2 Physical Methods and Techniques Part (ii)N.M.R. Spectroscopy By R. F. M. WHITE Department of Chemistry City of London Polytechnic Jewry Street London EC3N 2EY 1 Introduction As in last year's article' this report concentrates on the use of ideas and techniques as means of solving chemical problems. Changes in some of the section headings reflect changes in the topics chosen for consideration. As in previous years the illustrations selected have not been narrowly restricted and in addition to organic applications examples drawn from studies of organometallic and inorganic com- pounds are included. 2 Chemical Shifts and Coupling Constants A computer program for the direct determination of the carbon backbone of molecules from natural abundance I3C n.m.r.data has revealed2 that in the earlier literature there is an erroneous assignment for C-3 and C-6 in (+)-limonene (1); the revised assignment gives C-3 at 30.9 and C-6 at 30.6 p.p.m. Natural abundance C-I3C one-bond coupling constants have been determined for patchoulo13 and 1,2-didehydrotestosterone17-O-a~etate;~ in both cases simple analysis of the spectral data was supported by the computer program CABSA Connectivity by AB Satellite Analysis. A summary of all known up to the begining of 1J13c-13c 1980 have been presented in a diagramatic form.5 ' R. F. M. White Annu. Rep. Prog. Chem. Sect. B 1980,77 3. ' R. Richarz W. Ammann and T. Wirthlin J. Magn. Reson. 1981 45 270. A. Neszrnelyi and G. Lukacs J. Chem. SOC. Chem. Commun. 1981,999. G.Lukacs and A. Neszrnelyi J. Chem. SOC. Chem. Commun. 1981 1275. P. E. Hansen and V. Wray Org. Magn. Reson. 1981,15 102. 15 16 R. F. M. White The 13C chemical shifts obtained6 under uniform conditions for selected secondary aliphatic compounds have been used in a linear regression analysis and two- parameter relationships describing substituent effects have been calculated. In aldopyranoses 'J13C-1H(-170 Hz) for an equatorial C-H adjacent to a ring oxygen exceeds that of a comparable axial C-H by about 10 Hz. When the ring oxygen is replaced by sulphur both orientations of the C-H bond have almost the same value' of this one-bond coupling constant. The 'H and I3C spectra of 5-thio-~-glucopyranose have been analysed* the 'H spectrum showing that two anomers are present in the ratio 85 :12 (a:@) in solution in 2H20 or 2H20-DMS0.The proton-proton vicinal coupling constants provide evidence that the ring of 5-thio-a-D-glucopyranose is slightly puckered compared with that of a-D-glucopyranose. The smaller C-S-C angle deforms the ring in such a way that the carbon atoms are further from the average plane. The I5Nfrequencies have been measured' for ""0 "'NO "NH3 15N2,and HC"N in the gas phase and for neat liquid Me"N02. From the temperature dependence at various densities it is possible to reduce the measured frequencies to the zero pressure limit at 300K providing a basis for an absolute nitrogen shielding scale. A number of N-alkyl formamides show" two resonances in their 15 N n.m.r. spectra the cis- and trans-isomers only slowly interconverting about the CO-N bond.For most of the compounds examined the "N shift in the cis-isomer was to high field of that in the trans- exceptions being when the N-substituent was was a tertiary alkyl group. For all the compounds studied was greater for 1J15N-H the trans-isomer. Substituent effects in "N chemical shifts have been determined" for a number of substituted pyridines and pyrimidines and (on the p.p.m. scale) are about three times more sensitive towards substituents than are 13C shifts; substituent shifts for 15N are additive and are useful for assignment purposes. The 1sN coupling constants to 13C and 'H have been reported'* for 2-(hydroxymethyl)-5- methoxy-1 -phenyl-4-pyridone (2) values of being consistent with the 1J15NT13c 0 nitrogen being sp2 hybridized.The molecule resembles a hybrid between an aromatic system and an a,@-unsaturated ketone; the authors expressed the opinion that there is a definite bias towards aromatic character. On protonation I3C shifts do not change much whereas 15Nshifts undergo a large change. A. Ejchart Org. Mugn. Reson. 1981 15 22. 