CHAPTER 1
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 standard in variable-temperature work is emphasised by reference to an example where external standardisation gave trends opposite to those found with an internal standard. Conversely the use of internal and external standards has suggested there may be a specific interaction between solvent benzene and RB(pz)3,M(CO)2N:N[??] where M = Mo or W, R = H or pyrazolyl.
A means of measuring susceptibility using a glass sphere microcell is described.
The rather unusual sets of asymmetric spinning side-bands produced by coaxial sample tubes in linear field gradients are described. The effect can be used to compare the susceptibilities of liquids and to minimise linear field gradients. A comprehensive paper describes a computational method whereby a spectrum containing a considerable proportion of dispersion mode signal can be converted to exact absorption mode and some remarkable illustrations are given. The effect of instrumental time constant and recorder response on peak shapes is discussed, an n.m.r. method for determining water in crystalline NH4ClO4 is described, and a note has been published on the use of the small PDP8 computer for spectral accumulation.
Proton n.m.r. spectroscopy of edta complexes of lead has been used to determine the isotopic abundance of 207Pb in various samples of different origin. A method is described for obtaining the molarity of solutions of Grignard reagents using n.m.r. and an apparatus is described for filling n.m.r. tubes under vacuum.
Coupling Constants and Chemical Shifts. — Papers are reviewed here in which the primary consideration has been the determination of the magnitudes and sign of coupling constant, or of the magnitudes of chemical shifts, and the relation of these to bonding and electronic structure. Papers dealing primarily with coupling are dealt with first, then papers dealing primarily with chemical shifts and finally a section dealing with nitrogen resonances, which form a coherent group on their own.
(a) Coupling Constants. Coupling in Group IV compounds is considered first followed by Group V, Group VI, and organometallic complexes. Much of the data in this section have been obtained using the very elegant INDOR technique.
The electronegativity of the SiMe group has been estimated as 2.25 from the extent of variations of 3J(H — H) for ortho-ring protons of 1-(Me3Si)-4-X-C6H4, where X = D, NH2, OMe, NO2, or F. pπ-dπ bonding may occur if X is a strong π-electron donor. The value of 2J(C — C) is reported for 13C-enriched methyl ether [(–)2.4 Hz], acetone (+16.1 Hz), and Me2Hg (+22.4 Hz). These are large values considering the small value of the magnetogyric ratio of 13C.
Detailed calculations have been made on the magnitude of the ring currents in phthalocyanine rings. Two models were used, one with a single large loop and a second with four small loops above the phenyl rings and a fifth above the central part of the molecule. The loops were in pairs, above and below the plane of the molecule. The calculated effects were compared with experimental results for systems with Group IV central atoms and were found to agree well, especially the results calculated using the second more complex model. The magnitude of the effect was also calculated along the fourfold axis of the ring and, as far out as 10 Å, still has a value of 0.5 p.p.m.
J(Si — H) has been correlated with v(Si — M) for 27 organosilanes. The reactivity of the methyl radicals of Me4M, where M = Si, Ge, Sn, or Pb, has been measured and 1J(C — H) and 1K(M — H) are found to correlate with the activation energy and rate constants determined. 1K(119Sn — F) has been shown to be negative as predicted by Pople and Santry. The determination was made on (PhMe2CCH2)3SnF which is a non-ionic fluoride, the values obtained being, 1J(Sn — F) = 2298 Hz, 2J(Sn – H) = + 50.5 Hz (assumed positive), and 3J(F — H) = +5 Hz. All the coupling and shift parameters have been obtained for Me6Pb2 and it is noted that 1J(Pb — C) is surprisingly small (28 Hz), thus suggesting high s character of the lead — lead bond. The 207Pb resonance is high field of its value in Me2Pb.
The coupling constants have been calculated in a series of A2X4 molecules for the gauche-, trans-, and cis-forms (N2H4, N2F4, P2H4, and P2F4) and compared with experimental values with some success. The spectra of a number of cyclic tervalent phosphorus compounds of fixed orientation have been analysed and it was found that 2J(P — CH) depends upon the dihedral angle, α, between the planes defined by the P, C, H atoms, on the one hand, and the P — C bond and phosphorus lone pair axis, on the other. 2JJ varies from + 26 Hz (α = 0) to a minimum of -6 Hz (α = 110°) to zero (α = 180°). Similar conclusions have been reached, though in less detail, from consideration of molecules such as ClCH2CH2P(S)Cl2. Measurements on a number of phosphine-type molecules indicate that 3J(P — H) and 4J(P — H) are positive in para-substituted phenyl derivatives of trivalent phosphorus; if the substituent is fluorine 5J(P — F) is found to be negative. All couplings became more positive as the phosphorus valency increased. 7J(P — H) was measured by a triple resonance experiment for the compound p-MeC6H4CH2P+Ph3 and is negative.
