Nuclear magnetic resonance spectroscopy. Ring-current effects upon carbon-13 chemical shifts

Abstract

Ring currents have played a crucial role in modern chemistry, but they have been analyzed and discussed almost entirely in terms of their effect upon proton nmr chemical shifts. The application of carbon magnetic resonance spectroscopy to this area has received relatively little attention. In fact, only a few alternate and nonalternate aromatic hydrocarbons have been studied in an attempt to elucidate the effect of ring currents on the shifts of carbon nuclei. Unfortunately, the previous conclusions regarding ring-current effects on ^(13)C shifts are not always unambiguous. For example, the observed cmr-shift data for biphenylene might be interpreted in terms of electronegativity and strain superimposed on a diamagnetic ring current, rather than by invoking a paramagnetic ring-current effect, interpretation of the cmr data from biphenylene, pyrene and the majority of other compounds studied suffer somewhat from the lack of suitable model compounds, which are needed to derive “normal” chemical shifts.