Nuclear magnetic resonance spectrometry - Analytical Chemistry

John R. Wasson, and Peter J. Corvan. Anal. Chem. , 1978, 50 (5), pp 121–130 ... M. A. Evenson and G. D. Carmack. Analytical Chemistry 1979 51 (5), 3...
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ANALYTICAL CHEMISTRY, VOL. 50, NO.

5, APRIL 1978

121 I?

Nuclear Magnetic Resonance Spectrometry John R. Wasson" Ellestad Research Laboratories, Lithium Corporation of America, P.O. Box 795, Bessemer City, North Carolina 280 16

Peter J. Corvan Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 275 14

This review covers the published literature from July 1975 to July 1977 although a few references to other work are also included. (Photographs and biographies of the authors of this review appear in the Electron Spin Resonance review.) As noted previously ( I ) , thousands of papers containing information on NMR spectrometry are published in the two-year period covered by this review, and space limitations preclude citing more than just a few of the publications. The Chemical Society (London)Specialist Reports on NMR spectrometry are the best continuing series of comprehensive reviews on the subject and are recommended for literature searching a topic of recent vintage or finding applications to particular systems. Computer searching Chemical Abstracts and the relatively new C A Selects-Nuclear Magnetic Resonance (Chemical Aspects) published by Chemical Abstracts Service, Columbus, Ohio 43210 afford convenient approaches to current awareness of the NMR literature. Herein, an attempt is made to note those publications which capture the flavor of the development and applications of modern NMR spectrometry to chemical systems. Many superb contributions to NMR spectrometry are not cited because of the vastness of the literature surveyed, the idiosyncrasies of the selection process and the ever important necessity for journal space conservation. However, it is hoped that where this review fails as a review, it succeeds as a useful guide to current advances in NMR spectrometry.

BOOKS (2-30) AND REVIEWS The book edited by Ivin (12) contains useful reviews of hi h resolution 13C studies of polymer structure, analysis of 1 3 8 NMR relaxation experiments on polymers, 1 3 NMR ~ studies of a-methylstyrene-alkane copolymers and the characterization of diene polymers by high-resolution proton NMR. Aspects of NMR spectrometry relevant to problems in defining aromaticity have been discussed (13). The book edited by Pintar (11) contains a collection of discussions, mainly pertaining to relaxation, which are rather lucid. An ACS symposium volume (18) covers many applications of magnetic resonance to colloid and interface science. Useful reviews of 29SiNMR (30),the nuclear Overhauser effect (31) and molecular structure determination by NMR in liquid crystals (32) have been presented. Opella (33) has presented an overview of biological NMR spectrometry stressing the utility of other than PMR techniques. Table I list reviews of various aspects of NMR spectrometry. For convenience, the references in Table I, and the other tables, are collected separately in the bibliography.

APPARATUS AND TECHNIQUES DeMarco (34) has presented data on the pH dependence of internal references, DSS, and TSP, and introduced some cautions on their applications. CMR shifts of methyl iodide, CHJ2 and cyclooctane relative to neat TMS have been reported (35) over a range of temperatures, and solutions suitable for use as NMR thermometers have been described. The use of microsample probes in PMR and CMR has been discussed and illustrated (36). The determination of naturally abundant 13C chemical shifts in the gas phase has been detailed (37)and compared with extrapolation from liquid phase measurements. The sample tube was clearly described. A simple inexpensive circuit which digitally converts the fre0003-2700/78/0350-12lR$Ol .OO/O

quency of an NMR spectrometer to Gauss (38) and a hybrid computing circuit which uses digital logic and analog techniques to provide on-line processing of pulsed NMR signals to give relaxation measurements a t low cost (39) have been described. A simple one-step H F preamplifier for improvement of the normally achievable signal-to-noise ratio on a 270-MHz spectrometer has been reported (40). The circuit presented may be useful with small alterations as a preamplifier down to a few megaHertz. A broadband system for the observation of NMR spectra of any resonant nucleus has been described (41). A quadrupole coil for NMR spin-echo diffusion measurements as a function of pressure has been reported (42). Apparatus and techniques which permit the observation of high-resolution NMR spectra in flowing, chemically reacting systems have been described (43). Transient chemical species and effects with lifetimes of less than 1 s can be detected and their associated kinetic parameters measured. Arrangements for the measurement of flow UV and NMR spectra under the same conditions have also been detailed. The sensitivity of PMR or CMR spectral measurements has been substantially increased by using pulsed-rf FT methods (44).Double Fourier Transformation in high-resolution NMR (45) and a new technique claimed to totally eliminate systematic noise in FT NMR (46) have been described. A quantitative analysis technique using FT PMR has been reported (47) which entails only minor modifications of normal operatin conditions. Alternative methods for two-dimensional PC spin-echo spectroscopy have been put forth (48). Two- and three-dimensional images of objects may be generated from their NMR signals in magnetic field gradients. The zeugmatographic technique makes possible nondestructive chemical analysis of the interiors of objects (49). A modification of the C60 HL spectrometer probe for photo-CIDNP studies has been reported (50). A method has been described for the prediction of proton chemical shifts using off-resonance decoupled and gate decoupled CMR spectra (51). An approach t o obtaining 13C T1 and NOE data from PMR measurements on I3C-enrichedmolecules has been examined (52). A progressive saturation method has been proposed for the quantitative measurement of deuterium-labeled carbon atoms that does not require resolution of signals from corresponding carbons labeled to different extents and which optimizes the sensitivity vs. time limitations which are inherent in CMR (53). The assignment of long-range carbon-proton couplings through selective proton decoupling has been detailed (54). The techniques MINDOR (modified INDOR) and TINDOR (transferred INDOR) which can be used to present spectra obtained under conditions of nuclear magnetic triple resonance, and which are appropriate when one of the irradiated species is not spin coupled to the observed one, have been described (55) and applied t o a study of the 63Cushielding in the [ (CH30)3P]4Cucation. Bloch-Siegert shifts imposed on the 'H resonance frequencies in 13C-(1H1single-frequency off-resonance decoupling experiments have been indirectly triple-resonance techniques (56). observed by means of 13C-{1HJ A method for correcting chemical shifts of adsorbate molecules for adsorbent susceptibility has been described (57) as has an improved broadband NMR spectrometer scheme (58) which yields excellent results at frequencies from 4 to 51 MHz. The observation of natural abundance 13C chemical shifts and indirect 13C-'H spin-spin coupling in crystalline pivalic acid, (CH,),CCOOH, at ambient temperatures using a conventional 1978 American Chemical Society

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Table I. Reviews Subject

Ref.

