MAS NMR

Volume 168, number 2

CHEMICAL PHYSICS LETTERS

AN EXTENSION OF THE CROSS-DEPOLARIZATION SUBSPECTRAL EDITING IN CP/MAS NMR

21 April 1990

METHOD:

J.S. HARTMAN ’ and J.A. RIPMEESTER Division of Chemistry, NationoiResearch Council of Canada. Ottawa, Ontario, Canada HA OR9 Received 8 May 1989; in final form 8 February 1990

A modification of the cross-depolarization method in CP/MAS NMR is described which allows simultaneous nuking of both methine and methylene carbon signals despite their different sensitivities to cross depolarization, Extension of the cross-depolarization time, partially inverting methine as well as methylene carbon signals, followed by a suitably chosen brief period of normal cross polarization (“cross repolarization”), nulls all signals arising from proton-bearing carbons in rigid-lattice environments. Since selective nulling of methylene or methine carbon signals is also possible by cross depolarization, spectral editing readily gives separate methine and methylene subspectra.

procedure a null in the signal of interest can be produced by adjusting 72 so that

1. Introduction Dipolar dephasing [ 1,2] is an important technique in “C CPjMAS NMR for the removal of signals arising from proton-bearing carbons, and has found extensive use in the analysis of complex materials such as coals [ 3 1. Recently [ 4-61 there has been a revival of interest in the technique of cross depolarization, first proposed in 198 1 [ 71, as an alternative means of separating proton-bearing from non-proton-bearing carbon signals. In this technique, also known as inversion recovery cross polarization [ 6,7] a 180” phase change in the 13C (or ‘H) spin locking pulse, follows normal cross polarization (fig. la). The signal intensity M(7,, 72) is given by (ignoring effects of T,,)

rz=Tc,ln[2-

exp(-r,/Tor)]

.

This reduces simply to 7z= Tcph 2 for long crosspolarization times rl >> TdCH 1, T,,(CH, ), T,( CH) and T,(CH2) are usually different, so that it is not possible to obtain a simultaneous null for both methylene and methine carbons. However, by adding an extra cross-repolarization step (fig. lb) this again becomes possible. The signal intensity becomes M(r,,~,,Z3)a1-2exp(-t,/T,,) +2ew[-(73+72)I~~Pl

(3)

-exp[-(73+7~+7~)ITCPI.

+ exp[ - (rr +r,)/Tor]

,

(1)

where Tcp is the characteristic cross-polarization time constant, and the resulting “C spin population is driven to the negative rather than the positive z axis (fig. la). Tcpfor proton-bearing carbons is much smaller than Top for quaternary carbons. With this

(2)

Again assuming that r, > T,,( CH), T, (CH2) a null can now be obtained with r3=TcP[ln2-2exp(-r2/TcP)].

(4)

A simultaneous null is obtained for methylene and methine carbons on equating two expressions of the type shown in eq. (4)) T,p(CH2){ln2-2exp[-7,lT,,(CH2)1}

’ Permanent address: Department of Chemistry, Brock University, St. Catharines, Ontario, Canada LZS 3A 1.

=To,(CH){ln2-2exp[

-r2/TcP(CH)]}.

(5)

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(b)

*Jk (d)

II

I1l’l’l1.I’I’J’I

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266

180

160

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IIJIII~ 120

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66

66

46

20

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Fig. 2.75.47 MHz 13CCP/MAS spectra ( 1600 scans) ofcstrone (I): (a) the standard CP/MAS spectrum; (b) cross-depolarization time of 47 us; (c) crossdepolarization time of 24 p; (d) cross-depolarization and cross-repolariration times of 50 and 20 ps, respectively. The small sharp peaks arc spinning sidebands.

sity, by using dipolar dephasing times of 30 us.

