The nature and types of lone-pair effects on nuclear spin coupling constants are reviewed in the context of a localized bond description of molecular electronic structure. Emphasis is placed on the importance of residual delocalization involving the otherwise lone-pair orbital, in terms of which the effect of an X lone-pair, when compared with an isoelectronic Y-H or X-H+ group or with an X-R group, and its orientational dependence can be interpreted. One-, two-and three-bond coupling constants are considered and the importance of lone-pair effects for configurational and conformational information is stressed and illustrated. They also serve for a better understanding of substituent inductive effects on coupling constants.A large collection of illustrative examples are presented, with particular attention paid to couplings involving H, C, N, F and P nuclei, organized in a systematic manner into nine categories. The signs of the lone-pair effects on the reduced coupling constants are found to be independent of the actual nuclei under study in the same category. This is taken as an indication that the electron lone-pairs mainly affect the Fermi contact contribution to the coupling and, accordingly, an interpretation is given in terms of simple sum-over-states models. In addition, symmetry-based relationship are established involving the sign of lone-pair effects in coupling constants between nuclei which are a different number of bonds apart.KEY WORDS NMR Multinuclear spin coupling Lone-pair effects Substituent effects Stereochemistry MO Sum-over-states models Electron delocalization CONTENTS

Transition metal NMR chemical shifts are readily measured by modern 1D and 2D pulse techniques and serve as a probe into electronic and steric effects of ligands and substituents in metal complexes. Quantitative correlations of metal chemical shifts with reaction rates and catalytic activities, both experimental and as a result of quantum chemical calculations, give new mechanistic insights and permit reactivity predictions and a screening of homogeneous catalysts. A variety of such examples involving the spin-1/2 nuclei 57 Fe, 103 Rh, and 187 Os as well as the quadrupolar nuclei 51 V, 53 Cr, 55 Mn, 59 Co, and 91 Zr are discussed.

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SummaryThe 15N-NMR spectra of azoles, with natural isotope abundance, have been measured under different experimental conditions, and chemical shifts are reported for imidazoles, pyrazoles, oxazoles, isoxazoles, thiazoles, and isothiazoles. General trends of substituent effects in this heterocyclic series are discussed based on the data of 67 substituted azoles, dihydro-and tetrahydroazoles. "N, 'H spin-coupling constants have been determined from spectra obtained by ['HI 4 I5N polarizationtransfer experiments, i.e. an application of INEPT and DEPT pulse sequences. Two-bond and three-bond coupling constants are fully assigned and are discussed in terms of the specific pathways in azoles. The potential of structural applications of the new data is illustrated for isomeric nitro-imidazoles and highly-substituted pyrazoles, and in the case of ring-chain tautomerism of 2-substituted tetrahydrooxazoles.

The nitrogen nucleus is the third most important probe (after ' H and I3C) for structural investigations of organic and bioorganic molecules by NMR spectroscopy. For a long time, however, the insufficient sensitivity and low natural abundance of the I5N isotope hampered detection of the I5N nucleus, and the quadrupolar I4N nucleus proved unsuitable for the study of larger molecules with several nonequivalent nitrogen atoms. The advent of new techniques, such as pulse sequences and polarization transfer, in conjunction with the use of high-field magnets and large-sample probe heads largely solved the detection problem. As a result, the last few years have seen a dramatic development of "N-NMR spectroscopy as a versatile method for studying molecular structure, both in isotropic (liquid) and anisotropic (solid) phases. The scope of chemical applications extends from inorganic, organometallic, and organic chemistry to biochemistry and molecular biology, and includes the study of reactive intermediates, biopolymers, enzyme-inhibitor complexes, and nitrogen metabolism. Two-dimensional NMR techniques offer additional possibilities for detailed studies of biological systems. Pages 383-486Angew Chem. Inr. Ed. Engl. 25 (1986) 383-413 0 VCH Verlagsgesellschajl mbH. 0-6940 Weinheim. 1986 0S70-0833/86/05~15-0383 . $ 02.50/0 Anyen, Chem. Int. Ed. Engl. 25 (1986) 383-413