NMR in flowing systems Theory of CIDNP CMR-organometallic and transition metal complexes NMR in enzyme research, especially, basic pancreatic trypsin inhibitor NMR in pharmaceutical analysis Polymer microstructures and CMR Biopolymers and CMR 13C-labeledcompounds in biosynthetic pathway research Solute-solute and solute-solvent interactions Organic structure assignments-CMR Theory of indirect nuclear spin-spin coupling constants Porphyrins and metalloporphyrins Use of hexafluoroacetone as an analytical tool in NMR spectrometry Determination of water in foods Viomycin and its derivatives Kondo effect Microstructure of polymer chains NMR and theoretical chemistry Hemoglobin Cyclic conjugated n-electron systems, CMR General Two-bond coupling between protons and carbon-13 Biosynthetic studies Inorganic and organometallic compounds Quantitative analyses Drug metabolites Nuclear shielding Superconducting state CMR-naturally occurring substances Phase transitions in RMX, and R,MX, compounds Photoproducts Hydrogen-bonded systems Peptides by INDOR, difference NMR and time-resolved double resonance Simple rotamers Alkaloids Mononucleotides, oligonucleotides, and t-RNA Membrane active peptides Surface analysis Testing of the theory of rubber elasticity Liquid crystals Alkali metal ions, solvation and complexation of alkali metal ions Domain structure of ferromagnets Pharmacy Compounds containing N-S--F bonds Fluorine coupling constants One- and two-dimensional systems Protons bound to phosphorus Transition metal dichalcogenides, spin waves Conformational analysis, alicyclic compounds Calculation of nuclear spin-spin coupling constants Fluorine-19 NMR Liquid semiconductors Proton spin imaging Lanthanide shift reagents Membranes Two-dimensional spectroscopy Hemes and hemoproteins, CMR Alkali naphthalene ion pairs Theory Chemical shift nonequivalence of prochiral groups Metal ions as probes in biochemistry Proteins Biochemistry Nuclei other than hydrogen Density matrix theory and lineshape calculations Through-space spin-spin coupling Phosphotransferases Line shapes in liquids Multiple resonance Spin-spin coupling Macromolecules Solids Medium effects Silicon-29 NMR

07A)

11a) OOA)

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Table I. (Continued) Ref.

Subject NMR at high pressures Spin-lattice relaxation, PMR Conformation analysis NMR and the periodic table Barriers t o rotation and inversion-theory Chemical applications of diamagnetism Fourier transform spectrometer has been reported (59). Increasing the pulse rate in WHH-type multiple pulse sequences results in qualitative improvement of the resolution in high-resolution PMR spectrometry of solids (60).

COMPUTER APPLICATIONS Herein we mention some developments of computer applications to NMR spectrometry. From the perspective of having reviewed this material for some years, it would appear that the time is about right for a monograph (or series) collecting and documenting various programs which have been developed as well as discussing hardware and software and interfacing problems. The rapid development of F T NMR in recent years has led to a plethora of additional information dispersed throughout the literature. A compendium of programs, etc., would be of genuine assistance to new and established investigators. An algorithm for simultaneous on-line computer analysis of multicomponent systems by quantitative NMR has been presented (61) along with the underlying theory. A crosscorrelation approach for sensitivity enhancement in quantitative NMR work has been proposed (62). A simple computational procedure has been described (63) by which it is possible to correct Fourier NMR spectra automatically for all amplitude and phase distortions resulting from signal-conditioning circuits and from anomalies in quadrature phase detection. The advantage of incorporating a complex interpolation program in the computer as a means of obtaining better-defined FT spectra has been pointed out (64). An iterative least-squares procedure for the lineshape fitting of high-resolution spectra making use of symmetry and magnetic equivalence factorization has been outlined (65). The program NMRCON handling up to six different chemical shifts was also described. The combination of group theoretical methods with a new computer-compatible representation of symmetryadapted functions yielded the program SYMTRY (66). The number and type of irreducible representations, the symmetry-adapted basis product functions, the energy level diagram and subspectra, all of which correlate with the corresponding symmetry species, are conveniently obtained. A general computer program which calculates NMR band shapes of spin-’/2 nuclei in spin systems containing quadrupolar nuclei (67) has been developed as has a FORTRAN IV program that interprets first-order PMR spectra in terms of molecular fragments and the probable molecular structure (68).

ANALYTICAL APPLICATIONS An egg-shaped microcell for CMR of trace level samples has been described (69). FT 31PNMR has been successfully employed as a trace analysis tool for the determination of inorganic phosphate pollutants in waste water (70). A NMR method for the quantitative analysis of the product of the transesterification process for the production of butyl acrylate has been developed (71). Difficulties encountered in obtaining accurate integral values for the individual signals owing to overlapping were overcome by setting up simultaneous linear equations. A simple, nondestructive CMR determination of the oil composition in individual plant seeds has been presented (72). The use of low-resolution NMR for determination of the hydrogen content of aviation turbine fuels is accurate, simple to apply, and requires a comparatively short time (73). The fraction of aromatic carbon in coal derivatives can be determined (74) from proton coupled CMR spectra. The pulsed NMR determination of hydrogen in coal samples using initial values of free induction decays has been reported (75). The approach may be adaptable to related systems. The dependence of the free-induction decay moments of the components enables the determination of the components by

