Calculating ESR G tensors of doubler radicals by the semiempirical INDO/S method

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 f ield of molecular biology, and we are particularly interested in a reliable method for modeling G tensors as a function of different structures. We be gin this study by examining simple organic radicals and testing how accurat e the intermediate neglect of differential overlap (INDO/S) semiempirical m odel 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 syste ms. Since this model is accurate in predicting electronic spectra, we suspe ct also that the propagator needed to estimate the orbital Zeeman-spin-orbi t cross terms might be quite accurate. In addition, since this model is als o parameterized for many elements of the periodic table, we expect this mod el to be of general utility. We begin here with a preliminary study of G te nsors calculated for simple radicals, testing the accuracy of the calculate d results and their sensitivity to geometry. We also report preliminary res ults on calculated G tensors for the special pair radical cation formed as the first step in the electron transfer in photosynthesis. (C) 1999 John Wi ley & Sons, Inc.