Computational Studies of Electron Paramagnetic Resonance Parameters for Paramagnetic Molybdenum Complexes. 1. Method Validation on Small and Medium-Sized Systems

Abstract

A variety of density functional methods have been evaluated in the computation of electronic g-tensors and molybdenum hyperfine couplings for systems ranging from the Mo atom through MoIIIN, [MoVOCl4]-, and [MoVOF5]2- to two larger MoV complexes MoXLCl2 (X=O, S; L=tris(3,5-dimethylpyrazolyl)hydroborate anion). In particular, the influence of the molybdenum basis set and of various exchange-correlation functionals with variable admixtures of Hartree-Fock exchange on the computed EPR parameters have been evaluated in detail. Careful basis-set studies have provided a moderate-sized 12s6p5d all-electron basis on molybdenum that gives hyperfine tensors in excellent agreement with much larger basis sets and that will be useful for calculations on larger systems. The best agreement with experimental data for both hyperfine and g-tensors is obtained with hybrid functionals containing approximately 30-40% Hartree-Fock exchange. Only for MoSLCl2 does increasing spin contamination with increasing exact-exchange admixture restrict the achievable computational accuracy. In all cases, spin-orbit corrections to the hyperfine tensors are sizable and have to be included in accurate calculations. Scalar relativistic effects enhance the isotropic Mo hyperfine coupling by approximately 15-20%. Two-component g-tensor calculations with variational inclusion of spin-orbit coupling show that the Deltag parallel components in [MoVOCl4]- and [MoVOF5]2- depend on higher-order spin-orbit contributions and are thus described insufficiently by the usual second-order perturbation approaches. Computed orientations of g- and hyperfine tensors relative to each other and to the molecular framework for the MoXLCl2 complexes provide good agreement between theory and single-crystal electron paramagnetic resonance experiments. In these cases, the hyperfine tensor orientations are influenced only slightly by spin-orbit effects.

Relativistic two-component calculations of electronic g-tensor for oxo-molybdenum(V) and oxo-tungsten(V) complexes: The important role of higher-order spin-orbit contributions

“…In case of Mo V and W V sites, metal and ligand hyperfine couplings, certain ligand nuclear quadrupole couplings, and the electronic g-tensor, are the most important parameters to be considered to describe the EPR spectra [14][15][16]25]. We have recently validated state-of-the-art quantum chemical approaches based on density functional theory (DFT) for the calculation of g-tensors and Mo hyperfine tensors in a series of Mo(V) complexes ranging from small models to larger complexes of low symmetry that resemble some of the enzyme active sites [26,27]. It turned out that hybrid functionals with an exact-exchange admixture between 30% and 40% provided the best agreement with experimental g-and hyperfine tensors, including the orientations of these tensors relative to each other and to the molecular framework (as compared to single crystal EPR data with 95 Mo and 97 Mo isotope-enriched samples) [26].…”

“…We have recently validated state-of-the-art quantum chemical approaches based on density functional theory (DFT) for the calculation of g-tensors and Mo hyperfine tensors in a series of Mo(V) complexes ranging from small models to larger complexes of low symmetry that resemble some of the enzyme active sites [26,27]. It turned out that hybrid functionals with an exact-exchange admixture between 30% and 40% provided the best agreement with experimental g-and hyperfine tensors, including the orientations of these tensors relative to each other and to the molecular framework (as compared to single crystal EPR data with 95 Mo and 97 Mo isotope-enriched samples) [26]. In cases of X-band powder spectra without isotope enrichment, the hyperfine tensors of model complexes and enzymes frequently cannot be determined accurately by experiment, and it was shown that assistance by quantum chemical calculations may allow improved spectra simulations in such cases [27].…”

Comparative Analysis of Electron‐Density and Electron‐Localization Function for Dinuclear Manganese Complexes with Bridging Boron‐ and Carbon‐Centered Ligands

“…While B3 LYP is clearly the most popular functional in main group chemistry (albeit it also has been shown to exhibit severe shortcomings even in organic chemistry), it appears that the optimum amount of exact-exchange admixture varies for different systems and different properties (see, e.g., discussions in refs. [27,64,[66][67][68][69][70][71][72]). B3 LYP incorporates 20 % and the BHLYP functional 50 % HF exchange, and thus a comparison of these two hybrid functionals with the BP86 and BLYP GGA functionals should provide insight to an interrelation between charge transfer between metal and bridging ligand (and vice versa) and the computed features in 5 2…”

Electronic Structure and Reactivity of a [1],[1]Disilamolybdenocenophane

“…For subsequent analyses, single-point calculations on the model structure 1 cs were performed at B3LYP/TZVP level by using a TZVP-based 12s6p5d all-electron basis set for Mo. [30] For all single-point calculations the Gaussian 03 program package [31] was used. NPA analyses and calculation of Wiberg bond indices for all complexes under consideration (1, 2, 8, 9) were performed with the NBO program (version 3.1) [32] as part of the Gaussian 03 program package.…”