(101A) (102Aj (103A) (113A) (115A) (116A)

pulsed NMR techniques (76). The NMR determination of the magnetic susceptibility of iron proteins has been described (77)-in such a fashion ihat it can be generally employed in the undergraduate laboratory. A straightforward and useful experimental technique for determining keto-enol tautomerism in /3-dicarbonyl compounds has been recommended (78) as an experiment in undergraduate physical organic chemistry and instrumental analysis courses. The use of NMR in qualitative and quantitative analysis has been discussed (79) with emphasis on analysis of pharmaceuticals, polysaccharides, and copolymers. An automated procedure for qualitative and quantitative analyses of mixtures by CMR has been reported (80)and applied to carbohydrate analysis. The simultaneous determination of vitamin D, isomers with the aid of an europium shift reagent has been described (81)as has a method for determining whether a graft polymerization yields a true graft polymer or only a mixture of polymers (82). Sequences in methyl methacrylate/methacrylic acid copolymers can be determined by PMR in different solvents and CMR (83). The use of FT PMR spectrometry to determine unsaturated structures in poly(viny1 chloride) (84) has been employed to partially explain PVC degradation. T z measurements for irradiated polymers can be employed to obtain number-average molecular weights (85). A method for determining the fraction of intensity attributable t o each constituent in an NMR spectrum consisting of two components having a common center and a width ratio of 3 or greater has been developed (86) and applied to a number of polymers. An approach for measurement of solvent polarity using NMR and ESR probes has been discussed (87) and the results correlated with other solvent polarity scales. An approach to the evaluation of stability constants of various complexes using NMR spectra has been reported (88). Errors in factor analysis have been assessed (89). Analysis of CMR data by means of pattern recognition methodology has been described (90). Simplex pattern recognition has been applied to CMR data (91). The determination of heats of physical adsorption on solids by NMR has been developed (92) and tested with success. The book by Leyden and Cox (16) affords numerous analytical applications of NMR spectrometry.

THEORY A general formula has been presented (93) for the calcu-

lation of the weighting of the sum of individual component spectra according to natural (or known enriched) isotopic abundance. A simplification of NMR lineshape calculations that arise for equivalent spins has been described (94) as has a general treatment of DNMR lineshape analysis by an iterative least-squares method for coupled spin systems undergoing intramolecular exchange (95). NMR lineshapes of exchanging first-order spin-spin multiplets have been calculated (96) taking the effect of T I spjn decoupling into consideration. Explicit expressions for line broadening and shift were derived for the system AX + BY and in one limiting case also for A,X, + B,Y, under the conditions where Swift and Connicks equations for single-spin systems are applicable. The physical and mathematical formulation of the “twofraction fast-exchange” model has been investigated (97) and its application to bound water systems in biological samples indicated. NMR powder patterns for integer spin nuclei, in particular spin-1 and -3 nuclei, in the presence of asymmetric quadrupole effects have been calculated (98). A general procedure for the calculation of theoretical NMR second moments in dipolar solids has been described (99)and applied to oxalic acid dihydrate. It has also been demonstrated that the relaxation time of a dipolar solid can be calculated from second moment tensors if the correlation times of the motional processes are known (100).

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A density matrix description of a spin system in terms of the basis operator representation of Fano and Hoffman has been developed (101) and a discussion of CW spectra of ABC, A B 2 and AX2systems as well as the double quantum transition in an AB system presented. The work has also been extended to a system containing a spin-1 particle (102). A theoretical description of intensity effects in A(X,) and A(X,) systems has been reported (103) and it was demonstrated how different relaxation mechanisms alter line intensities. A semiempirical method has been developed (104) for the calculation of electric field effects in CMR. NMR pseudocontact shifts have been calculated for d' and d5 transition metal complexes (105). Molecular orbital studies of hydrogen bond NMR shifts have been described (106). Calculation for 'J(PH), 'J(SiH), 'J(PCH) and 2J(SiCH) a t the INDO level of sophistication have been reported (107). There was no compelling evidence for including 3d orbitals in the calculations. INDO-SCF MO calculations for CsHnFs-, compounds have been found (108) to reproduce the signs, trends and magnitudes of the couplings. 13C chemical shifts and their tensors in hydrocarbons have been calculated (109) by means of the finite perturbation method together with a semiempirical INDO-MO method. The effective paramagnetic shieldings for the 13Cnucleus in many organic compounds are deduced by subtracting the diamagnetic shieldings calculated by semiempirical MO theory from the experimental total shieldings. The effective excitation energies E required to reproduce these paramagnetic terms using the Pople-Karplus approach have been deduced (110). The SCF perturbation theory of 13C chemical shifts, using a modified INDO framework, has been extended (111) to include nitrogen-containing compounds. The calculation of spin-spin coupling constants using standard LCAOSCF-MO methods has been critically assessed (112) and a number of recommendations have been made for improved agreement between theory and experiment.

RELAXATION A simple method for evaluation of relatively short spinlattice relaxation times using a CW spectrometer operating under rapid passage conditions has been described (113)which appears capable of evaluating T1 values of the order of 5 X s. A single-scan Fourier transform spectrometer method for measuring spin-lattice relaxation times has been detailed (114). Using no equipment other than a normal Fourier spectrometer, methods have been developed (115) for measuring the relaxation times T1and T, and the equilibrium magnetization Mo of slowly relaxing systems with weak NMR signals. NMR spin-lattice relaxation time data, obtained with the conversion-recovery pulse sequence and subsequent Fourier transformation, have been analyzed by various statistical estimation techniques (116). The different estimation methods often give markedly different T1values. The most reliable method was shown to be iterative nonlinear estimation. A low cost programmable impulse sequences generator for NMR relaxation studies on a CW spectrometer has been reported (117). A "rapid" modification of the progressive saturation technique for studying spin-lattice relaxation has been described (118) and applied to several liquid aromatic compounds. An error analysis for optimized inversion recovery spinlattice relaxation measurements has been developed (119) and the superiority of exponential fitting with regard to T , error and expenditure of measuring time emphasized. The choice of pulse spacings for accurate T , and NOE measurements in NMR spectrometry has been assessed (120). A theory has been worked out for the influence of cross relaxation between protons on the relaxation behavior of the protons in a protein (121). The theory has been tested on papain and ribonuclease a t 100 MHz and good agreement between theory and experiment found. The water proton spin-lattice relaxation data for normal and sickle erythrocytes at temperatures between -20 and -80 "C can be adequately described by a log-normal distribution of correlation times for water proton motion with a single peak centered at approximately lO-'s at -35 "C (122). PMR shifts, spin-lattice, and spin-spin relaxation times have been measured (123) for low-spin pyridine complexes of Fe(II1)-protoporphyrin(1X) dimethyl ester and Fe(II1)tetraphenylporphyrin in chloroform between 203 and 253 K. A theory for spin-spin and spin-lattice relaxation of Zl,2nuclei in chain methylene groups of a phospholipid bilayer mem-