'V. S. Rao and A. S. Perlin Curbohydr. Res. 1981,92 142. J. B. Lambert and S. M. Wharry J. Org. Chew. 1981,46 3193. C. J. Jameson A. K. Jameson D. Oppusunggu S. Wille P. M. Burrell and J. Mason J. Chem. Phys. 1981 74 81. lo H. Nakakishi and J. D. Roberts Org. Mugn. Reson. 1981 15 7. W. Stadeli and W. Von Philipsborn Org. Mugn. Reson. 1981,15 106. '* C. A. Kingsbury M. D.Cliffton S. Rajan D. L. Durham and J. H. Looker Heterocycles,1981,16,343. Physical Methods and Techniques -Part (ii) N.M.R. Spectroscopy 17 OR" R = Me,But,or H R' = Me. R = MeorEt R' (3) Both 13C and 'lo n.m.r. spectra have been ~btained'~ for 2-alkoxytetrahy- dropyrans (3) ten of which were in the form of cis- and trans-isomeric pairs; when both isomers were available a consistent pattern of shifts was observed. In the trans-isomers where the alkoxy-group is exclusively or predominantly axial both the ring oxygen and alkoxy-oxygen are more shielded than the corresponding atoms in the (equatorial) cis-compounds. The three carbons attached to oxygen atoms are also shifted upfield in the axial isomers. Substituent effects and hydrogen- bonding interactions in 170n.m.r.spectra have been investigated for mono- and poly-substituted acetophenones and benzaldehydes the 170 shifts being14 highly sensitive to electronic perturbation induced by substituents. Rate constants for carbonyl oxygen exchange in H2170-dioxane solvent are reported for several acetophenone derivatives. Magnetic non-equivalence of diastereotopic oxygen atoms has been detected by means of 170 n.m.r. spectroscopy. Oxidation of phenyl 1-phenylalkyl sulphides with iodobenzene trichloride and H2170 gives" dias-tereomeric phenyl 1-phenylalkyl ['70]sulphoxides (4) of known configurations (RRISS)-(4) and (RS/SR)-(4). Further oxidation with rn-chloroperoxybenzoic acid gives the [I6O,'70]sulphones (5) in the forms (RR/SS)-(5)and (RS/SR)-(5) and Ph Ph I I PhSOCH PhS02CH I I R R (4;R = Me or Et) (5) a solvent-dependent chemical shift of 4 to 10 pqm.between the diastereotopic oxygen atoms was found for these sulphones. Both diastereomers of cyclic 2'- [170 '80]deoxyadenosine 3',5'-monophosphate have been synthesized and configurations at phosphorus determined. The 170spectra show16 that the chemical shift 1J31p-170 and the line widths are different for the diastereomers the chemical shift being large enough for two resonances to be detected in ihe 31P decoupled 17 0 spectrum of a racemic mixture. The compound previously thought to be a hydroxyphosphazene has been shown17 to exist in solution as an oxophosphazene (6).The product of condensing (6) with P3N3ClPh5 has an oxygen bridged structure with 3Jp-o-p= 37.9 Hz the largest value so far obtained for this coupling.l3 R. D. McKelvey Y. Kawada T. Sugawara and H. Iwamura J. Org. Chem. 1981,46,4948. l4 T.E.St. Amour M. I. Burgar B. Valentine and D. Fiat J. Am. Chem. SOC.,1981,103 1128. lJ K. Kobayashi T. Sugawara and H. Iwamura J. Chem. SOC.,Chem. Commun. 1981,479. 16 J. A. Coderre S. Mehdi P. C. Demou R. Weber D. D. Traficante and J. A. Geret. J. Am. Chem. SOC.,1981,103 1870. " D. S.Rycroft V. R. Miller C. D. Schmulbach and R. A. Shaw Phosphorus Sulfur 1981,10 121. R. F.M. White Isomeric mixtures of compounds Me,M(CH :CHMe)4-, with M = Si or Pb and It = 0 to 3 have been studied18 by 'H 13C 29Si and *"Pb n.m.r. spectra. The results obtain from isomers Me3M(C3H5) were used to calculate shifts for the other compounds.The lead derivatives gave good agreement between observed and calculated values but for silicon they diverged with decreasing n because of steric factors. The existence of the two tin isotopes "'Sn and '19Sn simplifies determina- tion" of the coupling constant between isochronous tin atoms. In (Me,Sn),CH 2Jsn-Sn is negative and the evidence points to this also being so in most of the trimethyl tin-(Group IV and V) compounds examined the results obtained for a number of stannylamines suggest a dependence of 2Jsn-Sn on the Sn-N-Sn bond angle and the geometry of the nitrogen atom. General features in the 'H spectra of 2-substituted 1,3-dithians (7) suggest that when X = SnMe or PbMe there should be observable coupling between H-4e and the metal atom.In the 270MHz 'H spectra of these derivatives H-4e and H-4a give well resolved multiplets2' in which the former is easily