3. Results and discussion Fig. 2 shows “C CP/MA!S spectra of estrone (I). All were obtained with the same number of scans and are normalized to the same peak height. Fig. 2a is the standard CP/MAS spectrum. Cross-depolarization spectra (figs. 2b and 2c) retain practically all the sensitivity of standard CP/MAS spectra, but cannot

null both methine carbon and methylene carbon signals simultaneously. The methylene carbons are inherently more susceptible to cross depolarization as well as cross polarization because of their two nearby protons, and are inverted when the cross depolarization time is sufficient to null the methine carbon signals (fig. 2b). If cross-depolarization time is decreased in order to null the methylene carbon signals, methine signals of diminished intensity remain (fig. 2~). While these patterns are useful in peak assignments (somewhat analogous to DEPT in solu221

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tion NMR), there is loss of quantitative information based on peak areas, especially serious in complex materials in which peaks of different kinds of carbon overlap.

I

II

This drawback of cross depolarization can be overcome by extending the cross-depolarization time to partially invert methine as well as methylene signals, followed by a brief period of normal cross polarization (“cross repolarization” ) , as shown schematically in fig. lb. The methylenes, which have become the most negative, also recover the fastest, and by suitable choice of cross-depolarization and cross-repolarization times it should be possible to a good approximation to null both the methine and the methylene carbon signals, giving a spectrum consisting solely of the non-proton-bearing carbons. Total cross-depolarization/cross-repolarization times are sufficiently short that, as in simple cross depolarization, there should normally be only a minor effect on non-proton-bearing carbon peak intensities. Fig. 2d shows the cross-depolarization/cross-repolarization spectrum of estrone. There is very effective nulling of both the methylene and methine carbon peaks, with only very minor imperfections. Thus it appears that quantitative information can be retained by the cross-depolarization/cross-repolarization technique, provided that the cross polarization times Tcp of the signals to be simultaneously nulled are within about a factor of two. This is generally the case for methylene and methine carbon signals in the same rigid lattice environment [ 8,9]. The clean separation of the three proton-bearing from the three quatemary aromatic carbon signals of estrone (fig. 2) is promising for analysis of the aromatic carbons of coals. Other estrogens (estriol, l3esttadiol, and P-estradiol-3-methyl ether) behave similarly except for doubling of the resonances of most carbon positions of estriol and P-estradiol-Z methyl ether, consistent with the presence of two non222

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equivalent molecules in the unit cell which is typical of steroids of the estrogen family [ lo], (Even a tripling was observed for the aromatic resonances of P_ estradiol-3-methyl ether; this is not unprecedented [ 81.) Chemical shifts are roughly in line with solution values [ 111. The chemical shift differences of up to 4 ppm of the doubled or tripled resonances are attributed to differences in crystal packing and/or conformation. A similar sequence of i3C CP/MAS spectra of cholic acid (II) shows similar effects (fig. 3). The cross-depolarization technique has previously been applied to obtained spectra of non-proton-bearing carbons only, in a polymer in which methine but no methylene carbons were present [ 5 ], and to an analogous situation in *‘Si NMR [ 41. However, previous cross-depolarization studies [ 4-61 have not addressed the nulling of methylene and methine carbon signals simultaneously, or the separation of methylene from methine carbon signals. Dipolar dephasing, the more highly developed alternative for eliminating proton-bearing carbon signals in CP/MAS NMR [ 1,2], is based on interrupted decoupling: during a short interval before acquisition no proton decoupling is applied, and the carbon magnetization decays exponentially by a T2 process at rates determined by the strength of each carbon’s C-H dipolar coupling. Here too, non-proton-bearing carbon magnetization decays more slowly than methylene or methine carbon magnetization, and here too, with our compounds, the dephasing time can be chosen to eliminate most methylene carbon signal intensity while leaving appreciable methine intensity. Although selective nulling both in dipolar dephasing and in cross depolarization is based on differences in C-H dipolar coupling, and hence the relative rates of approach of proton-bearing and non-proton-bearing signal intensities to equilibrium should be similar under both techniques, S/N should be inherently poorer in dipolar dephasing spectra. This is because equilibrium under dipolar dephasing occurs at zero rather than negative intensity. Approaching zero intensity for the proton-bearing carbon peaks requires a time interval of several T2’s. This can appreciably attenuate even the non-pmtonbearing carbon signals which decay more slowly, usually by far more than a factor of two, but which are still on the steep part of their decay curve. Fur-

