CHAPTER 1
Nuclear Magnetic Resonance Spectroscopy
By B. E. MANN
1 Introduction
This year no further reductions in literature coverage have been introduced, but this has the consequence that the treatment of each reference is even briefer than before. Once again, 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 Reports on 'N.M.R. Spectroscopy', where a complete list of books and reviews is given. Reviews which 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.
Volume 6C of 'Annual Reports on N.M.R. Spectroscopy' has finally appeared, and is devoted to a referenced list of organic-metal carbonyl complexes that have had n.m.r. data reported on them during the period 1965 — 71. The book 'N.M.R. and the Periodic Table' is a comprehensive discussion of n.m.r. data on all nuclei other than 1H, 11B, 13C, 14N, 15N, 19F, and 31P up to the end of 1977. In addition to these books a number of reviews of direct relevance to this chapter have appeared, i.e. 'N.M.R. Spectra of Liquid Crystalline Solutions: A Route to Molecular Structures', 'Spectroscopy of Molten Salts', 'Multi-nuclear N.M.R. and the Preparative Coordination Chemist', 'C.I.D.N.P. from Bimolecular Reactions of Organometallic Compounds', 'Deuterium Magnetic Resonance, Applications in Chemistry, Physics, and Biology', 'Deuterium, Nitrogen-15, and Oxygen-17 N.M R and their applications', '17O N M.R. Spectroscopy as a Structural Probe', 'Fluorine-19 N.M.R. Spectroscopy of Fluoroalkyl and Fluoroaryl Derivatives of Transition Metals', 'Use of a Fluorine-19 N.M.R. Method for Studying Fluoro Complexes of d0-Transition Metals', 'Sodium-23 N.M.R. Spectroscopy', 'Alkali Metal, Magnesium-25, and Silver-109 N.M.R. Studies of Complex Compounds in Nonaqueous Solvents', and a review of tertiary phosphine ligands in co-ordination and organo-metallic chemistry includes the use of P n.m.r. data, especially 1J(M, 31P).
A number of papers have been published which are too broadly based to fit into a later section and are included here. The 1H n.m.r. shift for a given ring proton of cyclopentadienyl, benzene, and cycloheptatrienyl complexes relative to that for the appropriate free ligand shows a good linear correlation with the corresponding change in charge density at that proton with an intercept of 2.5 p.p.m. This intercept was attributed to an appreciable diminution in aromatic character of ligand rings on complex formation. 13C Chemical shifts for a variety of [M(CN)5X]n- complexes have been reported. δ(13C) Correlates extremely well with v (M — C) and v(C — N), and in the case of the iron complexes with Mössbauer isomer shifts and quadrupole splitting values. The 13C n.m.r. spectra of M(acac)n (M = H, Li, Na, K, Tl, Be, Mg, Ca, Cd, Al, Pt, Pd, Zn, In, Sc, Co, Zr, or Hf) have been recorded and the signals move to high frequency. J(195Pt, 13C) was recorded for Pt(acac)2. Be, Mg, Ca, Ba, and Zn β-diketonates have been investigated by 1H n.m.r. spectroscopy. Mg-, Ca-, and Ba-(acac)2 are polymeric while Mg- and Zn-(dpm)2 and the PhCHCOCHMe derivatives are monomeric. The 77Se n.m.r. spectra of twenty-nine selenium-containing compounds have been reported. The Se chemical shifts cover a range of ca. 100 p.p.m. In dialkyldiselenocarbamato metal complexes, the magnetic aniso-tropy associated with da8 nickel triad complexes contributes significantly to the 77Se chemical shielding, giving rise to upfield shifts with respect to the anionic ligands and zinc and cadmium complexes. Electronic effects arising from the ligand also significantly contribute to the shielding. Solvent temperature and concentration dependence studies were also carried out on a few of the diseleno-carbamate complexes. Both J(77Se, 31P) and J(195Pt, 77Se) and the n.m.r. trans influence argument were used in making peak assignments; the T1 range is 0.46 — 5 s. With Zn(Se2 CNEt2)2 and Pd(Se2CNBu12), the chemical shift anisotropy appears to be the dominant relaxation mechanism for the Se nuclei. An approximate formula for the second-order perturbation energy previously obtained has been used to calculate the geminal spin-spin coupling constants in ABn type molecular systems. The signs of cis- and trans-geminal coupling constants have been discussed for AB6-type molecular systems. The dependence of the nature of the A — Xi bond in octahedral and square planar complexes AX1X2 ... Xn - 11L on the properties of a variable ligand L has been analysed and used to interpret more than 600 experimental i.r., n.m.r., and n.q.r. spectral data. The use of 19F chemical shifts in the evaluation of the mutual effects of ligands in six-co-ordinate complexes has been examined.
2 Stereochemistry
This section is subdivided into ten parts which contain n.m.r. information about lithium, sodium, potassium, rubidium, caesium, magnesium, calcium, strontium, barium, and transition-metal complexes, presented by Groups according to the Periodic Table. Within each Group, classification is by ligand type.
Complexes of Group IA and IIA. — The n.m.r. frequency of 113Cs+ ions along with the Zeeman transition frequency of 113Cs has been measured in a magnetic field of ca. 0.06 T. This gave the ratio of the nuclear g factor of the 133Cs+ ion to the electronic g factor in the ground electronic state of free 133Cs. Using this value and results of other researchers' calculations yield g1 (133Cs+)/g1(133Cs) = 0.9999917(21) as the ratio of the nuclear g factor of the Cs ions to that of the free Cs atoms; σ(Cs) - σ(Cs) = 7.7 (2.0) x 10-6, the difference in the dia-magnetic shielding constants of the free atoms and ions. 6Li, 7Li, and 13C T1 and n.O.e. data have been reported for MeLi, BunLi, and PhLi. 6Li T1 values are in the order of tens of seconds and their relaxation is two — three orders of magnitude less than that for 7Li. 6Li is substantially relaxed by the intramolecular 6Li — 1H mechanism whereas both quadrupolar and 6Li — 7Li dipolar relaxation are minor processes. The non-linearity of the Arrhenius curve...
