Nuclear Magnetic Resonance Spectroscopy

BY B. E. MANN

1 Introduction

Following the criteria established in earlier volumes, only books and reviews directly relevant to this chapter are included, and the reader who requires a complete list is referred to the Specialist Periodical Report 'Nuclear Magnetic Resonance',' where a complete list of books and reviews is given. Reviews that are of direct relevance to a section of this report are included in the beginning of that section rather than here. Papers where only 1H n.m.r. spectroscopy is used are only included when the 1H n.m.r. spectra make a non-routine contribution, but complete coverage of relevant papers is still attempted where nuclei other than the proton are involved.

Several relevant books have been published during 1979. The book 'Phosphorus-31 and Carbon-13 N.M.R of Transition Metal Phosphine Complexes' provides a welcome selective source of data. In addition, 'Nitrogen-15 Nuclear Magnetic Resonance Spectroscopy' and 'N.M.R. in Inorganic Fluorides: Structure and Chemical Bonding' have also appeared. 'N.M.R. Spectra; Its Application to the Organometallic Compounds' has also been reviewed.

A number of papers have been published that are too broadly based to fit into a later section and are included here. N.m.r. spectroscopy has been applied to condensed systems with asymmetric centres. The relaxation, including effects of chemical exchange, of spin-5/2 and -7/2 nuclei has been examined. An analytical perturbation extension of the Pople-Santry theory of the Fermi contact term in reduced spin-coupling constants, 1K(M-L), has been developed for the substituted compounds MLnL'm, where the ligands L have a valence ns orbital. It was found that changes in 1K(M–L) under substitution should typically follow changes in s-contribution to the E–L bond overlap population. Subsequently this treatment was applied to phosphorus(III) compounds and good agreement with experiment was found.

1H relaxation times have been measured for H2 in liquid neon, argon, and krypton. The results of coupled Hartree–Fock computations for the n.m.r. shielding constant have been reported for LiH, HF, and PH3. 13C n.m.r. spectra have been reported for diamagnetic cyano complexes in D2O. Except for (Fe(CN)6]4- the 13C resonance in all the cyano complexes examined is at lower frequency than uncomplexed |CN|- and the range of chemical shift spans 92.5 p.p.m. Chemical shift differences were discussed in terms of changes in σ and π-bonding of |CN|- to the metal ion. 1JMC was observed for [59Co(CN)6]3- (126 Hz), [103Rh(CN)6]3- (33.6 Hz), [195Pt(CN)6]2- (808 Hz), and [195Pt(CN)4]2- (1034 Hz). T1, values were also reported for selected compounds and interpreted as being consistent with scalar coupling to 14N.

The equivalent dipole model of the benzene ring current has been applied to both inter- and intra molecular geometries to give detailed conformational information in n.m.r. studies of metalloporphyrins. 13C chemical shifts have been measured for [SCN]-, [NCO]-, and [SeCN]- in a variety of transition-metal complexes. The effects of cation and solvent changes upon the chemical shifts of the ionic pseudo-halides were determined. Changes in hybridization are more important than electron excitation energy changes and substitution changes in determining chemical shifts for both ionic and complexed species. The ordering of the ions in terms of increasing shielding is [SCN]-< [OCN]-< [SeCN|-. The ordering of [SCN]- and [SeCN]- in terms of increasing shielding as a function of bonding mode is M-NCS < [NCS]-< M-SCN. Unsupervised 13C n.m.r. pattern recognition has permitted the qualitative classification of NN-dialkyldithio-carbamate complexes into five classes: free ligands, complexes of main-group elements, and complexes of transition metals, with the latter two groups being divided by a free oxidation number of 0.5 or greater than 0.5. Within the transition-metal complexes metals with electron configuration d6 or d8 and d0 or d]10 can be further distinguished. The class of main-group elements is subdivided into organic dithiocarbamates and a subclass of main-group element complexes. Changes of 31P chemical shifts during co-ordination of phosphoryl compounds have been discussed.

The 17O n.m.r. shifts of some metal carbonyls are 400 to 300 p.p.m. to high frequency from 17OH2. The δ(17O) trends are generally opposite to those for the carbon monoxide 13C chemical shifts and this is explained by metal π-back-bonding in carbon monoxide π*-orbitals. A metal-triad effect is observed for 17O chemical shifts. There is a low-frequency shift of δ(17O) on descending a given group in the Periodic Table. Over 100 17O n.m.r. chemical shifts have been reported for 27 diamagnetic polyoxoanions of the early transition metals. Efficient procedures for obtaining 17O-enriched compounds were described and the factors controlling sensitivity and spectral resolution are examined and discussed in detail. Comparison of chemical-shift values with structural data shows that chemical shifts are determined largely by metal-oxygen bond strengths. As complexes of 3-mesitylacetylacetone show very small chemical-shift differences between their o- and p-methyl signals, 0.11 to 0.23 p.p.m., it was concluded that they do not possess magnetic anisotropies in the chelate rings comparable with those of benzene. The nature of the ligand L and the position of the resonance lines of non-equivalent fluorine atoms in the 19F n.m.r. spectra of pseudo-octahedral fluoro complexes have been reviewed.

2 Stereochemistry

This section is subdivided into ten parts containing n.m.r. information about lithium, magnesium, and transition-metal complexes, presented by Groups, according to the Periodic Table. Within each Group classification is by ligand type.

Complexes of Groups IA and IIA. — 1H n.m.r. spectroscopy has been used to determine the concentration of n-butyl-lithium. In the anionic polymerization of 1-phenyl-1,3-butadiene the propagating species was shown by 1H and 13C n.m.r. spectra to be the 4,1-anion. The 13C n.m.r. spectra of (1) and related compounds have been reported, while for LiCBr31J(13C, 7Li) is 43 Hz. Low-temperature 13C n.m.r. spectra of some lithiated chiral oxazolines show the presence of two isomers. 1H n.m.r. spectra have been used to show chair conformations for complexes of sparteine and α-isosparteine with Li+.

From 1H n.m.r. spectroscopy it has been...