Toward a Broadly Applicable Force Field for d6-Piano Stool Complexes

Direct simulation of electron transfer in the cobalt hexammine(ii/iii) self-exchange reaction

We present an atomistic simulation of electron transfer in a transition metal complex system using path integral methods.

Copper Oxidation/Reduction in Water and Protein: Studies with DFTB3/MM and VALBOND Molecular Dynamics Simulations

We apply two recently developed computational methods, DFTB3 and VALBOND, to study copper oxidation/reduction processes in solution and protein. The properties of interest include the coordination structure of copper in different oxidation states in water or in a protein (plastocyanin) active site, the reduction potential of the copper ion in different environments, and the environmental response to copper oxidation. The DFTB3/MM and VALBOND simulation results are compared to DFT/MM simulations and experimental results whenever possible. For a copper ion in aqueous solution, DFTB3/MM results are generally close to B3LYP/MM with a medium basis, including both solvation structure and reduction potential for Cu(II); for Cu(I), however, DFTB3/MM finds a two-water coordination, similar to previous Born-Oppenheimer molecular dynamics simulations using BLYP and HSE, whereas B3LYP/MM leads to a tetrahedron coordination. For a tetraammonia copper complex in aqueous solution, VALBOND and DFTB3/MM are consistent in terms of both structural and dynamical properties of solvent near copper for both oxidation states. For copper reduction in plastocyanin, DFTB3/MM simulations capture the key properties of the active site, and the computed reduction potential and reorganization energy are in fair agreement with experiment, especially when the periodic boundary condition is used. Overall, the study supports the value of VALBOND and DFTB3(/MM) for the analysis of fundamental copper redox chemistry in water and protein, and the results also help highlight areas where further improvements in these methods are desirable.

Toward force fields for atomistic simulations of iridium-containing complexes

The structural and energetic characterization of metal complexes is important in catalysis and photochemical applications. Unraveling their modes-of-action can be greatly assisted by computation, which typically is restricted to computationally demanding methods including electronic structure calculations with density functional theory. Here, we present an empirical force field based on valence bond theory applicable to a range of octahedral Ir(III) complexes with different coordinating ligands, including iridium complexes with a chiral P,N ligand. Using an approach applicable to metal-containing complexes in general, it is shown that with one common parametrization 85% of the 116 diastereomers--all within 21 kcal/mol of the lowest energy conformation of each series--can be correctly ranked. For neutral complexes, all diastereomers are ranked correctly. This helps to identify the most relevant diastereomers which, if necessary, can be further investigated by more demanding computational methods. Furthermore, if one specific complex is considered, the root mean square deviation between reference data from electronic structure calculations and the force field is ≈1 kcal/mol. This, together with the possibility to carry out explicit simulations in solution paves the way for an atomistic understanding of iridium-containing complexes in catalysis.