An Integrated Computational Approach to Modelling of Esr Observables

Interpretation of dynamic behaviour of molecules is of fundamental importance to understand their stability, biological functions and activities, catalytic action. In particular, electron spin resonance measurements are highly informative and the wealth of dynamic information which can be extracted from a CW-ESR or ENDOR spectrum can be interpreted by a basic theoretical approach defined within the Stochastic Liouville Equation (SLE) formalism, i.e. the direct inclusion of motional dynamics in the form of stochastic (Fokker-Planck / diffusive) operators in the super Hamiltonian H governing the time evolution of the system [1]. Modelling based on the SLE approach requires the characterization of magnetic parameters (e.g. hyperfine and Zeeman tensors) and the calculation of ESR observables in terms of spectral densities. The magnetic observables can be pursued by the employment of Density Functional Theory (DFT) which are apt, provided that hybrid functionals are employed, for the accurate computation of structural properties of molecular systems[2]. Recently, an ab-initio integrated computational approach (ICA) to the in-silico interpretation of CW-ESR spectra of multi-labelled systems has been discussed [3]. In this presentation I shall be discussing the basic ingredients of the ICA i.e. 1) determination of geometric and local magnetic parameters by quantum-mechanical DFT calculations taking into account solvent and, when needed, vibrational averaging contributions; 2) numerical solution of a stochastic Liouville equation in the presence of diffusive rotational dynamics, based on 3) parameterization of diffusion rotational tensor provided by a hydrodynamic model. CW-ESR spectra are simulated with minimal or no resorting to fitting procedures, proving that the combination of sensitive ESR spectroscopy and sophisticated modelling can be highly helpful in providing 3Dstructural and dynamic information on molecular systems in complex environments.