brane has been developed and applied to PMR and CMR data for phosphatidylcholine and phosphatidylethanolamine (124). The PMR bandshape of phospholipid bilayer vesicles has been analyzed (125) theoretically using a detailed density matrix description of the transverse relaxation process. It was found that slow vesicle rotation should give rise to an NMR signal which is a superposition of Lorentzian curves with different widths in contrast to the results of previous treatments. The 14N spin-lattice and spin-spin relaxation times have been measured (126) for aqueous micellar solutions of n-hexadecyltrimethylammonium chloride and bromide. From comparison with models for the motion of rodlike aggregates, it was concluded that the large micelles probably are flexible. An anisotropic diffusion model has been proposed (127) to account for methylene deuteron relaxation in syndiotactic methylene deuterated poly(methacry1ic acid). The temperature dependent spin-lattice relaxation of 6Li in aqueous LiCl has been reported (128). 6Li possesses most nucleus and it is anticipated that 6Li NMR virtues of a 11/2 will afford high-resolution spectra irrespective of the symmetry of the field gradients a t the site of the nucleus. A theoretical analysis of nuclear relaxation by ZlI2heavy atom nuclei in electrolyte solutions which may occur via transient anisotropic chemical shielding interactions and transient spin-rotational interactions has been presented (129). T,'s and NOE factors at two or more temperatures have been reported (130) for triethylphosphine, tetraethylphosphonium iodide, triphenylphosphine, tetraphenylphosphonium bromide, and triphenylphosphine oxide. A PMR study of the solid propellane, [4,4,4]prop-3,8,12-triene, affords a value of 7 . 5 f 0.5 kcal/mol for the rotational activation energy from T , measurements. Linewidth and second moment measurements gave significantlyhigher results (131). The saturation behavior of three and four 11,spectra of the type AMX and AMRX has been discussed (132) for relaxation by the random field and intramolecular dipole-dipole mechanisms. Spin-lattice relaxation mechanisms of 9Be in aqueous beryllium nitrate have been investigated (133)and the powder spectrum of solid Be(N03)2.4H,0 has been interpreted. A PMR relaxation study of molecular reorientation in ",Reo4 has found (134) no evidence for a previously postulated order-disorder phase transition in the vicinity of 200 K and involving ammonium ions (134).

ORGANIC Tables I1 and I11 list selected investigations of rotational barriers and conformations of various molecules. Increasingly, multinuclei NMR and solvent dependence studies are reported in order to more closely relate experimental data and developments in molecular orbital approaches to these problems. Note again that the references for the tables are collected separately in the Literature Cited. PMR spectra of the nearly tetrahedral molecules CH3MC13 and the tetrahedral molecules (CH3)4M(M = C, Si) in smectic liquid crystal solvents have been reported (135) and interpreted. The relatively large magnitudes observed for the orientation parameter S,,in the smectic phase enable the 'H chemical shielding anisotropies of CH3MC13compounds to be obtained to good accuracy. A study describing the anisotropy and orientation behavior of selected nematic phases using PMR has been described (136). Table IV list studies of various systems in liquid crystal solvents. Books (26,30) and reviews cited in Table I afford additional accounts of NMR studies of molecules in liquid crystal mesophases. Low temperature NMR spectra have confirmed the presence of a two-step intramolecular exchange process in aminodifluoro(perfluoropinaco1yl)phosphoranes (137). Energy barriers for these processes have been estimated. Complete 13C, '9,and 'H spectral analysis of fluorobenzenes, CsHnF6,, has been undertaken and results for 1,2-difluorobenzenealong with INDO-MO calculation of J" have been reported (138). CMR spectra of a number of pefluorinated organic compounds using wide-band fluoride decoupling have been described. Multiple decoupling of widely spaced fluorine resonances is complicated by large Bloch-Siegert shifts but these can be compensated for empirically (139). Fluorine chemical shifts have been reported (140)for 1,1,1,10,10,10-hexafluorodecane in 14 monochloroalkanes and 13 structurally related alcohols. Sizable shielding contributions attributable to the formation of weak C-F- - -H-0 hydrogen bonds were ruled out. Fluorine

ANALYTICAL CHEMISTRY, VOL. 50, NO. 5, APRIL 1978

, Table 111. Conformational Analysis

Table 11. Rotational Barrier Studies Compound Para substituted benzaldehydes 3,5-Dibromoisopropyl benzene Benzenethiol Peptide bond isomerization Styrene P-N ylides

Compound

Comments PMR, CNDOI2 calculations PMR, SP’ C-SP’ C barrier of 2.0 * 0.2 kcal/mol PMR Magnetization transfer NMR Internal barrier 1.6 i 0.3 kcal/mol ‘’C and 31PNMR

\o

Acetyl- and 1,l’-diacetyl. ruthenocene l-Arylsubstituted-l,2dihydro-s-triazines 2-Phenvl-4.6-dioxo1,3-dioxanes

=

0 . S . S e . NH H , Br. Me. MeZN. N O p

( I C , 14C) (2C)

2,2,6,6-Tetrabromo-4-methylcyclohexanone

1,3,2-Dithiaphosphorinanes

9B 1 1 OB)

11B) 12B) 13B)

Effect of metal ion bonding PMR and CMR

=

R

Ref.

