Using a Molecular Modeling Program to Calculate Electron Paramagnetic Resonance Hyperfine Couplings in Semiquinone Anion Radicals

Abstract

This paper presents a laboratory exercise in computational chemistry in which molecular orbital theory is used to calculate hyperfine couplings in semiquinone anion radicals found by EPR spectroscopy. The exercise can be used in conjunction with a classic experiment described in Experiments in Physical Chemistry by Shoemaker, Garland, and Nibler, or it can be used as a stand-alone computational exercise. It is designed to be carried out using the program HyperChem for the personal computer, but other molecular modeling programs can be used. Because of the depth of this exercise, a preparatory analysis of the diatomic NO molecule is suggested. The student uses semiempirical molecular orbital calculation methods to optimize the geometry of the semiquinone anion and then performs a single point calculation on the optimized molecule while recording to a log file. The student then examines the log file to find the a and b electron populations for the H atom orbitals, from which the spin densities and finally h...