NMR Nomenclature: Nuclear Spin Properties and Conventions for Chemical Shifts. IUPAC Recommendations 2001.

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 YH or XH+ group or with an XR 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 relationships are established involving the sign of lone-pair effects in coupling constants between nuclei which are a different number of bonds apart.

Effect of electron lone-pairs on nuclear spin–spin coupling constants

Publisher Summary This chapter reviews that the impact of 119Sn nuclear magnetic resonance (NMR) parameters, determined either by 119Sn NMR or in the NMR spectra of other nuclei, on tin chemistry is remarkable. Nowadays, tin chemical shifts coupling constants and nuclear spin relaxation, with respect to the physical principles, is growing. At the same time, the application of more or less sophisticated NMR techniques enables one to monitor the reactions, identify the unstable intermediates, discover their relevance for mechanistic implications, assign the structural features, investigate the intra- and intermolecular dynamic processes, or to compare the structures in the solid and liquid state. 119Sn NMR parameters, in addition to those of the more common nuclei, for example, 1H, 13C, or 31P, add to the attempts at a more complete description of the bonding situation. The chapter suggests that it must be emphasized that the great potential of modern NMR spectrometers can be exploited by a multinuclear approach to obtain a large self-consistent set of NMR data. This also includes the isotope induced chemical shifts and long-range coupling constants that have been neglected in the past.

Application of 119Sn NMR Parameters

Great advances have been made in the past decade in the field of NMR spectroscopy. Apart from the development of completely new areas of application, such as in solid-state chemistry, in materials science, in physiological chemistry, and in medicine, with the introduction of new pulse spectroscopic methods and the application of high magnetic field strengths important progress has also been made in the traditional field of high-resolution NMR spectroscopy. Thus, among other things, the observation of metal resonances has been facilitated and new areas of application have been opened up in inorganic and organometallic chemistry. In this review, recent detection methods for spin-1/2 and quadrupolar metal nuclei are presented and discussed. The use of metal-NMR spectroscopy with respect to problems of a typical chemical nature, mainly from the field of organometallics, is demonstrated for a number of selected metal nuclei (25Mg, 27Al, 49Ti, 57Fe, 59Co, 61Ni, 91Zr, 103Rh, and 195Pt). Relations found empirically between chemical shifts and coordination number, oxidation number, and electronic configuration of a metal bound in a complex are emphasized. Furthermore, cases in which the chemical shifts of metal nuclei can be interpreted in terms of the energy difference of frontier orbitals are presented. This aspect leads to the establishment of a relationship between chemical reactivity and NMR parameters for a series of related compounds.

High‐Resolution Metal NMR Spectroscopy of Organometallic Compounds [New Analytical Methods (30)]

An Addendum has been published for this article in Magnetic Resonance in Chemistry 40(9) 2002, 622.

A unified scale is recommended for reporting the NMR chemical shifts of all nuclei relative to the 1H resonance of tetramethylsilane. The unified scale is designed to provide a precise ratio, Ξ, of the resonance frequency of a given nuclide to that of the primary reference, the 1H resonance of tetramethylsilane (TMS) in dilute solution (volume fraction, ϕ < 1%) in chloroform. Referencing procedures are discussed, including matters of practical application of the unified scale. Special attention is paid to recommended reference samples, and values of Ξ for secondary references on the unified scale are listed, many of which are the results of new measurements. Some earlier recommendations relating to the reporting of chemical shifts are endorsed. The chemical shift, δ, is redefined to avoid previous ambiguities but to leave practical usage unchanged. Relations between the unified scale and recently published recommendations for referencing in aqueous solutions (for specific use in biochemical work) are discussed, as well as the special effects of working in the solid state with magic-angle spinning. In all, nine new recommendations relating to chemical shifts are made. Standardized nuclear spin data are also presented in tabular form for the stable (and some unstable) isotopes of all elements with non-zero quantum numbers. The information given includes quantum numbers, isotopic abundances, magnetic moments, magnetogyric ratios and receptivities, together with quadrupole moments and linewidth factors (where appropriate). Copyright 2001 IUPAC. Reprinted with permission from Pure Appl. Chem. 2001; 73: 1795.

NMR nomenclature: nuclear spin properties and conventions for chemical shifts. IUPAC Recommendations 2001. International Union of Pure and Applied Chemistry. Physical Chemistry Division. Commission on Molecular Structure and Spectroscopy

Twenty-two organo-phosphorus compounds of a variety of structural types have been examined by $^1$H-{$^{13}$C} and $^1$H-{$^{31}$P} magnetic double resonance spectroscopy. The signs and magnitudes of the $^{31}$P-H and $^{31}$P-$^{13}$C spin-spin coupling constants are sensitive to the valency of the phosphorus atom, and the nature of the groups attached to it. Parallel behaviour is noted between two types of coupling constant. The $^{31}$P chemical shifts agree with results obtained by conventional $^{31}$P single resonance spectroscopy, and the $^{13}$C chemical shifts depend on the polarizability of the phosphorus atom and its associated groups.

W. McFarlane

Papers

A $^{13}$ and $^{31}$P Magnetic Double Resonance Study of Organo-Phosphorus Compounds