Nuclear Magnetic Resonance Spectroscopy

1 Introduction

The third year of our review has seen a further substantial increase in the application of n.m.r. to inorganic chemistry, the number of papers this year having increased some 21% over the number reported in 1968, and having nearly doubled in the three years. At the same time it has been necessary to limit any increase in the size of the chapter. This has inevitably meant that the writing has had to be compressed and this has been achieved by increasing the proportion of references assigned to tables at the end of the chapter. These tables in their turn have been presented in a more compact form, almost all structural formulae having been omitted. This unfortunately detracts somewhat from their readability but it is hoped that this will be compensated to some extent by the convenience of having the compounds recorded rather than deleted from the review.

Proton n.m.r. as usual provides the bulk of the experimental information but a considerable array of other nuclei have also been investigated and references will be found in which direct use has been made of the following nuclei: 2D, 7Li, 10B, 11B, 13C, 14N, 15N, 17O, 19F, 23Na, 27Al, 31P, 33S, 35Cl, 53Cr, 55Mn, 71Ga, 81Br, 127I and 133Cs. Of these, 35Cl has had perhaps a surprising amount of attention and has given in return considerable information.

The subject matter is arranged in a similar manner to last year, though the section on adducts and solvent effects has been omitted, most work on these topics being now found in the Group III section. As before we begin with the technique-oriented sections and follow with those dealing with particular elements. There is inevitably some overlap between sections, e.g. paramagnetic ions are found in the section on ionic solutions and in the section on contact shifts; the solution chemistry of aluminium is discussed extensively in the former section but many references also appear in the section on Group III elements. In general, papers were placed where the most coherent n.m.r. picture would result and the order within sections and subsections follows the order of vertical groups of the Periodic Table, main groups usually coming first. Although this chapter has been written so as to emphasise the application of n.m.r. to each problem, it should be borne in mind that other techniques frequently play an important part and all of these should be considered in a balanced appraisal of the work.

Several books and reviews have appeared during the year. Volume 2 of 'Annual Reviews of N.M.R. Spectroscopy' includes articles on, 'Solvent effects in proton magnetic resonance spectroscopy' by J. Ronayne and D. H. Williams, 'Nitrogen magnetic resonance spectroscopy' and 'Carbon-13 n.m.r. chemical shifts and coupling constants' both by E. F. Mooney and P. H. Winson, 'Boron-11 n.m.r. spectroscopy' by W. G. Henderson and E. F. Mooney, and 'Phosphorus-31 n.m.r. spectra of co-ordination compounds' by J. F. Nixon and A. Pidcock. Volume 5 of 'Progress in N.M.R. Spectroscopy' contains articles on 'The INDOR technique in high resolution n.m.r. spectroscopy' by V. J. Kowalewski, 'Magnetic non-equivalence related to symmetry considerations and restricted molecular motion' by T. H. Siddall and W. E. Stewart, and 'Applications of H-1 n.m.r. spectroscopy to the conformational analysis of cyclic compounds' by H. Booth.

Of textbooks, the 'N.M.R. of Boron Hydrides', which mainly consists of a review of the topic, is of particular relevance to this review though it is expensive, while R. M. Lynden-Bell and R. K. Harris have provided a readable introduction to the physicochemical basis of the subject. The second edition of L. M. Jackman's well known book has also just been issued, and, while this is written specifically for the organic chemist and with a non-mathematical approach, it contains a very large amount of data and gives a very full and up-to-date account of factors affecting chemical shifts and coupling constants in organic substances. Two treatments of n.m.r. at intermediate levels have been published' and the proceedings of the Brighton Conference on Molecular Spectroscopy are now available.

Reviews have also appeared on 'Nuclear spin–spin coupling between directly bound elements' in which the theory of this coupling is discussed; on 'Magnetic double resonance techniques in chemistry'; on 'A survey of various methods currently used for analysis of n.m.r. spectra'; on 'Correlation of interproton spin-spin coupling constants with structures'; on 'N.M.R. at high magnetic fields'; on 'Organometallic amines and imines', including n.m.r. of elements of Groups IV, V, and VI bonded to Me3Sn — N groups; and on 'Organo-germyl, -stannyl, and -plumbyl phosphines, arsines, stibines, and bismuthines', which includes some n.m.r. data.

Four new journals have also appeared during this and last year. These are 'Journal of Magnetic Resonance', Vol. 1, 1969, published by Academic Press, edited by W. S. Brey, and which will prove of particular interest to n.m.r. specialists; 'Chemical Instrumentation' which carries a few n.m.r. papers; 'Spectroscopy Letters', Vol. 1, 1968, and 'Organic Magnetic Resonance', Vol. 1, 1969/70, published by Heyden & Son, edited by E. F. Mooney. To the reviewer's consternation and despite its title, the last journal contained papers of relevance to this review. Its first issue also contains a note on the presentation of n.m.r. data. It is proposed that, based on a consideration of frequency, there is a justifiable basis for presenting all low field shifts as positive.

Instrumental Techniques. — A number of miscellaneous papers of general interest have appeared in the inorganic literature. The way in which the crossed-coil n.m.r. spectrometer works has caused some discussion. Two papers, one extensive, show how to measure the relaxation times of individual lines of high resolution spectra and a method for following slow relaxation processes of individual nuclei is described.

Several notes deal with standardisation. A new cyclosilane [— CH2 — Si(CD3)2 —]3 with [delta;] = -0.327 has been produced commercially; it is claimed to be ideal for high-temperature work while a water-soluble deuteriated salt (Me3Si=CD2CD2CO2N2) with δ = 0.00, which has only one proton resonance, is more soluble than Tiers salt, and can also be used up to 200 °C, has been made. The single fluorine resonance of CCl3F has been separated into three components at -80 °C arising from molecules containing different isotopic abundances of 35Cl and 37Cl and its suitability as a fluorine standard is questioned.

The importance of making bulk volume susceptibility corrections when using an external...