N,N’-Dimethylpiperazine 2-Methylbutyllithium Tetraalkylhexahydro-l,2,4,5-tetrazines

PMR and CMR P-N, PMR

X

Methyl sulfide, methyl sulfoxide, and methyl sulfone groups-conformational energies 1,2-truns-Disubstituted cyclohexanes

N -Acylimidazoles

Substituted benzaldehydes Phosphonamido thioic and phosphonamidous chlorides 1-(9-Fluorenyl)-2naphthyl aroates Destabilized ketones 3,3’-Dichloro-bithienyl M-Allylbis(q -cyclopen tadienyldicarbony1)iron cations Amides

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14B)

Hexahydropyridazine derivatives Hydrogen-bonded amino alcohols Methylated cycloheptanones Cyclic dipeptide rings Cyclo(prolyl-leucyl) N-Inversion in hindered piperidines Alkyl-substituted 5-oxo-1,3-oxathiolanes 9-Substituted 9,lO-dihydrophenanthrenes 1-[2-( 1,3-Dimethy1-2-butenylidene)hydrazino Iphthalazine-LSR Highly substituted ethanes Esters of 2-isothiocyanatocarboxylic acids 1,3,5,7-Tetroxacane 2,B-Diketo[ 8 ]( 2,5)furanophane 4,6-Dioxo-l,3-dioxanes Acylated 5,6,7,8-tetrahydropterins tris( Diamine)cobalt(111) complexes-CMR Os,(C,H,)(CO),,-fluxional behavior Tetra-tert-butyldiphosphine Chelated S- and Se-containing ligands

5‘-AMP dimer 2-Chloro-l,3,2-dioxarsolane

chemical shifts have been determined (141) as a function of solvent composition for dilute solutions of l , l , l , l O , l O , l O hexafluorodecane, hexafluorobenzene, and difluorodichloromethane in a number of mixed solvent systems each consisting of n-heptane and one of several electron donor co-solvents. The results provide no evidence for the existence of an appreciable reaction-field contribution to the solvent shift. The 13C chemical shifts of acetonitrile and acetone, as calculated by Pople’s GIAO-MO theory and application of Klopman’s “solvation” model to the MINDO/2-MO method, agree well with the observed solvent effects, including the temperature dependence of the chemical shift (142). Isotropic proton hyperfine splitting constants for two cationic nitroxide radicals have been obtained (143) from the NMR spectra in D20. The probing of the electronic structure of radical complexes by NMR has been developed experimentally and theoretically (144). A method of determination of the lifetimes and distances between the unpaired electron and ligand protons in paramagnetic complexes with stable radicals using NMR linewidths has been described (145). Three different methods have been employed (146)for the determination of indirect 13C-lH coupling constants in some monosubstituted allenes. This paper should be found useful to others confronting similar problems. Long-range spin-spin coupling of -0.5Hz over four bonds between N-methyl and adjacent ring C-H protons is observed for a wide variety of purines and pteridines (147). Factors which permit this coupling have been discussed. A general method for determining deuteron quadrupole coupling constants from direct 13C-D couplings in DMR has been described (148) and is simple to apply to any molecule regardless of symmetry and requires only the C-D bond length and DMR spectrum of a monodeuterated molecule in a nematic solvent. The 13C chemical shifts for 15 of the 19 possible phenyl-substituted methanes, ethanes, and ethylenes have been reported (149). In general, there are no specific trends established for the ortho, meta, and para aromatic carbons with increasing substitution but the C-1 aromatic carbon shows a regular substitution effect and the aliphatic and olefinic carbons show excellent qualitative agreement with data compilations for the effects of alkyl substitution. The high-resolution proton coupled CMR spectra of several monosubstituted benzenes have been completely analyzed and all the signs and mag-

M(CO),L; M = Cr, Mo, W, L = 1,2-bis(benzy1thio)ethane (1,4,8,1l-Tetrathiacyclotetradecane)-

nickel(I1) tetrafluoroborate nitudes of the 13C-lH coupling constants have been determined. INDO-MO calculations only reproduce the substituent trends for 1J44, ‘J12, 3J13, 3526, and 353, and the couplings of the ring carbons with the exocyclic protons in toluene (150). A CMR study of the protonation of aliphatic ketones in sulfuric acid solution has demonstrated that CMR is a suitable alternative to PMR studies of such processes (151). 13C chemical shifts have been reported (152) for the alkyl carbons and the carboxyl carbons of a number of amines, carboxylic acids, and amino acids in aqueous solution. The complete high-precision analysis of proton-coupled CMR spectra of cyclopentadiene has been presented (153) and the J(13C-’H) coupling constants have been compared with data previously reported for five-membered heterocycles. Substituent and electric field effects in the CMR spectra of estr-4-en-17 derivatives have been assessed (154) and the chemical shift dependence of olefinic carbons correlated with INDO-calculated charge densities. The kinetics of deuteration of a series of carcinogenic and noncarcinogenic methylbenzanthracenes has been examined (155)by an NMR method and related to bioactivation. PMR and ‘jN NMR studies of ‘jN-labeled meso-tetraphenylporphyrin lead to a good accounting of the intramolecular proton exchange between nitrogen sites (156). Phenothiazine, 2,3-diazaphenothiazine, and numerous derivatives have been investigated (157) by CMR and PMR. The psychopharmacological activity of phenothiazine derivatives seems to be related both t o the electron donor properties and to the side chain attached a t N-10.

