Nitrogen-14 and -15 nuclear magnetic resonance spectroscopy. Part III. The diamagnetic correction and the absolute scale of nitrogen chemical shift

Abstract

We show that significant errors arise if uncorrected shifts are used as estimates of the paramagnetic term, in a comparison of compounds (such as ammonia and the methylamines) which have atoms of different atomic number attached to nitrogen, and in a comparison of compounds (such as amines and amine oxides) in which nitrogen has different co-ordination numbers. With the appropriate diamagnetic corrections, the shifts of some representative nitrogen compounds are arranged on an absolute scale (relative to the bare nucleus), based on Ramsey's molecularbeam magnetic resonance measurements of the nitrogen molecule. Some of the factors that determine nitrogen paramagnetic shifts are discussed.