identified from the couplings to other ring hydrogen atoms; it also shows satellites due to "7*119Sn or 207Pb whereas the metal to H-4a coupling was not identifiable and was presum- ably small. If the long range metal-proton coupling is favoured by a zig-zag arrangement the metal groups must favour a predominantly equatorial orientation. Vicinal couplings (,JC4between 13C and 117~119Sn or 207Pb are dihedral-angle dependent; in the cyclohexyl compounds an axial SnMe group gives 3JSn-C of 10 to 12 Hz whereas the equatorial group gives a coupling constant of 65 Hz.The conformationally homogeneous trans-5 -t-butyldithian-derivatives were prepared. In the tin compound 3JSn-C = 32.4 Hz was comparable with the coupling of 28 Hz found for (7;X = SnMe,); when M = Pb 3J207'pb-13c = 38 Hz. The chemical shifts low temperature spectra and JM-13c in 2,2-disubstituted derivatives indicate that the M-Me3 group (M = Si Ge Sn or Pb) has a large preference (A > 2.0 kcal mol-') for the equatorial orientation at position 2 in the 1,3-dithians a preference that is larger than in the corresponding M-cyclohexyl compounds. T. N. Mitchell and H. C. Marsmann Org. Mugn. Reson. 1981 15 263. l9 W. Biffar T. Gasparis-Ebeling H. Noth W. Storch and B. Wrackmeyer J. Mugn. Reson. 1981 44 54. 2o G. M. Drew and W. Kitching J. Org. Chem.1981,46,558 Physical Methods and Techniques-Part (ii) N.M.R. Spectroscopy 19 Relatively narrow line widths-of 50 to 150 Hz have been obtained” in the 33S (I= 3/2 abundance 0.74%) n.m.r. spectra of a number of sulphones and sulphonic acids. The range of 33Sshifts is wide enough to allow their use for quantitative analysis particularly in the examination of petroleum. Resonances from ”Fe (I= 1/2 abundance 2.2%) in organo-iron complexes mainly of the type Fe(CO),(diene) show a range of 3000 p.p.m. in chemical shifts,” mostly to high frequency of Fe(C0)’. Values of were from 23 to 32 Hz ‘J57~~-13~ for carbonyl carbon atoms whereas for carbon m-bonded to iron this coupling was less than 6Hz. Values of have been obtained from the 13C spectra of 2J13c-13c highly 13CO-enriched metal carbonyls.At low temperatures the rearrangement of the carbonyl groups of (8) is frozen so that separate peaks (intensities 1:2) 0 C (8) arise from axial and equatorial groups. At -100 “C the spectrum of a 65% 13C0 enriched sample has carbonyl peaks split into 5 and 3 lines because of the different isotopomers present. The five-line feature arises from overlap of a singlet (no 13C in the basal position) with a doublet (one basal 13C) and with a triplet (two basal 13 C) while three lines arise from overlap of a singlet and a doublet. Values of 2J13c-13c are 30-35 Hz for trans-carbonyl groups while the cis-couplings are an order of magnitude smaller. 125 Te n.m.r. has been usedz4 to follow the stereochemistry of orthotelluric acid solvolysis in hydrogen fluoride.When (H0)6Te is dissolved in 48% aqueous HF the product is cis-(H0)4TeFz; further fluorination in anhydrous HF leads to equal proportions of sym- and asyh- (HO),TeF,. 3 Isotope Shifts The deuterium-decoupled ‘H n.m.r. spectrum of a mixture of pentadeuteriobenzene and benzene showed” a chemical shift for (’HS)benzene 0.0032 p.p.m. to low field of benzene. Such a down-field 2H isotope shift is uncommon and is attributed to the increased electron density affecting the n-electron ring current. Downfield ’H isotope shifts have been detected26 over three bonds in monodeuterated cyclopen- tane and over four bonds in cyclopentane. The fate of deuterium in biosynthesis can be followed directly by 2H n.m.r.or indirectly from the 13C spectra. The indirect method has been used with monitoring of 13C directly bonded to 2H, the resonance of such a carbon atom being shifted to high field (the a-shift) and showing coupling to deuterium. In this form the method suffers disadvantages of a reduced signal to ” R. Faure E. J. Vincent J. M. Ruiz and L. LCna Org. Magn. Reson. 1981 15,401. ‘’ T.Jenny W. Von Philipsborn J. Kronenbitter and A Schwenk J. Organomet. Chem. 1981,205,211. 23 S. Aime and D. Osella J. Chem. SOC., Chem. Commun.. 1981 300. 24 W.Totsch P. Peringer and F. Sladky J. Chem. SOC., Chem. Commun. 1981 841. 25 T.Yonemitsu and K. Kubo J. Chem. SOC.,Chem. Commun. 