Anisotrophy in Arenes: Calculation of NMR Chemical Shifts

Abstract

<p>The local anisotropic shielding contribution to proton NMR chemical shifts, due to electron circulation around each carbon, in benzene is calculated on the basis of the free electron in a loop model. The circulating current was estimated from the experimentally measured ¹³C shielding tensors. The ring current contribution is recalculated, using the Waugh and Fessenden approach but no loop separation is assumed.</p> <p>The sum of the local anisotropic and ring current contributions in conjunction with an appropriately chosen standard shift predict chemical shift values which compare favourably with experimental values for benzene and toluene. The predicted chemical shifts for the methylene protons of [9]-paracyclophane are given.</p> <p>Contour maps of the local anisotropic contributions in various planes in and around a benzene ring are presented. Also presented is a contour map for the ring current contribution, similar to the Johnson and Bovey plot, with the assumption there is no separation of the current loops.</p>