Nuclear Magnetic Resonance Spectroscopy

BY B. E. MANN

1 Introduction

The growth in papers containing n.m.r. data on inorganic and organometallic compounds has continued with the result that only papers containing information on nuclei other than H, or papers wherein H n.m.r. spectroscopy makes a significant contribution, are included. A more selective approach has also been applied within the area of solid-state n.m.r. spectroscopy, and papers which are considered to be principally of interest to physicists have been omitted. As usual, no attempt is made to cover n.m.r. spectroscopy in depth, and any reader who requires such an approach is referred to the excellent Specialist Periodical Report on 'Nuclear Magnetic Resonance'.

Volume 8 of 'Advances in Magnetic Resonance' has appeared and contains chapters on 'The Theory of Chemically Induced Dynamic Spin Polarization', by H. H. Freed and J. B. Pedersen, and 'N.M.R. in Flowing Systems', by D. W. Jones and T. F. Child. In addition, the first supplement to this series on 'High Resolution N.M.R. in Solids, Selective Averaging', by U. Haeberlen has appeared. Volume 6B of 'Annual Reports on N.M.R. Spectroscopy' contains chapters on 'N.M.R. and Conformations of Amino Acids, Peptides, and Proteins', by W. A. Thomas, and 'Fluorine-19 N.M.R. Spectroscopy', by L. Cavalli. The series 'N.M.R., Basic Principles and Progress' has proved to be prolific with books on 'Van der Waals Forces and Shielding Effects', by F. H. A. Rummens, 'High Resolution N.M.R. Spectroscopy in Solids', by M. Mehring,'Chlorine, Bromine, and Iodine N.M.R. Physico-chemical and Biological Applications', by B. Lindman and S. Forsén, and 'Introductory Essays', edited by M. M. Pintar, containing 11 essays on various n.m.r. topics, none of which is of direct relevance to this chapter. 'Progress in Nuclear Magnetic Resonance Spectroscopy' has produced a review on 'Spin-Spin Coupling and the Con-formational States of Peptide Systems', by V. F. Bystrov. The second volume of 'Topics in Carbon-13 N.M.R. Spectroscopy' has now appeared and contains a number of chapters on various aspects of 13C n.m.r. spectroscopy, and in particular 'Carbon-13 N.M.R. Studies of Organometallic and Transition Metal Complex Compounds', by O. A. Gansow and W. D. Vernon.

A number of books devoted to n.m.r. spectroscopy have been published, including 'Proton Nuclear Magnetic Resonance', by P. Joseph-Nathan; 'The Dynamics of Spectroscopic Transition: Illustrated by Magnetic Resonance and Laser Effects', by J. D. Macomber; 'Spectroscopic Methods for Determining the Structure of Organic Compounds. Section "Nuclear Magnetic Resonance Spectroscopy", by J. Prochorow; 'Magnetic Resonance', edited by C. A. McDowell, which contains chapters on 'High Resolution N.M.R. Studies on Solids', by E. R. Andrew, 'The Studies of Ionic Processes at Membranes by N.M.R. Methods', by L. W. Reeves, and 'Tunnelling Processes as Studied by Magnetic Resonance Techniques', by R. Srinivasan; 'Fourier Transform N.M.R. Spectroscopy', by D. Shaw; 'Use of Nuclear Magnetic Relaxation in the Analysis of Inorganic Compounds', by A. A. Popel; 'Interpretation of Carbon-13 Nuclear Magnetic Resonance Spectra', by F. Wehrli and T. Wirthlin; 'Spectroscopic Methods in Pharmacy, Vol. 2: Nuclear Magnetic Resonance Spectroscopy and Mass Spectrometry', by G. Ruecker; 'Magnetic Resonance of Biomolecules', by P. F. Knowles, D. Marsh, and H. W. E. Rattle; 'Nuclear Magnetic Resonance in Biological Research: Peptides and Proteins', by K. Wuethrich ; 'Nuclear Magnetic Resonance in Biochemistry', by T. L. James; a collection of lectures in 'Magnetic Resonance in Chemistry and Biology'; and 'Corso Teorico-Pratico di Resonanza Magnetica Nucleare', edited by A. Frigerio.

Several other books have appeared which contain sections or chapters devoted to n.m.r. spectroscopy. These books have articles on 'Physical Methods and Techniques; 11. Nuclear Magnetic Resonance', by R. B. Jones and L. Phillips; 'Nuclear Magnetic Resonance Spectroscopy', by I. O. Sutherland; 'N.M.R. of Nuclei other than Proton and Fluorine-19', by G. A. Webb; 'Correlations in Nuclear Magnetic Shielding, Part l', by J. Mason; 'Si Nuclear Magnetic Resonance', by J. Schraml and J. M. Bellama; 'The Nuclear Overhauser Effect', by J. K. Saunders and J. W. Easton; 'Molecular Structure by N.M.R. in Liquid Crystals', by L. Lunazzi ; and 'N.m.r. Parameters of the Proton Directly Bonded to Phosphorus'.