1981,309. 26 R.Aydin and H. Guenther J Am. Chem. SOC.,1981,103 1301. R.F.M. White noise ratio of the shifted resonance because of poor relaxation signal multiplicity and loss of nuclear Overhauser effect (n.0.e.). A variation of the indirect method has been suggested2' in which 13Cin a position @ to the deuterium is monitored. The @-shift is upfield and as 2J2H-13c is negligible the proton-decoupled spectrum gives a singlet for the 13Cpeak monitored and difficulties arising from poor relaxation and n.0.e. are avoided. A comparison between the direct 'H n.m.r. and the indirect @-isotope shift methods has been made by measurement of incorporation of deuterium into the polyketide 6-methylsalicyclic acid. Poor resolution is often a limitation of 'H n.m.r a problem which becomes worse as the molecular size increases; on the other hand the @-shift method offers better resolution which is not lost with increased molecular size.Exchange of 'H for 'H in carbohydrate hydroxy-groups also causes 13Cisotope shifts. Parameters have been determined empirically for a number of such isotope shifts and these may be used to calculate shifts to assist in the assignment of 13C spectra. The 13Cn.m.r. of psicose has been re-examined2* by the differential isotope shift (DIS) to clarify some of the assignments for this ketose which in aqueous solution exists as a mixture of a-and p-furanose and pyranose forms. In the DIS technique samples of the isotopomers (a) all -02H in 'H20 solution and (6)all -OH in H20 are contained in co-axial n.m.r. tubes. When the isotope shift was large enough two lines were obtained for each 13Cresonance and comparison of calculated and observed shifts clarified assignments for psicose.Deuterium isotope effects in I3Cn.m.r. have been usedz9 to investigate tautomeric equilibria in enamine ketones which may exist in the three forms (9a b and c) although no evidence has been observed for (9c). Samples of compounds [(lo) (ll) and (12)] were examined in co-axial n.m.r. tubes 2H,0 being added to one half and H20 to the other. An isotope shift was observed for C-2 in (lo) whereas in (11)and (12) both C-1 and C-2 showed isotope shifts. As deuterium isotope shifts are usually detect- able over not more than three bonds it was concluded that forms (9a) and (9b) are present in equilibrium for (11)and (12) and that there is no tautomeric equilibrium for (10).This is consistent with the conclusion that the double bond prefers to be em-to a five-membered ring system for P-diketone derivatives. 0JrJ/ 0 (10) (11) (12) 27 C. Abell and J. Staunton J. Chem. SOC.,Chem. Commun. 1981,856. 28 K. M. Valentine L. W. Doner and P. E. Pfeffer Carbohydr. Res. 1981,96 293. 29 G. M. Coppola R. Damon A. D. Kahle and M. J. Shapiro J. Org. Chem. 1981 46 1221 Physical Methods and Techniques -Part (ii) N.M.R. Spectroscopy Double isotopic substitution ("C and l80) has been used3' in an investigation of the thermal and the acid-catalysed rearrangements of trispiro (tricycl0(3.3.3.0'*~) undecane-2,2';8,2";9,2-tris[oxirane]} (13) to the triether 2,5,14-trioxahexa-c yclo[ 5.5.2.1.2 .4*10 O4*l7O10*17]heptadecane (14).(13) (14) The trioxide (13) labelled with 13C (90%) and l80(45%) in one of the three epoxide groups showed two resonances with intensities 55(low field) :45(high field) in part of its 13C n.m.r. spectrum. The rearrangement to (14) could proceed either by breaking 13C-'80 bonds (15; pathway 1)with loss of the l80isotope-shifted 1 (15) peak or by breaking oxygen-quaternary carbon bonds (16; pathway 2) with retention of '*O shifted peaks and hence two 13C resonances with relative intensities 55:45. The products of both the thermal and the acid catalysed reactions showed 18 0 shifted peaks hence the reactions proceeded by pathway 2. (16) Mono- and di-labelled '80-acetals [(17)-(19)] show3' additivity of isotope shifts in their 13Cn.m.r.spectra. Thus the shifts at C-2 for the doubly labelled compounds (19a-c) are from 0.046 to 0.055 p.p.m. twice those for the singly labelled analogues (Ma+) which are from 0.023 to 0.028 p.p.m. It is usually found that "0 P-isotope shifts are not resolvable however the ethylene glycol derived acetals (18) and (19) are favourable cases in which this shift can be resolved. These show that within experimental error the &shifts (ca. 0.007 p.p.m.) are also additive. 