Differential ring proton NMR shieldings and cyclic stabilization energies

Ring currents are thought to be responsible for unusual magnetic susceptibility effects in organic ring systems and represent a method of measuring cyclic stabilization/destabilization (aromaticity/antiaromaticity). They should, in principle, also affect nuclear shieldings in such systems, especially proton shieldings. By studying a variety of four-, five-, and six-membered organic ring systems, we investigate the question here by theoretical means as to whether proton shieldings reflect such cyclic stabilization/destabilization effects as measured by homomolecular homodesmotic reaction energies. While absolute shieldings show only a slight hint of correlation, it is found that differences in ring proton shieldings between the fully unsaturated species and its monoene counterpart show a clear correlation with this energy. In particular, mean differential shieldings exhibit a good linear relationship with the homomolecular homodesmotic reaction energy that is quantitatively comparable to other measures of aromaticity/antiaromaticity. (C)1998 Elsevier Science B.V. All rights reserved.