Once again numerous reviews have appeared. Some of the reviews are of n.m.r. spectroscopy in general. Other reviews cover 'High-resolution N.M.R. Method', 'N.M.R. as an Analytical Technique', 'Nuclear Magnetic Resonance as the Stereochemists' Tool', 'Determination of Molecular Conformation in Solution', 'Conformational Analysis using High-resolution N.M.R. Spectroscopy', 'Nuclear Magnetic Resonance Techniques for the Determination of the Structure of Molecules', 'Recent N.M.R. Applications in Organic Chemistry', 'Nuclear Magnetic Resonance at High Pressures', 'Basics of N.M.R. Spectroscopy', 'Practical Interpretations of N.M.R. Spectra', 'Pulsed N.M.R. Spectrometry', 'Proton N.M.R. Spectrometry', 'Analysis of N.M.R. Spectra', 'Allylic Interproton Spin-Spin Coupling', 'Influence of Solvents on Spectroscopy', 'Solute-Solute and Solute-Solvent Interactions: N.M.R. Studies', 'Forbidden Transition in Magnetic Resonance', 'Spin Relaxation in Fluids', 'Magnetic Relaxation', 'Reorientation and Dipole Relaxation', 'N.M.R. and the Periodic Table', 'The Application of Carbon-13 N.M.R. to Oil Chemistry. Part 2. Review of Recent Topics',7 'Carbon-13 N.M.R. Spectroscopy'," 'Carbon-13 Fourier Transform N.M.R. An Important New Analysis Tool', 'Applications of C and lH N.M.R. and I.R. Spectro-scopies in Structural and Quantitative Analysis', 'c N.M.R. Use in Organic Chemistry, Biochemistry, and Technology',6 'The Application of Carbon-13 N.M.R. Spectroscopic Techniques to Biological Problems', 'C N.M.R. Spectroscopy in Organic Chemistry', 'The Use of Alkali Metal N.M.R. in the Study of Solvation and Complexation of Alkali Metal Ions', 'Dynamic N.M.R. of Carbon', 'Unusually High Barriers to Rotation Involving the Tetrahedral Carbon Atom', 'Nuclear Magnetic Resonance Studies of Coordination Compounds', 'Nuclear Magnetic Resonance Spectroscopy of Porphyrins and Metalloporphyrins', 'Investigations of Hydrides of the Group IVa and Va Metals by Nuclear Magnetic Resonance', 'N.M.R. Spectroscopy of Iron-Sulphur Proteins', 'Apicophilicity and Ring Strain in Five-coordinate Phosphoranes', 'Organic and Inorganic Reaction Mechanisms Using Oxygen-17', 'Chemically Induced Dynamic Nuclear Polarization and Chemically Induced Dynamic Electron Polarization', 'N.M.R. in Drug Metabolism', 'N.M.R. in the Field of Biochemistry', and 'N.M.R. Spectroscopy in Biological Sciences', 'N.M.R. Spectroscopy in Biological Research', 'Use of N.M.R. for the Study of the Structure and Function of Enzymes', and 'The Calculation of NM.R. Lineshapes of Glasses'.

I.U.P.A.C. have published revised recommendations for the 'Presentation of N.M.R. data for publication in chemical journals - B. Conventions relating to spectra from nuclei other than protons'. All workers in this area are strongly urged to read these recommendations. Difficulties are commonly encountered as a result of the use of the wrong chemical shift sign convention and not defining the sign convention. These difficulties are particularly common for P n.m.r. spectroscopy where a 31P chemical shift of say δ10 can mean a shift to either high or low frequency (high frequency would be assumed) of an unknown reference, normally assumed to be 85% H3PO4. Similar difficulties can be encountered with other nuclei, especially 1H, 13C, and 19F. Throughout this chapter, authors permitting, the sign convention that high frequency is positive is adopted.