'O S. A. Benner,J. E. Maggio and H. E. Simmons 111 J. Am. Chem. SOC.,1981,103 1581. 31 R.N.Moore J. Diakur T.T. Nakashima S. L. McLaren and J. C. Vederas J. Chem. SOC.,Chem. Commun. 1981,501. R. F.M. White *o= 180 (a) R1 = Me(CH&,R2 = H (b) R'-R2 = -(CH&- (c> R' = R2 = Me(CH2)3 The l80isotope shifts in 13C spectra have been used in the in~estigation~~ of the acid-catalysed exchange of oxygen between water and [l-13C,'802]acetic acid the isotope shift facilitating the direct observation of the three different 0-isotopomers of acetic acid.In water the exchange proceeds by the pathway shown in Scheme 1to give (20) the slow stage involving a tetrahedral intermediate formed by nucleophilic attack of H20 on RC02H2+. A similar use has been made of "0 induced shifts in the "N n.m.r. spectrum of the nitrite ion. Here33 the isotope shift depends on the number of **Oatoms in the ion and is about 0.138 p.p.m. per '*O; in the nitrate ion the corresponding shift is 0.056 p.p.m.per l80.The acid catalysed exchange of oxygen between water and "0 nitrous acid can be monitored by "N n.m.r.. At pH 6.26 and 28°C a sequential mode of oxygen atom exchange was found in agreement with a mechanism postulated earlier on the basis of indirect evidence. 4 Interactions in Solutions Reactions of phenylphosphonic acid and phenylphosphonic dichloride with the strong acid solvents 100% H2S04,25% and 65% oleum and chlorosulphuric acid have been inve~tigated~~ by 31P n.m.r. which shows that initial protonation of the phosphoryl oxygen varies with solvent and solute and except for 100% sulphuric acid may be followed by sulphonation of the aromatic group and/or condensation of the phosphonic acid or solvolysis of the dichloride. Slow chlorination occurs for phenylphosphonic acid dissolved in chlorosulphuric acid.Attempts to prepare the mono-chloride PhPClO(0H) did not yield the pure compound but the mixtures obtained reacted in a way that was consistent with the mono-chloride participating in the reactions mentioned above. 32 J. M. Risley and R. L. Van Etten J. Am Chem. SOC.,1981,103,4389. 33 R. L. Van Etten and J. M. Risley J. Am.Chem. SOC.,1981,103,5633. 34 K.B.Diilon M. P. Nisbet and T. Waddington J. Chem. SOC., Dalton Trans. 1981,212. Physical Methods and Techniques -Part (ii) N.M.R.Spectroscopy 23 As the temperature of the sample is lowered to -74 "C the 'H n.m.r. spectrum of 1-methoxycarbonylpiperidine(21) in FS03H shows3' the appearance of a broad peak at 66.7 (1 H) and an increase in the complexity of the signal from the C-2 protons.In contrast the amide (22) gives a sharp (1H) peak at 69.2 (a shift characteristic of 0-protonation) and no change in the C-2 proton signal. It was concluded that (21) is predominantly N-protonated in fluorosulphuric acid. The variation of 13C chemical shifts with acidity in sulphuric acid indicated that a switch to predominant 0-protonation occurs at lower acidities. Me-C-N'3 (21) (22) Low temperature 'H and 13C n.m.r. spectra that in super-acid media (SbF,-FSO,H) the mono- and dialkyl-substituted thioanisoles investigated were protonated at the sulphur atom. A doublet at about 63.2 can be assigned to the methyl thio-group while the integrals and multiplicities of peaks indicate that the signal due to H' on sulphur is hidden in the aromatic region of the spectrum.In the 13C spectrum the carbon atom of the methyl thio-group is most affected by protonation showing an upfield shift of about 20 p.p.m. In 1:1SbFs-FS03H solvent 2,6-dimethyl-y-pyrone (23; X = 0)and its sulphur and nitrogen analogues (23; X = S or NMe) are doubly protonated on the exocyclic oxygen atom37 while the 4-thione derivative is singly protonated at the exocyclic sulphur atom. 0 (23) N.m.r. line-shape analysis has been to study the kinetics of proton exchange between fluorosulphuric acid and protonated solute bases in super-acid media. The rates of proton exchange have been used to show that over the range from 0 to 90 mol% of SbF the acidity increases monotonically despite the low concentration of acidic protons in such solutions.The value of Ho for 90mol% SbFS is about -26.5 which is the most acidic liquid Bronsted acid measured so far though there are indications that mixtures of hydrofluoric acid with an excess of SbFs may be even stronger acids Differences in pK between the bases examined were also derived. In these media water appears to be a slightly weaker base than acetone the solute species produced being very different from the hydrated oxonium ion formed in aqueous media. The effects of solvent temperature and concentration on 170and 14N n.m.r. spectra have been determined39 for some simple amides. Hydrogen bonding causes 35 P. J. Battye J. F. Cassidy and R. B. Moodie J. Chem. SOC.,Chem.Commun. 