SHIFT REAGENTS Enthusiasm for shift reagents appears to show little abatement. The number and types of shift reagents continue to increase as do their applications. Effects of errors in analyses of lanthanide-induced shifts in the PMR spectra of cis- and trans-pinocarveol have been evaluated (158). A useful analysis of line broadenings induced by lanthanide shift

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Table IV. NMR in Liquid Crystal Solvents Compound 2,6-Difluoropyridine and 2,4,6-trifluoropyridine CCl,F, CC1,CF3 CH,X (X = F, C1, Br, I) 1,3-Dioxolane 1,3,5-Trichlorobenzene 4-Meth ylpyridine

Spin

=

3/2 nuclei

(CH3).tC, (CH3)(CD3)3C9 (CD,),C Potassium laurate/decanol/KCl/H, 0 D2 0 Fluorobenzenes NH,', Na+, and D,O Nematic MBBAiEBBA mixtures N-Methyl formamide

Comments PMR and 19FNMR in nematic phases CMR, nematic phases PMR, smectic phases PMR, nematic phase PMR, spiral mesophase PMR, lyotropic mesophase Lyotropic liquid crystals, multiple quantum transitions Nematic phases, PMR, DMR Nematic-like mesophase, DMR DMR, lyotropic phase PMR, lyotropic phase Lyotropic phase Angular dependence of T, and T, Nematic phase, isotopically enriched

p-Ethoxy-benzylidene-p-N-octyloxyaniline

TTF and TCNQ N-p-Cyanobenzylidene-p-N-octyloxyaniline

Hexamethylenetetramine s-Triazine Hexaphen ylcyclohexaphosphine Transient phenomena MBBA CF,X ( X = C1, Br, I, CCl,) Acetophenone 2.4.6-Trifluoronitrobenzene

Nematic phase Nematic and isotropic phases Lyotropic phase Nematic phase Nematic phase T , proton relaxation Smectic phase Nematic phase 19Fshielding anisotropy

reagents (concentration, frequency, and temperature effects) has been reported (159). The temperature dependence of lanthanide-induced NMR shifts has been examined in detail (160) and it is likely that for the majority of adduct systems the most realistic description of the proton shifts is in terms of the well-established T 2dependence. The temperature dependence of lanthanide-induced shifts has also been the subject of further study (161). The lanthanide-induced PMR, CMR and 31P NMR shifts for (EtO),PO, (EtO),P(=O)Et, E t 3 P 0 and (EtO)3Pwith Eu(dpm) , E ~ ( f o d )P~r, ( d ~ mand )~ Pr(fod), have been reported (162j. The use of G d ( f ~ d ) ~ induced changes in 'H relaxation times in conformational studies of the adducts between lanthanide chelates and borneols has been presented (163). The method should be applicable to conformational studies of other molecules which form well-defined complexes with L n ( f ~ d ) ~The . chemical exchange effects in the PMR spectra of solutions containing a L n ( f ~ dand ) ~ 1.2-dimethoxyethane have been assessed (164). The utility of lanthanide P-diketonate complexes as shift reagents has been extended to alkyl fluorides (165). A determination of the sequence of peptides using paramagnetic ions (Gd3+and Cu2+)and PMR and CMR has been reported (166). The nitroxide radical DTBN(di-tert-butylnitroxide) has been found (167) to be a useful paramagnetic shift reagent in PMR, inducing downfield shifts for C-H protons. Measurements of 13C spin-lattice relaxation times for a number of representative organic compounds in the presence of paramagnetic P-diketonate derivatives of Cr, Mn, Fe, Ni, and Gd have been reported (168) and discussed in terms of several possible interaction mechanisms. A relaxation time study of a number of metal acetylacetonates shows that electron-nuclear dipolar coupling dominates T1 while the hyperfine contribution can be important for Tz (169). The elimination of spin-spin coupling in the PMR spectrum of N-vinylpyrazole by paramagnetic additives, e.g., N i ( a ~ a c )has ~, been investigated (170).

INORGANIC The development of probes for various nuclei with the concomitant growth of F T techniques is leading to an increasing number of investigations in which the metal NMR spectra are obtained rather than just PMR or CMR studies of the matrix in which the metal ion is found or the ligands bound to the metal. Solvent shifts of the PMR spectrum of molecular hydrogen have been reported (171). The anisotropy of the chemical shift tensor for 99.5% 15N-enriched solid nitrogen has been described (172) and found to be in good agreement with an

estimated value based on a molecular beam measurement of the spin-rotational interaction and ab initio calculations. Similar results have been found for solid carbon monoxide (173).

Boron-boron coupling constants for B(l)-B(3) of B4H10, B(l)-B(4) of CB5Hg,and several other compounds have been determined (174). From both J("B-H) and J("B-"B) values for B2Hs, B4Hlo,and B Hg a set of SB-orbitalpopulations, comparing favorably wit\ PRDDO wavefunctions, have been derived. A bond length of 1.30 A was suggested for the unique bridge hydrogen bonded to the apex boron of B5Hll. An extensive study of 13C and ''B shielding values of icosahedral heteroatom boranes has been reported (175). A linear relationship between 13C and "B shielding values of isoelectronic and isostructural molecules was demonstrated. %i and CMR studies of the chemical shifts, coupling constants and relaxation times for Me3SiSMe and (Me3Si)&Nhave been described (176). Proton-decoupled 19Fand P NMR spectra of a series of dialkyltetafluorodiphosphetidines, (RF2PNMe),, have been recorded (177) and analyzed for temperatures high enough for gauche e trans exchange to be rapid on the NMR time scale and a t low temperatures when exchange is slow. The 19FNMR of ErF3 and LiErF, powders and a single crystal of ErF, have been reported (178) in detail. The Knight shift of 7Li in Li-methylamine solutions has been employed to characterize the metal-nonmetal transition (179). This approach to solutions of metals in nonaqueous solvents provides a useful complement to conventional electrical measurements. 39K NMR of potassium salts in various solvents has demonstrated (180) that 39KNMR is a sensitive probe of the immediate environment of the potassium ion. Numerous biological applications of this technique suggest themselves. Results tending to show that medium effects in 'C NMR are generally appreciable have been presented (181). The 75AsNMR spectra of a series of arsonium salts have been recorded (182). The changes in the linewidths of the 75As resonances in the fully symmetrical species R4As+Br-(R = Me, Et, Pr, Bu) may be accounted for in terms of changes in the molecular correlation time. The existence of the ASH,+ ion in H F solution has also been demonstrated. A comparison of lanthanide and cobaltate and nickelate ion pairs with organic cations suggests (183) that the dominant shift mechanisms is the axial (through space) interaction in the cobalt complexes. Scandium-45 NMR spectra for aqueous solutions containing SCFs3- and NMR for aqueous solutions of TiF2- and GeF, have been described (184) and the metal quadrupole coupling constants evaluated. 55MnNMR linewidths in solution have

