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.

The mechanisms of seven reactions between keteniminium cations and olefins have been theoretically explored at BHandHLYP/6-31G* level. It is found that these seven reactions always form a relatively stable hydrogen-bonded type of ion-molecule complex first except for reactions 1d+2a and 1e+2a, which have no hydrogen atom attached to nitrogen atom in keteniminium cations. Some reactions take place via a concerted but unsynchronous mechanism, and the others are stepwise processes. The substituent effects are also studied. The data reveal that the electronpushing substituents on keteniminium cations disfavor the reaction, and the electron-attracting substituents on keteniminium cations favor the reactions. The substituent effects on ethene are contrary to the former case.

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).

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.

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.