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.

Density functional (B3LYP) calculations, using the 6-31G** basis set, have been employed to study the title reactions. For the model reaction (H2CdC-NH+dCH2 + H2CdCH2), a complex has been formed with 6.2kcal/mol of stabilization energy and the transition state is 4.0kcal/mol above this complex, but 2.1kcal/mol below the reactants. However, the substituent effects are quite remarkable. When ethene is substituted by electron-withdrawing group CN, the reaction could also yield sixmembered-ring products, but the energy barriers are all more than 7 kcal/mol, which shows that CN group unfavors the reaction. The other substituents, such as CH3O and CH3 groups, have also been considered in the present work, and the results show that they are favorable for the formation of six-membered-ring adducts. The calculated results have been rationalized with frontier orbital interaction and topological analysis.

The geometries of the reactant, products, and transition state of the title reaction have been optimized at UHF, UMP2, and UMP4 levels with the double and triple zeta basis sets as well as polarization functions using the energy gradient method. The best potential barrier height for this reaction was calculated to be 9.05kcal/mol with MP-SAC4 theory. The intrinsic reaction coordinate (IRC) was traced at UHF/6-3 1G and UMP2(FU)/6-311G** levels. The energy profile along the IRC was refined with subsequent MP-SAC4//MP2 calculations using UMP2(FU)/6-3 1 1G** geometries. The vibration modes and the couplings between the IRC and the normal modes were analyzed along the IRC. The calculated rate constants at the level of MP-SAC4//MP2 theory are in good agreement with the most recent experimental values in the temperature range from 2200 to 2800 K.

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).