Calculating ESR G Tensors of Doublet Radicals by the Semiempirical INDO/S Method

Abstract

The interpretation of the EPR spectra of radicals can be a powerful tool in obtaining structural information. There are many examples of this in the field of molecular biology, and we are particularly interested in a reliable method for modeling G tensors as a function of different structures. We begin this study by examining simple organic radicals and testing how accurate the intermediate neglect of differential overlap (INDO/S) semiempirical model can be when put to this purpose. The INDO/S model is fast and has been used with some success in examining the photochemistry of very large systems. Since this model is accurate in predicting electronic spectra, we suspect also that the propagator needed to estimate the orbital Zeeman-spin-orbit cross terms might be quite accurate. In addition, since this model is also parameterized for many elements of the periodic table, we expect this model to be of general utility. We begin here with a preliminary study of G tensors calculated for simple radicals, testing the accuracy of the calculated results and their sensitivity to geometry. We also report preliminary results on calculated G tensors for the special pair radical cation formed as the first step in the electron transfer in photosynthesis. © 1999 John Wiley & Sons, Inc.