DIFFERENTIAL RING PROTON NMR SHIELDINGS AND CYCLIC STABILIZATION ENERGIES

Ring currents are thought to be responsible for unusual magnetic susce ptibility effects in organic ring systems and represent a method of me asuring cyclic stabilization/destabilization (aromaticity/antiaromatic ity). They should, in principle, also affect nuclear shieldings in suc h systems, especially proton shieldings. By studying a variety of four -, five-, and six-membered organic ring systems, we investigate the qu estion here by theoretical means as to whether proton shieldings refle ct such cyclic stabilization/destabilization effects as measured by ho momolecular homodesmotic reaction energies. While absolute shieldings show only a slight hint of correlation, it is found that differences i n ring proton shieldings between the fully unsaturated species and its monoene counterpart show a clear correlation with this energy. In par ticular, mean differential shieldings exhibit a good linear relationsh ip with the homomolecular homodesmotic reaction energy that is quantit atively comparable to other measures of aromaticity/antiaromaticity. ( C)1998 Elsevier Science B.V. All rights reserved.