+H2O NH2+NO state partition function for the recombination reaction is evaluated with a N2H]OH direct implementation of variable reaction coordinate (VRC) transition state theory (TST). The orientation dependent interaction energies are directly determined at the CAS+1+2/cc-pvdz level. Corrections for basis set limitations are obtained via calculations along the cis and trans minimum energy paths employing an Daug-pvtz basis set. The calculated rate constant for the recombination is found N2H+OH+HNNOH to decrease signi-cantly with increasing temperature, in agreement with the predictions of our earlier theoretical study. Conventional transition state theory analyses, employing new coupled cluster estimates for the vibrational frequencies and energies at the saddlepoints along the reaction pathway, are coupled with the VRC-TST analyses for the NH2+NO channels to provide estimates for the branching in the reaction. N2H+OH NH2+NO Modest variations in the exothermicity of the reaction (1-2 kcal mol-1), and in a few of the saddlepoint energies (2-4 kcal mol-1), yield TST based predictions for the branching fraction that are in satisfactory agreement with related experimental results. The unmodi-ed results are in reasonable agreement for higher temperatures, but predict too low a branching ratio near room temperature, as well as too steep an initial rise.