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Methods to include dynamical and environmental effects in EPR/NMR parameter calculations

Figure 1. Probability distribution for T-stacking hydrogen bonds to aqueous benzosemiquinone radical anion from Car-Parrinello molecular dynamics simulations (cf. [2]).

Most quantum chemical calculations presume static molecules at 0 K. Obviously this meets reality very rarely, and it neglects the important effects of molecular dynamics (MD), with severe effects also for magnetic properties. In the field of open-shell systems, it is difficult to use classical MD simulations, as corresponding accurate force fields do not exist. We have recently evaluated dynamical effects on the electronic g-tensors of semiquinone radical anions in water, by combining ab initio MD simulations within the framework of the Car-Parrinello method with our accurate approaches to calculate g-tensors [1-4]. Initial work was on the parent benzosemiquinone radical anion, and substantial insight has been obtained on dynamics of the system (cf. Figure 1) and on the effects of solvation on g-tensors [1,2] and hyperfine tensors [3]. In particular, we discovered that T-stacked hydrogen bonding to the pi-system is significant [1]. This is interesting, as it has been discussed in the context of photosystem I [5]. The studies have also been extended already to the biologically more relevant ubisemiquinone radical anion (again in aqueous solution) [4]. Note that the cluster snapshot calculations used to extract EPR parameters along the CPMD trajectory may also include bulk dielectric solvent effects via continuum solvent models [6,7]. Most recently, the g-tensor of the radiation-induced glycyl radical in solid glycine has been studied by similar methods [8].

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[1] Ab Initio Molecular Dynamics Simulations and g-Tensor Calculations of Aqueous Benzosemiquinone: Effects of Regular and “T-Stacked” Hydrogen Bonds J. Asher, N. Doltsinis, M. Kaupp J. Am. Chem. Soc., 2004, 126, 9854-9861.

[2] Extended Car-Parrinello molecular dynamics and electronic g-tensors study of benzosemiquinone radical anion J. Asher, N. Doltsinis, M. Kaupp Magn. Reson. Chem 2005, 43, S237-S247.

[3] Hyperfine coupling tensors of benzosemiquinone radical anion from Car-Parrinello molecular dynamics J. R. Asher, M. Kaupp ChemPhysChem 2007, 8, 69-79.

[4] Car-Parrinello Molecular Dynamics Simulations and EPR Property Calculations on Aqueous Ubisemiquinone Radical Anion J. R. Asher, M. Kaupp Theor. Chem. Acc., online, DOI 10.1007/s00214-007-0408-1.

[5] The Function of Photosystem I. Quantum Chemical Insight into the Role of Tryptophan-Quinone Interactions M. Kaupp Biochemistry 2002, 41, 2895-2900.

[6]  Solvent Effects on g-Tensors of Semiquinone Radical Anions: Polarizable Continuum vs Cluster Models I. Ciofini, R. Reviakine, A. Arbuznikov, M. Kaupp Theor. Chem. Acc. 2004, 111, 132-140.

[7] Understanding Solvent Effects on Hyperfine Coupling Constants of Cyclohexadienyl Radicals M. Straka, M. Kaupp, E. Roduner Theor. Chem. Acc. 2005, 114, 318-326.

[8] Cluster or periodic, static or dynamic – the challenge of calculating the g tensor of the solid-state glycine radical E. Pauwels, J. Asher, M. Kaupp, M. Waroquier Phys. Chem. Chem. Phys. 2011, 13, 18638-18646.

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Prof. Dr. M. Kaupp
Theoretical Chemistry
Quantum Chemistry