Molecular structures of nickel adducts in zeolites – Interpretation of experimental EPR g-tensors guided by DFT calculations

Abstract

Computational electron paramagnetic resonance (EPR) spectroscopy is a combination of robust simulation techniques of complex spectra with density functional theory (DFT) calculations of spin-Hamiltonian parameters for the adopted models of the investigated paramagnetic species. This approach was used for guiding interpretation of the experimental EPR data of selected adducts of nickel(I) and nickel (II) centers in high-silica zeolites with gas-phase small molecules of environmental significance (CO, NO, C2H2, NH3, O2). As a result, a quantitative connection between the spectroscopic fingerprints, in particular g-tensors, and structure of the molecular clusters mimicking the real intrazeolitic species was obtained. Possible calculation schemes for assessing g-tensor values were explored. Various levels of relativistic effects (including relativistic corrections to the composition of electronic ground state and spin-orbit approximations) were studied. Selection of a proper exchange-correlation functional was also discussed underlying its influence for the systems for which spin density is delocalized over a metal core and a ligand. The conformation analysis of g-tensor anisotropy for amine adducts in assessing its structure was shown. Finally, molecular interpretation of electronic g-tensor in terms of the magnetic field-induced transitions between involved orbitals was shown taking as an example a diamine Ni(I) adduct.