In addition to these books and reviews, papers have appeared which are too broadly based to fit into any of the later sections of this chapter and are included here. The valence bond treatment of the Fermi contact term for nucleus-nucleus coupling constants gave simplified expressions in which a constant term for each nucleus was introduced. The order of the bond separating the coupled nuclei was estimated. This approach was applied to long-range hydrogen-metal and carbon-metal coupling constants in organometallic compounds. 17O, 35Cl, 51V, 53Cr, 55Mn, and 95MO n.m.r. spectra have been determined for aqueous solutions of [FORMULA NOT REPRODUCIBLE IN ASCII] and [MoO4]2-, partly enriched in 70. The following coupling constants were determined: [FORMULA NOT REPRODUCIBLE IN ASCII]. A linear dependence of the reduced coupling constant on the atomic number of the central nucleus in the isoelectric series [FORMULA NOT REPRODUCIBLE IN ASCII] and [MnO4]- was found. The mutual influence of ligands in main-group element co-ordination compounds ALn has been analysed on the basis of the orbitally deficient scheme of the structure of these compounds. It was shown that a decrease of the A-19F bond strength can be accompanied by an increase of [FORMULA NOT REPRODUCIBLE IN ASCII]. A statistical method has been suggested to analyse the temperature-dependent n.m.r. relaxation in order to determine structural and kinetic parameters of metal ion complexes in solution. F Solvate isotope shifts of several inorganic fluoro-complexes of Group II, III, IV, V, and VI metals have been carried out. Isotope shifts were compared with the effective charge and their distribution in fluoride-containing anions. The possibility of using F solvate isotope shifts to establish unfamiliar complex anions and their hydration was discussed. The 19F n.m.r. spectra of m- and [FORMULA NOT REPRODUCIBLE IN ASCII] derivatives show that 19F shielding is a useful probe of [FORMULA NOT REPRODUCIBLE IN ASCII] π-bonding effects in ArN2] complexes as long as the compound is confined to complexes of the same structural type (singly-bent ArN2]+ in the present case). For the complexes studied a total range of 10 p.p.m. was recorded for (δp - δm] the shielding parameters becoming progressively more positive as conjugative transfer of electron density along the [FORMULA NOT REPRODUCIBLE IN ASCII] chain decreased. Arranging complexes in order of increasing δp - δmgives a sequence which is in excellent agreement with the ranking of [FORMULA NOT REPRODUCIBLE IN ASCII] π-bonding anticipated on the basis of relative donor and acceptor powers of co-ligands and the formal oxidation state of the metal atom. The 19F n.m.r. technique was extended to pprovide the first unambiguous evidence that the [ArN2] + ligand is a poorer π-acceptor than [NO]+. The 13C n.m.r. spectra of several metal enolates have been examined in various aprotic solvents and compared with those of enol acetates and Me3SiO ethers. The chemical shift difference between enolates and enolacetates appears to be related both to the π-electron density and reactivity at the enolate π-carbon atom. Diamagnetic metal complexes of propane 1,3-diamine-NNN'N'-tetra-acetate ([pyta]4-) have been studied by 3C n.m.r. spectroscopy. The results confirm previous studies in showing that complexes in which the ligand is quinquedentate are not formed (except possibly with HgII) but also show that this is due not to decreased ring strain in the six-co-ordinate complex but rather to steric compression of the glycinate chelate arms arising from the bunched backbone methylene groups. At low pH the complex formed between [pyta4- and AlIII is more inert than the analogous [edta]4- complex despite the lower equilibrium formation constant, and this is in line with the reduced tendency of [pyta]4- to form complexes in which the ligand is quinquedentate.

2 Stereochemistry

This section is subdivided into ten parts which contain n.m.r. information about hydrogen, lithium, sodium, beryllium, magnesium, and transition-metal complexes, presented by Groups, according to the Periodic Table. Within each Group classification is by ligand type. As far as possible, cross- references are given at the beginning of each sub-group to compounds discussed elsewhere in this chapter. In this cross- referencing, it has not proved possible within the space available to include the many compounds that occur within the sections on dynamic systems, paramagnetic systems, and solid-state n.m.r. spectroscopy. Thus many more compounds of relevance to this section appear elsewhere.

Complexes of Group I and II Elements. — Information on [FORMULA NOT REPRODUCIBLE IN ASCII] is given later (ref. 1136). A review entitled 'Nuclear Magnetic Resonance and its Use in the Study of Chlorophylls' has appeared.

