Chemistry and enzymology of vitamin B12.

Catalytic chain transfer in free-radical polymerizations.

The homolytic Co−C bond dissociation energy (BDE) is central to the understanding of the function of vitamin B12, an important coenzyme of many proteins. We investigate why earlier density functional (B3LYP) estimations of the BDE in methylcobalamin have given so poor results (91−117 kJ/mol) compared to the experimental estimate (155 ± 13 kJ/mol). Improving the basis set increases the discrepancy, as does a proper treatment of basis set superposition error (∼3 kJ/mol) and inclusion of zero-point energy corrections (−21 kJ/mol). On the other hand, relativistic (+6 kJ/mol), solvation (+7 kJ/mol in water), and thermal corrections (+6 kJ/mol) increase the BDE. However, neither of these corrections can explain the discrepancy. Instead, the problem seems to be the B3LYP density functional, which gives a corrected BDE of 78 kJ/mol, whereas the density functional Becke−Perdew-86 method and second-order perturbation theory (MP2) give BDEs of 134−139 kJ/mol. A comparison with other methods indicates that the error ...

Theoretical Prediction of the Co−C Bond Strength in Cobalamins

Theoretical Study on Monoligated Pd-Catalyzed Cross-Coupling Reactions of Aryl Chlorides and Bromides

Nanoparticles supported on high surface area materials are commonly used in many industrially relevant catalytic reactions. This review examines the existing literature of the mechanisms of formation of practical, non-ultra high vacuum, supported-nanoparticle heterogeneous catalysts. Specifically, this review includes: (i) a brief overview of the synthesis of supported-nanoparticles, (ii) an overview of the physical methods for following the kinetics of formation of supported-nanoparticles, and then (iii) a summary of the kinetic and mechanistic studies of the formation of supported nanoparticle catalysts, performed under the traditional synthetic conditions of the gas–solid interface. This review then also discusses (iv) the synthesis, (v) physical methods, and (vi) the extant kinetic and mechanistic studies under the less traditional, less examined conditions of a liquid–solid system. A summary of the main insights from each section of the review is also given. Overall, surprisingly little is known about the mechanism(s) of formation of the desired size, shape and compositionally controlled supported-nanoparticle catalysts.

Thomas W. Koenig

Papers

Cage effects in organotransition metal chemistry: their importance in the kinetic estimation of bond dissociation energies in solution