Molecular orbital treatment of differential Overhauser enhancements of solvent nuclei

Abstract

A molecular orbital perturbation approach has been developed to explain the varying Overhauser enhancements observed for chemically shifted nuclei within a solvent molecule. Assuming that these chemically inequivalent nuclei have the same dipolar interaction with the unpaired electron on the free radical solute, the differential enhancements can be ascribed to varying scalar interaction. This methodology allows the estimation of the induced spin density at a particular nucleus N, in the solvent molecule due to the nucleus—unpaired electron scalar interaction. The solvent molecule is considered as a closed-shell system in the CNDO/INDO framework, and the free radical is modelled by a single 2p-orbital of the pi-type centred on the oxygen atom. The Coulomb interaction between the unpaired electron on the free radical and those in the solvent molecule is considered as a perturbation. As a test of the theory we report our results for the induced spin density at the 19F nuclei for the system 1,2,4-trifluoroben...