The contact contribution to [FORMULA NOT REPRODUCIBLE IN ASCII] in HD has been calculated to be 39 Hz, cf. 42.94 Hz (experimental). 13C N.m.r. spectroscopy and [FORMULA NOT REPRODUCIBLE IN ASCII] have been used to investigate α-lithio-sulphoxides and sulphones. CNDO/2 Calculations have been performed on (Me2O)2Li (pentadienyl), and the calculated charge densities when compared with 13C n.m.r. data gave qualitative agreement. The optical pumping method of alkali molecules by atom-molecule exchange collisions has been applied to obtain the magnetic shielding difference [FORMULA NOT REPRODUCIBLE IN ASCII] between sodium atoms and Na2 molecules, and the scalar nuclear spin-spin coupling constant [FORMULA NOT REPRODUCIBLE IN ASCII]. Magnetic quantum transitions for spin 3/2 nuclei in n.m.r. spectra in lyotropic liquid crystals have been observed for 23Na+. The magnetic shielding of 133Cs in Cs+, Cs2, and free atoms has been determined. [FORMULA NOT REPRODUCIBLE IN ASCII] and [FORMULA NOT REPRODUCIBLE IN ASCII] have been reported for [FORMULA NOT REPRODUCIBLE IN ASCII]and related molecules. The results indicate that neither the 13C chemical shift nor [FORMULA NOT REPRODUCIBLE IN ASCII] may be used as a criterion for assigning a π or σ-bonded structure. For [FORMULA NOT REPRODUCIBLE IN ASCII]. 1H Spin-lattice relaxation measurements have been used to assign signals of metalloporphyrin and chlorophyll n.m.r. spectra. 1H and C n.m.r. spectroscopies have been used to demonstrate that aggregation of MgII porphyrins involves the magnesium in one molecule and the propionate carbonyl in another, whereas in zinc porphyrins, aggregation is independent of the side-chains. A similar 13C n.m.r. study of the relative donor properties of the three carbonyl groups of chlorophyll a has provided evidence that the donor-acceptor interactions that bind the chlorophyll dimer together involve a substantial participation by the ring V keto-carbonyl and minimal participation by the Z ester carbonyl groups.

Complexes of Sc, Y, U, Ti, and Zr. — Information concerning complexes of these elements can be found at the following sources: [FORMULA NOT REPRODUCIBLE IN ASCII](M = Ti, Zr, or Hf), [(η5-cycloheptadienyl)(η7 -cycloheptatrienyl)Ti], and [Ti(tetra-phenylporphyrin)O].

The 1H and 13C n.m.r. spectra of (1; M = Sc or Y) have been measured. For [FORMULA NOT REPRODUCIBLE IN ASCII], and [FORMULA NOT REPRODUCIBLE IN ASCII]. For the [FORMULA NOT REPRODUCIBLE IN ASCII] group, the first [FORMULA NOT REPRODUCIBLE IN ASCII] was observed. The 100 MHz n.m.r. spectra of Sc, T1, and Pb octaethylporphyrin and of a μ-oxo dimer of Sc octaethylporphyrin show the methylene protons of the ethyl side-chains to be anisochronous. The 100 and 300 MHz n.m.r. spectra of TI aetioporphyrin show that the methylene protons of the four ethyl side-groups of this porphyrin are also anisochronous. It was suggested that these phenomena result from the inherent asymmetry of metalloporphyrins rather than from hindered rotation of the side-chains. 19F N.m.r. spectra have been reported for [FORMULA NOT REPRODUCIBLE IN ASCII]and [FORMULA NOT REPRODUCIBLE IN ASCII].

The 1H and 13C n.m.r. spectra of [FORMULA NOT REPRODUCIBLE IN ASCII] show [FORMULA NOT REPRODUCIBLE IN ASCII]. The Lewis acidity of RTi(OCMe3)3 has been related to the 1H shifts of the methyl group and attributed to π-bonding. The 13C chemical shifts of a large series of cyclopentadienyl derivatives of titanium and iron have been measured. Substituent- and trans-effects were investigated. The 13C n.m.r. spectra of [FORMULA NOT REPRODUCIBLE IN ASCII] have yielded information on the charge on the carbon atoms. For M = Cr, the carbon atoms of the five-membered ring are more shielded than those of the seven-membered ring. For M = Ti, the reverse is true. Chemical shift considerations make it clear that for M = Cr, the carbon atoms of the five-membered rings are more negatively charged than those of the seven-membered ring. For M = Ti, the highest negative charge is on the carbon atoms of the seven-membered ring. The replacement of the 3d metal by the corresponding 4d metal causes a high-frequency shift for the signals of the carbon atoms of the five-membered ring and a low-frequency shift for the carbon atoms of the seven-membered ring. This results in an inversion of signals for M = Mo with respect to M = Cr. However, it was not clear in this conference abstract how the charge effect on chemical shifts was separated from all the other contributions. The solution N n.m.r. spectrum of [FORMULA NOT REPRODUCIBLE IN ASCII] supports the solid-state X-ray structure with two signals with [FORMULA NOT REPRODUCIBLE IN ASCII] and at δ = 160.4 a low-frequency singlet due to [FORMULA NOT REPRODUCIBLE IN ASCII].