The signs and magnitudes of the coupling between geminal protons on a phosphorus atom have been measured, using deuterium substitution. 2J(H — H) is about — 13.4 to — 12.5 Hz for RPH2 (R = Me, H, Ph, or CF3) where the bond angle is about 93 — 97° and is zero or slightly positive for the ions PH4+ and Me2PH2+ where the angle is larger. The value of + 35.1 Hz is obtained for H2PO2-. The methyl groups in Me2ECHMePh+ where E = PhN, PhP, S, or Se are non-equivalent but the shift difference is very dependent on solvent, concentration, and temperature. All the data have been reported for the molecule P*(— CH2 — O –)3P which has a fixed conformation. The values 2J(P* — H) = +9.3 Hz and 1J(P* — C) = — 24 Hz suggest p3 hybridisation of P+ with small interbond angles. Data are reported for Me4P+, Me3P, Ph2PXME3, and Ph3SiXMe3 where X = C, Si, or Sn, the compounds being isoelectronic with the phosphonium ions. 1X(Se — X) and 1K(P — X) are positive in the last two types of molecule whereas 1K(P — X) is negative in the phosphines. |1K| increases strongly with the atomic number of X.
The proton spectrum of (p-NO2C6H4O)P(O)(Me)(OCH2COC6H4-p-Y), where Y = H, Cl, Me, NO2, or OMe, has been studied as a function of solvent and temperature. The CH2 protons are equivalent in solvents of high dielectric constant (e.g. DMSO) but are non-equivalent and give an ABX spectrum in solvents of low dielectric constant (e.g. CDCl3). A two-conformer model is found to fit the results and the activation energy found for exchange is comparable to that found from the temperature dependence of the dielectric constant. The sign of 2J(P — C — F) may reverse between F2C=CFPCl2 and F2C=CFPF2. 2J(P — P)gem in the compounds F2PSPF2, Me2Ge(PH2)2, and Me3Si(PH2)2 changes by 25% as the temperature is changed from 0 to -120 °C and may indicate changes in bond angle or conformation.
1J(P — H) has been measured for thirteen compounds [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] obtained by adding R3P to concentrated sulphuric acid. The value varies from 457 Hz (R = C4H9) to 870 Hz (R = PhO). The change is attributed to the inductive effect of R which alters the effective nuclear charge, and this mechanism is also suggested as the source of variation of 1J(P — W) in R3PW(CO)5. 1J(P — W) in these complexes has been found to correlate linearly with the electronegativity of the phosphine, leading to essentially the same conclusions. Data are also given elsewhere on the correlation between 1J(W — P) and the E mode carbonyl stretching frequency. Analysis of [AXn]3 spin systems has been discussed in detail with reference to the molecules N3P3(NMe2)6 and cis-Mo(CO)3(PF3)3.
Data are reported for the Group VI compounds Me2Se2, PhSe2Me, and CH2CH2CH2Se. The latter gives a complex spectrum since rapid ring flexing does not cause the cis- and trans-coupling constants to become equal. The change of 136 Hz in 1J(P — Se) in going from PIII to PV in Me2PSeMe and Me2P(S)SeMe is accompanied by 77Se chemical shifts of 138 ppm and may arise mainly because of changes at Se. The contributing structures [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] are suggested. Extensive coupling data are given for the series of compounds Me2O, Me2S, Me2Te, Me2Se2, Me2Te2, MeSeTeMe, (SiH3)2S, (SiH3)2Se, MeOH, MeSeH, MeSH, and HSeD, and trends associated with the changes in atomic number of the Group VI atoms noted. 1K(Te — F) is negative in Et2NTeF5, as had been predicted.
Data have been obtained on a number of transition-metal complexes in addition to those already mentioned for tungsten and molybdenum. Coupling between the ring protons and 51V is thought to have been observed at high temperatures for the complex (π-C7H7)V(CO)3 and a value of quadrupole coupling constant (~2 MHz) and 2J(V — H) = 3 Hz calculated. The sign of 2J(P — P) has now been determined for the cis- and trans-isomers of (OC)4Mo[P(OMe)3]2; C double resonance was used and it was found that 2J is negative (-40-5 Hz) and 2Jtrans is positive (+ 162 Hz). Data have been obtained for the complexes (C5H5)Mn(CO)2L where L is a large range of mainly cyclic olefin ligands. A good correlation was obtained between the cyclopentadienyl chemical shift and k(C [equivalent to] O]).
The long-range proton–carbon coupling constants in the cyclobutane complex (1) indicate that the bond orders of the four ring bonds are the same. The MeCO group produces no real change in the ring when compared with the unsubstituted complex. The AA'BB' spectra of compounds (2) — (5) where R = Fe(CO)3 and the parent hydrocarbons have been analysed. Changes in J(AB) upon complexing for (2) and (3) can be explained by changes which occur in the A — B bond length while for (4) some bond fixation in the phenyl ring is indicated.