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ANALYTICAL CHEMISTRY, VOL. 50, NO.

been found t o be linearly related to measured nuclear quadrupole coupling constants (185). Heteronuclear magnetic double resonance spectra of some tin and mercury cyclopentadienyl derivatives have been used to probe fast intramolecular exchange (186). Vanadium-51 chemical shifts and 51V-31P coupling constants have been reported (187) for a number of vanadium carbonyl complexes. The value of 1J(51V-31P) is influenced mainly by inductive effects of the substituents on phosphorus. CMR spectra of several carbonyl complexes of Mn and Co show that line broadening due to the spin-spin coupling of carbon nuclei with metals having 1.2 1 / 2 can be eliminated by thermal decoupling or by using viscous solutions (188). Vanadium-51 and manganese-55 NMR studies of a number of metal carbonyl complexes afford additional data on the electronic structure of such materials (189). Fourier transform NMR measurements of 95Moand 97Mo have been recorded (190) for several molybdenum compounds in different oxidation states. Using the molybdate ion as a reference, chemical shifts from about +900 ppm to about -190 ppm were observed. " 0 and 33SNMR studies of aqueous sulfate, thiosulfate, molybdate, and thiomolybdate solutions have been described (191). A technique for obtaining lg5PtFT NMR spectra has been shown (192) to be readily applicable to measuring platinum spectra and yields chemical information not readily available using indirect methods of observation. The relative acceptor strengths of ten Lewis acids have been determined (193)from the 'J(Pt-H) values of their 1:l and 1:2 adducts with trans-HPt(PEt,)CN. An extensive collection of NMR spectra of trihalogeno(triha1ogenophosphine) latinate(I1) compounds has been presented (194). Trends in P coordination shifts, 195Pt-31P coupling constants, and lg5Ptchemical shifts are discussed. Rhodium-103 chemical shifts, measured by 1H-11@3Rh[ heteronuclear double resonance, have been reported (196)for a number of octahedral Rh(II1) complexes. Silver-109 NMR studies of a variety of organic and inorganic complexes have appeared (196). The Cu(I1) complexes of glycine in aqueous solution have been examined by monitoring the PMR linewidths of the solvent (197). NMR studies of the interaction of cytosine and gold complexes show that cytosine coordinates to gold(1) through the N-3 atom (198). The solution dynamics of Cr( a ~ a c )frequently ~, employed as a relaxation agent, have been investigated (199) as have the dynamics of pyridine base adducts of bis(N,N-dialkyldithiocarbamato)nickel(II)(200). An investigation of ethylenediamine complexes of Cu(I1) and Ni(I1) in solutions of dimethyl sulfoxide has explored DMSO exchange rates (201). The increase in exchange rate upon successive addition of ethylenediamine can be explained by a n amine trans effect. The 100-MHz PMR spectra of thallium, lead, and scandium octaethylporphyrins and of a p-oxo dimer of scandium octaethylporphyrin show the methylene protons of the ethyl side chains to be anisochromous. The 100- and 300-MHz PMR of a thallium aetioporphyrin show that the methylene protons of the four ethyl side groups are also anisochromous. It was suggested (202)that these phenomena result from the inherent asymmetry of the metalloporphyrins rather than from hindered rotations of the side chains. The 13C shifts of the porphyrin skeleton observed in porphine and Zn(I1) porphine provide no support for the 16-membered path of the .r;-electron delocalization in the porphyrin nucleus (203). Natural abundance "Zn chemical shifts have been reported for a variety of aqueous zinc(I1) solutions (204). The technique should find applicability to a number of systems of biochemical interest. The 13Cd magnetic shielding and Cd-H coupling constants of several dialkyl cadmium compounds have been obtained by 1H-{1'3CdJdouble resonance experiments. The 'I3Cd shielding increases in the order Cd(CH3)* < Cd(i-C4H9)*< Cd(n-C,H& < Cd(n-C3H7)*< Cd(C H5)2< Cd(i-C3H7),which parallels the trend which has been oiserved in dialkyl mercury compounds (205). lI3Cd spin-lattice relaxation in aqueous cadmium perchlorate solutions a t different temperatures has been described (206). In D I7O solution a scalar coupling constant of 248 MHz between313Cd and 170 is obtained. Solvent effects on the spin-lattice relaxation time ion in eleven solvents have been recorded (207). The of 2oT1+ solvent dependence of the *@jTlNMR chemical shift, the '05Tl-H coupling constant, and the chemical shift and 2@5Tl-13C coupling constant have been reported (208). The chemical shift solvent dependence for (CH3),T1C10, and

P

5, APRIL 1978

127 R

(CH,)2T1N03 is over 200 ppm.

