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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]).- © M. Kaupp

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].

### References

[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.