A method for the simulation of powder electron spin resonance spectra when S > ½; application to the iron–molybdenum–sulphur protein of nitrogenase and of some [Fe4S4(SR)4]3– clusters

Abstract

A method for simulating powder e.s.r. spectra of systems with S > ½ in the absence of hyperfine interactions is described. The method involves diagonalisation of the spin-Hamiltonian matrix, curve-fitting the resulting energies to obtain the resonance fields, calculating the transition probabilities of the resonances, and weighting the energy states from which transitions are being stimulated according to their relative thermal occupation. The simulation process is applied to published e.s.r. spectra of the FeMo protein and FeMo cofactor from Azotobacter vinelandii and Clostridium pasteurianom, and to the powder spectra of [NR′₄]₃[Fe₄S₄(SR)₄](where R′= Prⁿ, R = CH₂C₆H₅; R′= Et, R = CH₂C₆H₅ or CH₂C₆H₄OMe-p; and R′= Me, R = C₆H₄Me-p).