The Virtual Electron Paramagnetic Resonance Laboratory

Our main objective in this Chapter has been to discuss the degree of advancement of the ICS to the interpretation of cw-EPR of organic radicals and biradicals in solvated environments, via combination of advanced quantum mechanical approaches and stochastic modelling of relaxation processes. The ICS ab initio prediction of cw-EPR spectra is able to assess molecular characteristics entirely from computational models and direct comparison with the experimental data. The sensitivity of the integrated methodology to the overall molecular geometry is proved, in all the cases discussed above, by the significant dependence of the calculated spectrum on arbitrary modifications of molecular geometry or dynamic properties. For instance, in the case of the heptapetide biradicals, the ICS is sensitive enough to distinguish between different helix conformations. Some adjustment of computed magnetic tensors is probably unavoidable for a quantitative fitting of experimental spectra, particularly for large systems where only DFT approaches are feasible. However, the number of free parameters (if any) is limited enough that convergence to the true minimum can be granted. At the same time the allowed variation of parameters from their QM value is well within the difference between different structural models. Thus, pending further developments of DFT models, the ICS is already able to predict cw-EPR spectra of large molecular systems in solvents starting only from the chemical structure of the solute and some macroscopic solvent properties. Implementation in a user-friendly package finally could spread the systematic usage of the ICS in current real-life EPR laboratories, as much as standard QM packages for structural molecular properties are already diffuse in most modern chemistry research facilities.