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.

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.

Density functional (B3LYP) calculations using the 6-31++g** basis set have been employed to study the title reaction between the cationic 1,3-dipolar 1-aza-2-azoniaallene ion (H2CdN+dNH) and ethene. Our calculations confirmed that [3 + 2] cycloaddition reaction takes place via a threemembered ring intermediate. In addition, solvent effects and substituent effects were also studied. For the reactions involving tetrachloroethene, there are two attacking sites. One is on the NH group in the 1-aza-2-azoniaallene ion, another is on its terminal CH2 group, and they are competitive for both approaching positions. Electron-releasing methyl substituents on ethene favor the reaction, and the potential energy surface is quite different from the previous one.

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.

This paper uses the properties of atom X enclosed within an adamantane cage, denoted by X@C10H16, as a vehicle to introduce the Ehrenfest force into the discussion of bonding, the properties being determined by the physics of an open system. This is the force acting on an atom in a molecule and determining the potential energy appearing in Slater's molecular virial theorem. The Ehrenfest force acting across the interatomic surface of a bonded pair atomssatoms linked by a bond pathsis attractive, each atom being drawn toward the other, and the associated surface virial that measures the contribution to the energy arising from the formation of the surface is stabilizing. It is the Ehrenfest force that determines the adhesive properties of surfaces. The endothermicity of formation for X=He or Ne is not a result of instabilities incurred in the interaction of X with the four methine carbons to which it is bonded, interactions that are stabilizing both in terms of the changes in the atomic energies and in the surface virials. The exothermicity for X=Be2+, B3+, and Al3+ is a consequence of the transfer of electron density from the hydrogen atoms to the carbon and X atoms, the exothermicity increasing with charge transfer despite an increase in the contained volume of X.

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 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).

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.

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.