1981 68. 36 M. Eckert-MaksiC J. Chem. SOC.,Perkin Trans.2 1981 62. 37 V. Gold and T. Mah J. Chem. SOC., Perkin Trans.2 1981 812. 38 V. Gold K. Laali K. P. Morris and L. Z. Zdunek J. Chem. SOC., Chem. Commun. 1981,769. 39 M. I. Burgar T. E. St. Amour and D. Fiat J. Phys. Chem. 1981,85 502. 24 R.F.M. White relatively large changes in amide 170 chemical shifts increased hydrogen bonding being associated with an increase in shielding. Smaller variation is observed for 14 N paramagnetic shifts being observed on increased proton donation of the directly bonded amide N protons. At elevated temperaturFs 14N-lH coupling constants were found to be 60 * 5 Hz (formamide) and 67 f5 Hz (N-methyl formamide). In addition to solvent polarity-polarizability and solvent-to-solute hydrogen bond- ing effects solute-to-solvent hydrogen bonding by the second protons of self- associated formamide leads to "N shifts being dependent on the solvent hydrogen bond acceptor basi~ity;~' self association of N-monoalkyl amides is sufficiently strong for this effect to be negligible.The results of infrared 'H and I9F n.m.r. spectroscopic on solutions of alkali-metal fluorides in amides! support the finding from LCAO-MO-SCF calculations estimating the strength of the amide- fluoride ion hydrogen bond at about 148 kJ mol-' this is the second strongest hydrogen bond known. The possible chair conformations of myo-inositol can be described as equatorial (24) (having five equatorial -OH groups) or axial (25).Changes in the 31P spectrum of myo-inositol hexaphosphate with pH have been interpreted4* in terms of conformational change with degree of protonation. Above pH 12 the equatorial structure [cf,(24)] is preferred while below this the axial conformation [cf. (25)] is favoured until pH 5 when on addition of a seventh proton there is a reversion to the equatorial structure. There is evidence that at lower pH the axial conformation is again favoured. Nitration of N,Ndimethylaniline with H"NO in 85-90'/0 sulphuric acid shows evidence43 of "N chemically induced dynamic nuclear polarization in the puru-nitro-product from which it is concluded that at least part of this product is formed by a reaction involving radical pairs probably the nitrous acid-catalysed reaction.Depending on the direction of the reaction 15N02-'4N02 exchange reactions of the ion (26) with nitric acid gives44 either enhanced absorption or emission in the kMMe* MefJMe Me NO2 (26) *O M. J. Kamlet C. Dickinson and R. W. Taft J. Chem. SOC.,Perkin Trans. 2 1981 353 41 J. Emsley D. J. Jones J. M. Millet R. E. Overill and R. A. Waddilove J. Am. Chem. Soc. 1981 103,24. 42 J. Emsley and S. Niazi Phosphorus Sulfur 1981 10,401. 43 J. H. Ridd and J. P. B. Sandall J. Chem. SOC.,Chem. Commun. 1981,402. 44 P.Helsby and J. H. Ridd J. Chem. SOC.,Chem. Commun. 1981,825. Physical Methods and Techniques -Part (ii) N.M.R. Spectroscopy 25 15 N spectrum showing that some if not all of the isotopic exchange proceeds via the radical cation (27).ye2 MQMe'. Me (27) 5 Solid-state Studies In last year's article' examples were given of the splitting into broadened doublets observed in the spectra of solid state samples in which 13C atoms are bonded to nitrogen. Magic-angle sample-spinning is unable to eliminate 13C-14N dipolar interactions because of the 14N quadrupole moment. Natural [14N]glycine has an asymmetrically split aC resonance4' while 15N labelled material gives a 13C solid- state n.m.r. spectrum with sharp lines for both of the carbon atoms. Using a 13C-14N bond length of 1.49 A gives the calculated value of the 14N quadrupole