A combined quantum-mechanical (QM) and molecular-mechanical (MM) method has been used to explore the potential of neutral azoligand-based Grubbs type Pd(Ⅱ) complexes as catalysts for ethylene-acrylonitrile copolymerization. The first part of the investigation ncludes complexation of the monomers to the catalyst, the first insertion step, and isomerization of the first insertion product 1. The results show that the ð-complexation energies of acrylonitrile and ethylene are comparable, but the acrylonitrile insertion has a lower barrier over ethylene insertion by ca. 5 kcal/mol in the first step. The leading product 1 might conduct a series of isomerizations with a lower barrier to form different chelate structures. However, the most stable isomers have the CN group in the R-position. The second part explores the further role of the kinetic insertion product 1. It is shown that 1 readily can complex another ethylene or acrylonitrile monomer. Unfortunately, it is even more favorable for 1 to coordinate its Pd(Ⅱ) center to a nitrogen atom on a R-cyano-alkyl group of another 1 complex, leading to inert oligomers of 1 that do not further react with ethylene or acrylonitrile. Trimers of 1 are preferred over dimers because they allow for an optimal linear Pd-NC bonding mode with an angle of 180°. The preference for palladium coordination to the nitrogen on a R-cyano-alkyl group over N-coordination to acrylonitrile stems from the fact that the acrylonitrile nitrogen is less electron rich due to donation to the ð* orbital of the olefinic bond.

Presented here is the application of a scheme for optimizing the structures of minima and transition states on the free energy surface (FES) for a path along a fixed reaction coordinate with the aid of ab initio molecular dynamics (AIMD) simulation. In the direction of the reaction coordinate, the values corresponding to the stationary points were optimized using the quasi-Newton method, in which the gradient of the free energy along the reaction coordinate was obtained by a constraint AIMD method, and the Bofill Hessian update scheme was used. The equilibrium values for the other directions were taken as the corresponding averages in the dynamic simulation. This scheme was applied to several elementary bimolecular addition reactions: (A) BH3 + H2O f H2O

Density functional theory (DFT) together with Car-Parrinello ab initio molecular dynamics (CP-AIMD) simulation has been used to investigate the free energy profiles of two representative SN2 reactions: (A) Cl-+ CH3Cl f ClCH3 + Cl-; (B) NH3 + H3BNH3 f H3NBH3 + NH3. The free energy profiles along the reaction coordinates at 300 K and 600 K were determined directly by a pointwise thermodynamic integration (PTI) technique. Comparison between the well-known double-well potential energy profile (PEP) and the free energy profiles (FEP) has been made. The results show that, for reaction A, the double-well profile is maintained for the FEP at 300 K due to the stronger ion-dipole interaction between chloromethane and the chloride anion. In comparison with the PEP, the FEP has a higher central barrier and a more shallow well depth. However, at 600 K the double wells almost disappear on the FEP, whereas the central barrier increases further. For reaction B, the 300 K FEP also presents a higher central barrier peak and a more shallow well depth compared to the PEP. However, when the temperature increases to 600 K, a saddle shape FEP is obtained, which indicates that the reaction has changed mechanism from an associative SN2 reaction to a dissociative SN1 reaction. This change is driven by entropy.

Combined gradient-corrected density functional theory and molecular mechanics (QM/MM) has been used to investigate the copolymerization of ethylene with the CH2dCHX R-olefins, where X)-H, -Me, -CN,-OOMe, -OC(O)-Me, -Cl. The cationic NNPd (Ⅱ) diimine complex and its modified neutral and anionic derivatives have been used as catalysts, where NN)-NRCR2CR3NR¢¢-, R) aryl group, and R2, R3) BH3-, H. The consecutive insertion steps of CH2dCHX into the Pd-CH3 bond and of ethylene into the Pd-C(X)HCH2CH3 bond have been investigated. Focus has been put on the role of the X functional groups and the effect of the cationic, neutral, and anionic environments on the Pd (Ⅱ)-diimine system. Calculations have been performed on the CH2dCHX monomers, model catalysts, precursor ð-complexes, and ó-complexes of the monomers, as well as the chelate and H-agostic insertion products. The transition state of the insertion reaction and the corresponding activation energy was determined for both investigated insertion steps. The results show that the X group has only a minor effect on the insertion of the CH2dCHX monomers into the Pd-CH3 bond. On the other hand, the barrier for insertion of ethylene into the Pd-CHXR bond revealed an increase with the electron-withdrawing ability of X. We predict that the application of neutral and anionic catalysts leads to a preference for ð-complexation over ó-complexation of the polar monomers. Unfortunately, for an anionic model system the barriers for the first and second insertion are significantly increased for ethylene, whereas the first insertion barrier for the polar monomers only is moderately increased. Thus, while anionic catalysts are highly tolerant toward polar monomers, they are nearly inactive toward ethylene insertion.

Conventional criteria and indices of aromaticity, including electronic, geometric, energetic and magnetic aspects have been applied to examine the aromaticity of five typical transition metal heterocyclic complexes, i.e. six-membered osmabezene 1 and iridabenzene 2, five-membered cobaltacyclopentadiene 3 and iridacyclopentadiene 4, and four-membered tungstacyclobutadiene 5. The results show that the cyclic, planar, conjugated and H€uckel 4n þ 2 rule s criteria in the transition-metal-containing heterocycles of the five complexes studied are all met. Five quantitative aromaticity indices, including Bird aromatic index (In), homodesmotic reaction aromatic stabilization energy (HASE), absolute hardness (g), diamagnetic susceptibility exaltation (K) and NMR chemical shift (dH), qualitatively lead to a consistent and affirmative conclusion that all of them are aromatic. However, they fail to draw a common conclusion for their relative magnitudes of aromaticity, which proves once again the multidimensional character of aromaticity.