The theory of chemical shifts in nuclear magnetic resonance II. Interpretation of proton shifts

Abstract

The interpretation of proton chemical shifts is discussed in terms of the magnetic field due to the types of electronic current discussed in part I. It is shown that local paramagnetic circulations on neighbouring atoms may have a considerable effect on the resonance frequency. The anomalous position of the acetylenic proton can be ascribed to this effect. For the series of molecules CH 4 , NH 3 , H 2 O, HF, calculations show that there is an effect of paramagnetic currents around the central atom which increases along the series and which operates in the opposite direction to the electronegativity effect. It is suggested that the comparatively small change in the proton frequency along this series is due to cancellation of these terms.