NOMENCLATURE FOR NMR SPECTROMETRY We have compiled the following list of terms, their definitions and abbreviations, which occur most frequently in papers on NMR spectrometry. Anisotropic motion. Preferred rotation about one axis of a molecule. Anisotropy. Variation of a property with direction of the applied magnetic field. C A T (computer of average transients). A signal averaging device. Chemical shift. The position of a n NMR line relative to a standard reference. It is determined by the degree of "shielding" of the applied field by the electrons surrounding the particular nucleus. CMR. Carbon-13 nuclear magnetic resonance. Diamagnetic. Possessing no net electron magnetic moment. Dipolar correlation time. The time taken for a spin system to rotate through one radian from its previous orientation due to "through space" interactions with neighboring spins. Dipolar interaction. Magnetic interaction between two magnetic moments by virtue of the effect of the magnetic field of one on the other. D S S . 2,2-Dimethylsilapentane 5-sulfonic acid. A watersoluble NMR standard. Gauss ( G ) . Strictly speaking, a unit of magnetic induction while the oersted(oe) is a unit of magnetic field intensity. The practical equivalence of the two units has led to the common usage of the gauss as the unit of magnetic field intensity. 1 kG = 103G. 1 Tesla (T) = lo4 G. Gaussian lineshape. Shape of a spectral line whose hei h t as a function of frequency u is given by I(v) = I,, exp [-b5(uo - u ) ' ] where uo is the frequency of the line center and the linewidth is 1 u = 2(ln 2)'I2/b. Hall probe. Magnetic field measuring device employed for stabilizing the field of electromagnets. Hertz (Hz). Frequency of one cycle per second (cps). 60 MHz = 60 X lo6 Hz. Heteronuclear decoupling. Elimination of spin-spin interactions between nuclei of different types by irradiation with high power a t the resonant frequency of one of them. Homonuclear decoupling. Elimination of spin-spin interactions between nuclei of the same type by irradiation with high rf power a t the resonant frequency of one of them. Isotropic. A property which is the same independent of direction of the applied magnetic field. Isotropic spectrum. NMR spectrum in which the overall molecular tumbling is so rapid that all anisotropy in the spectrum is averaged out. Linelcidth. The width of a spectral line, normally defined as the distance (in Hz) between the two points of half-maximum height. Lorentzian lineshape. Shape of a spectral line whose hei ht as a function of frequency u is given by I(v) = /I, [l + aB(uo - u ) * ] where uo is the frequency of the line center and the linewidth is I u = 2 / a . Nuclear magnetic resonance ( N M R ) . Absorption spectrometry involving transitions between the energy levels corresponding to the different orientations of a nuclear magnetic moment in a magnetic field. Magnetic shielding. The reduction of the applied magnetic field at the nucleus below that applied to the entire sample. Modulation. Superposition of a varying wavelike component onto some steady quantity, e.g., magnetic field modulation: introduction of a small component of the magnetic field whose direction varies with time in a wavelength manner at a particular frequency. Nuclear Overhauser Effect (NOE). A change in the integrated NMR absorption intensity of a nuclear spin when the NMR absorption of another spin is saturated. The spins involved may be either heteronuclear or chemically shifted homonuclear spins. Paramagnetic. Possessing a net electron magnetic moment, Le., a t least one unpaired electron. Relaxation. Process by which a n atom or molecule in an excited state falls back into its ground state. Saturation. Situation in which the rates of upward and downward energy level transitions induced by radiation are equal, so that no net energy is absorbed.

128R

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ANALYTICAL CHEMISTRY, VOL. 50, NO. 5, APRIL 1 9 7 8

Signal aueraging. Improvement of the signal-to-noise ratio of a spectrum by adding up repeated scans through the spectrum. The noise tends to average out whereas the spectral lines reinforce. Spin-lattice relaxation time. A measure of the time taken for the spin population to return to its equilibrium value through interaction with fluctuating internal fields which surround it (the lattice). Spin-spin relaxation time. A measure of the time to lose phase coherence, Le., return to equilibrium through interaction with neighboring spins. It is inversely proportional to the linewidth. Spin-spin splitting. Splitting in the lines of an N M R spectrum arising from the interaction of the nuclear magnetic moment with those of neighboring nuclei. TMS. Tetramethylsilane. A water-insoluble NhlR reference standard.

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ACKNOWLEDGMENT The authors gratefully acknowledge the support and en couragement of Professor William E. Hatfield of the University of North Carolina-Chapel Hill. LITERATURE CITED

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Literature for Table I V (1D) M. Duchene. J. W. Emsley, J. C. Lindon, D. S. Stephenson, and S. R. Salman, J . Magn. Reson., 22, 207 (1976). (20) 6. R. Appleman and B. P. Dailey, ibid., 22, 375 (1976). (313) A. J. Montana and B. P. Dailey, ibid., 21, 25 (1976). (4D) C. A . Lange, ibid., 21, 37 (1976). (513) N. Kamezawa, ibid., 21, 211 (1976). (6D) S. A. Spearman, R. C. Long, Jr., and J. H. Goldstein. ibid., 21, 457 (1976). (7D) G. Lindblom, H. Wennerstrom and B. Lindman, ibid., 23, 259 (1976). (8D) I. Y. Wei and C. S. Johnson, Jr., ibid., 23, 259 (1976). (9D) R. C. Long and J. H. Goldstein, ibid., 23, 519 (1976). (10D) J. P. Jacobsen and K. Schaumberg, ibid., 24, 173 (1976). (11D) S. A. Spearman and J. H. Goldstein, ibid., 26, 237 (1977). (12D) J. Charvoiin, A. Loewenstein and J. Virlet, ibid., 26, 529 (1977). (13D) I.Zupanic, V. Zagar, et al., Solid State Commun., 18, 1591 (1976). (14D) C. L. Khetraoal. C. A. Kunwar. and S. Ramawasad, Mol. Cwst. Lia. Cryst., 34, 123 (1976). (15D) J. S. Prasad. J . Chem Phys., 65, 941 (1976). (16D) T C. Wong. E. E Burnell, and L. Weiler. Chem. Phys. Lett., 42, 272 (1976) (17D) J. Visintainer. E. Bock, R. Y. Dona, and E. Tomchuk. Can. J . Phys., ' 54, 2282 (1976). (18D) A. Amanzi, P. L. Barili, P. Chidichimo, and C. A. Veracini, Chem. Phys. Lett., 44, 110 (1976). (19D) J. P. Marchal and D. Canet, J . Chem. Phys., 66, 2566 (1977) (20D) J. P. Aibrand, A. Cogne, and J. B. Robert, Chem. Phys. Lett., 42, 498

2.

1197m _,. ~

(21D) C. E. Tarr, M. E. Field, and L. R. Whalley, Mol. Cryst. Li9. Cryst., 35, 225 (1976). (22D) V. Graf, F. Noack, and M. Stohrer. Z . Naturforsch. A , 32, 61 (1977). (23D) A. J. Montana, 6. Appleman, and B. P. Dailey. J. Chem. Phys.. 66, 1850 (1977). (24D) J. W. Emsley, J. C. Lindon. J. M. Street, and G. E. Hawkes, J . Chem. Soc., Faraday Trans. 2 , 72, 1365 (1976). (25D) G. J. Den Otter, J. Bulthuis, C. A. De Lange, and C. Maclean, Chem. Phys. Len., 45, 603 (1977).