coupling constant as 1.18MHz with an asymmetry parameter of 0.54. From the 13C-13C and 13C-14N dipolar splittings in the 13C n.m.r.spectrum of a single crystal sample of gly~ine~~ bond $stances rC4 = 1.543 f 0.008 A rC-N = 1.509 f 0.009 A and the bond angle CCN 111.1f 1.0" were obtained. The 14N spectrum was used to find the 14N quadrupole coupling constant (1.18 f 0.01 MHz) and the asymmetry parameter (0.54 f 0.01); Vzzis approximately along the C-N bond and V is almost perpendicular to the CCN plane. In the vapour phase and in solution the diazoles pyrazole and imidazole exist in tautomeric equilibria (28) and (29). Proton exchange in imidazole has not been slowed down sufficiently for separate signals to be observed for protons H-4 and H-5, for carbons C-4 and C-5 or for nitrogens N-1 and N-3. The high resolution solid-state 13C n.m.r. spectra of pyrazole and imidazole each show4' three carbon resonances the shifts for pyrazole being similar to those obtained for pyrazole in solution at low temperatures.The shifts for solid imidazole compare with those for N-methylimidazole in CDC13 (C-2 137.6 C-4 129.3 and C-5 119.7 p.p.m.); the assignments for solid imidazole are given in (30). (30) 45 J. G. Hexem M. H. Frey and S. J. Opella J Am. Chem. Soc. 1981,103,224. 46 R. A. Haberkorn R. E. Stark H. Van Willigen and R. G. Griffin J.Am. Chem. SOC.,1981,103,2534. '' J. Elguero A. Fruchier and V. Pellegrin J. Chem. Soc. Chem. Commun. 1981 1207. 48 M. Begtrup R. M. Claramunt and J. Elguero J. Chem. Soc.. Perkin Trans. 2,1978,99. R. F.M. White Introduction of a paramagnetic impurity has been suggested49 as an effective aid to obtaining 13C spectra from solid-state biomolecules which as pure compounds may have unfavourably long proton-relaxation times.Small (i.e. about 0.4%) amounts of Cu2' do not produce any observable shift or broadening in the 13C spectra of imidazole uracil thymine and cytosine.H,O whereas spectra could be obtained easily from the doped samples. These authors also report that imidazole shows three 13C peaks as a result of freezing out of tautomerism in the solid state. Assignment of the 13C peaks on the basis of broadening by the 14N quadrupole moments gives the correct assignment for C-2 but other factors prevent an exact assignment of the C-4 and C-5 resonances on this basis. A procedure has been described" for the preparation of unstable carbonium ions and their subsequent manipulation for high-resolution solid-state n.m.r.at about 77 K. The s-butyl cation EtCHMe' could be obtained in the solid state by high vacuum co-deposition of EtCHMeCl and SbFs on a liquid-nitrogen cooled surface. At the temperature of formation any 13C label was scrambled over the four-carbon linear unit but scrambling is too slow to cause coalescence of the s-butyl signals at -60 "C. Rearrangement to t-butyl cation occurs to a limited extent under the conditions of ion formation. Over the temperature range -60 to -190 "C there was no evidence for a static structure suggesting that the barrier for the degenerate hydride shift Me-CH-CH2-Me + + Me-CH2-CH-Me + may be less than 2.4 kcal mol-'. Comparison of 29Si n.m.r.spectra obtained for solid silicon polymers with results for the liquid state showS1 no evidence of special solid-state effects on 29Si chemical shifts so that relationships between shifts and molecular structure obtained for liquid samples can be used for the interpretation of solid-state 29Si spectra. This should offer detailed information about structural units in siloxane resin networks. The 29Si n.m.r. spectra of aluminosilicates show distinct signals for five possible numbers of AlO tetrahedra connected to the SiO tetrahedra in building units Si(OSi)4-n(OAl)n and a number of studies of zeolite structures have been rep~rted.~~-~~ Zeolites containing five-membered rings of silica tetrahedra give a 29Si peak at about -113 p.p.m. whereas other zeolites which do not contain this building unit give55 resonances in the range -80 to -106 p.p.m.6 Conformational Studies The crystal structure of the dinuclear complex (31)shows that the six-membered cyclic ligand adopts a boat conformation. The 'H n.m.r. spectrum of the compound cooled below -30°C is that predicted for a static structure that is three signals 49 S. Ganapathy A. Naito and C. A. McDowell J. Am. Chem. Soc. 1981,103 6011. "P.C. Myhre and C. S. Yannoni J. Am. Chem. Soc. 1981,103,230. '' G. Englehardt H. Jancke E. Lippmaa and A. Samoson J. Orgunomet. Chem. 1981,210 295. 