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Combined quantum mechanical (QM) and molecular mechanical (MM) calculations are reported for the triosephosphate isomerase-catalyzed conversion of dihydroxyacetone phosphate into glyceraldehyde 3-phosphate. The minima and transition states for the relevant proton-transfer reactions have been located on QM/ MM potential surfaces. The primary objective of this work is to study the sensitivity of optimized structures and relative energies toward variations in the QM/MM model, including the choice of the QM method, the size of the QM region, the size of the optimized MM region, and the treatment of the QM/MM boundary. The QM methods that have been applied in combination with the CHARMm force field range from semiempirical (AM1) to density functional (BP86, B3LYP) and ab initio (MP2) methods, the most extensive QM calculations involving 275 atoms and 2162 basis functions at the density functional level. Implications of the different choices of QM/MM options on the energy profile are discussed. From a mechanistic point of view, the present QM/MM results support a four-step proton-transfer pathway via an enediol, with involvement of neutral His95 acting as a proton donor, since the alternative direct intramolecular proton transfer in the enediolate is disfavored by the protein environment.

What's new about complexes 1 and 2, textbook examples of coordination compounds? Quantum‐chemical simulations reveal an exceptionally strong sensitivity of their 57Fe NMR spectroscopy chemical shifts to the FeC bond length, which, in turn, changes noticeably on going from the gas phase to aqueous solution.

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The mechanism of copper(I)‐catalyzed olefin cyclopropanation with diazomethanes has been studied at the BP86/AE1 level of density‐functional theory. For the model system Cu(diazabutadiene)+ + ethene + diazomethane, copper carbene complexes are confirmed as viable intermediates, with rate‐determining barriers of the order of 25 kcal/mol (energies including zero‐point corrections) or 14 kcal/mol (when entropic contributions are included). For another model system, Cu(2,9‐dimethyl‐1,10‐phenanthroline)+ + styrene + diazoacetate, very small anti/syn selectivities (resulting in trans/cis‐cyclopropanes) have been found both computationally and experimentally. Cu(carbene) complexes with macrocyclic phenanthroline‐based ligands 1 (aryl bridgeheads and ether linkages) and 2 (calix[6]arene) have been optimized at the BP86/SDD level. A qualitative explanation for the trans selectivity observed with 1, based on the tilted, cleft‐like conformation of 1·(CuCHCO2Me)+, is put forward. Similar conformations are found in structures of related acyclic mono‐ and diarylphenanthrolines (either free or complexed with Cu2+ or Pd2+), which have been determined by X‐ray crystallography. The observed cis selectivity of 2 is probably related to the fact that in 2·(CuCHCO2Me)+ the calixarene macrocycle effectively blocks one hemisphere of the catalyst.