52 G. Engelhardt E. Lippmaa and M. Magi J. Chem. SOC.,Chem. Commun. 1981,712. 53 J. M. Thomas L. A. Bursill E. A. Lodge A. K. Cheetham and C. A. Fyfe J. Chem. SOC.,Chem.Commun. 1981,276. "J. Klinowski J. M. Thomas M. Audier S. Vasudevan C. A. Fyfe and J. S. Hartman J. Chem. SOC. Chem. Commun. 1981,570. 55 J. B. Nagy J. P. Gilson and E. G. Derouane J. Chem. SOC.,Chem. Commun. 1981,1129. Physical Methods and Techniques -Part (ii) N.M.R. Spectroscopy +s'cYs s-c \I Pt ;Pt=Pt< I 101-I __. -Pt /Qt-T /4 a\ S-S -/ LS' from the different Pt-Me groups and two AB quartet patterns (intensities 2 1) from the two types of ring-methylene In addition all these peaks show coupling to one or two 19'Pt nuclei. Between -30 and +30"C signals from the Pt-Me trans-to the bridging chlorine atoms coalesce to a single line and the two AB quartets coalesce to a single AB quartet coupling to '"Pt being maintained throughout.These changes are interpreted in terms of a series of intramolecular 1,3-shifts in which the dinuclear metallic moiety commutes between pairs of sulphur atoms (32) a change in ring conformation from boat to chair almost certainly being involved. The "N n.m.r. spectrum of 5-azacytidine (33) in DMSO consistss7 of peaks at 207.9 (N-1 doublet 5 Hz); 184.4 (N-3 doublet 5 Hz); 159.2 (N-5 doublet of 4' Ribosyl (33) doublets 5 and 12 Hz); and 273.4 p.p.m. (N-4' triplet 91 Hz)from external H"NO in D20. Selective proton decoupling showed that the 5 Hz splittings of N-3 and N-5 each involve a single proton of the NH2 group indicating that rotation about the C-4-N-4' bond is slow. At 52°C the couplings to N-3 and N-5are lost as a s6 E.W. Abel M. Booth K. G. Orrell G. M. Pring andT. S. Cameron J. Chem. SOC.,Chem. Commun. 1981 29. '' J. D. Roberts G. R. Sullivan P. P. Pang and N. J. Leonard J. Org. Chem. 1981,46 1014. R.F. M. White result of either rapid rotation or intermolecular exchange of N-4' protons. If the former is the reason then AG" for the rotational process would be about 17 kcal-mol-' which is in the range observed for rotation of dimethylamino-groups in other cytosine derivatives. Like cytidine Sazacytidine is protonated largely at N-3. Increasing pressure leads to a low-field shift in the 'H spectrum of "N enriched formamide dissolved in acetones8 indicating a concomitant increase in hydrogen bonding. From variable pressure spectra the activation volume for rotation A V" (= dAG*/dp) was found to be (3.5 f 0.5) x m3 mol-' a value explained by steric effects.Breaking of a hydrogen bond is accompanied by an increase in volume of about 4 X m3 mol-'; if amide rotation involved simultaneous breaking of hydrogen bonds between amide and solvent molecules the expected value of A V" would be significantly larger than that actually observed. The observations9 of s7Fe satellites in the 13Cspectra of Fe2(C0)6S2 syn-and ~nfi-Fe~(CO)~(sMe)~ isomers and Fe3(CO)12 has given further understanding of the dynamic behaviour of these molecules in which carbonyl exchange occurs. Among the molecules containing one 13C atom in the carbonyl groups 4.4% will have one s7Fe atom; in half of these there will be a one-bond separation '3C-57Fe and in the other half there will be a two-bond separation 13C-Fe-s7Fe.Possible mechanisms of carbonyl exchange are (a) a polytopal rearrangement within each Fe(CO) moiety or (b) exchange between metal centres. For (a),the '7Fe satellite sub-spectrum would be two doublets with separations 'J57Fe-13c and 2J57Fe-13c. As the latter coupling is probably small the satellites arising from it would probably be hidden. If on the other hand (6) is the exchange process the satellite sub- spectrum should be one doublet only with a maximum separation of 3[1J57Fe-13c + 2J57Fe-13c 3. The values obtained for the sulphur containing compounds above range from 29.3 to 26.9 Hz favouring mechanism (a). '* J. Hauer G. Volkel and H.-D. Ludemann Chem. Phys.Letr. 1981,78,85. s9 S. Aime and D. Osella J. Organornet. Chem.,1981 214 C27.