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It

I 160

I

I 140

I

I 120

I

I 100

I

I 80

I

I 60

I

I, 40

I

,I

20 ppm

Fig. 3. 75.47 MHz lJC CP/MAS spectra of cholic acid (II): (a) the standard CP/MAS spectrum; (b) crossdepolarization time of 45 ks; (c) cross-depolarization time of 25 ps; (d) cross-depolarization and cross-repolarization times of 60 and 25 ps, respectively. (a) was obtained with 1600 scans, and (b)-(d) with 3200 scans.

thermore, decays of non-proton-bearing carbon intensity do not follow simple exponential behaviour in dipolar dephasing [2]; there is a fast-decaying component which can be lost quickly. There can be wide variations in relative rates of dipolar dephasing of proton-bearing and non-proton-bearing carbons in different systems [2], and hence wide variations in S/N of dipolar dephasing spectra of the non-proton-bearing carbons. In addition, since dipolar dephasing requires delayed acquisition, there is a large linear phase shift across the spectrum, which can be

adjusted only with difficulty, especially for samples with broad lines. In contrast, in cross depolarization (a T,, process), the equilibrium is at inverted rather than zero intensity and therefore the proton-bearing carbon signals are nulled’in less than one TCHperiod while still far from equilibrium and still on the steep pa? of their decay curve. This short time interval allows relatively less attenuation of the non-proton-bearing carbon signals, which can have Tcp values not vastly greater than those of the proton-bearing carbons. 223

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(Ratios of Tcp values for non-proton-bearing

versus

though f%s. 4b and 4c arise from diminished signals and therefore have relatively poor signal strength and any imperfections of nulling are magnified, nevertheless quite adequate subspectra are obtained. The potential advantages of such subspectra are obvious in the study of complex materials. Methylene and methine subspectra are obtainable provided that the T,, values of a material vary in the normal order, methylene carbon < methine carbon < non-proton-bearing carbon [ 8,9], and provided that the TCH values of all methylene and methine carbons do not cover such a wide range as to prevent simultaneous cross repolarization. Further work is required to probe these limitations. (Cethoxyphenyl)acetic acid, in which Ton values of methylene carbons are not clearly separated from those of

proton-bearing carbons range from 1.7 to 4 in the compounds listed in table IV of ref. [ 81.) This can account for the fact that we obtained far better S/N in our cross-depolarization tion/cross-repolarization

and cross-depolariza-

spectra than we have been

able to obtain with the same number of scans in dipolar dephasing spectra

of our model compounds. Further subspectral analysis is possible using crossdepolarization/cross-repolarization spectra, as shown in fig. 4. Subtraction of different fig. 3 spectra gives a proton-bearing carbon only spectrum (fig. 4a), a methylene-carbon-only spectrum (fig. 4b), and a methine-carbon-only spectrum (fig. 4c) of cholic acid. (Methyl signals cannot be completely eliminated but are normally readily identifiable.) Al-

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CHEMICALPHYSICSLETTERS

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II 160

I 146

I

III 120

I 100

I 80

I

I 66

I

I 40

I

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I

I

wm

Fig. 4. Cholicacid subspectraobtainedby subtractionof spectraof fig. 3. (a) Proton-bearingcarbonspectrum(fig. 3a-fig 3d); (b) 3b); (c) metbine carbon spectrum (fig. 3c-fig. 3d).

methylene carbon spectrum (fig. Jd-fig.

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methine carbons [ 8 1, is an example in which methylene and methine subspectra may not be obtainable, although it should be straightforward to null at methylene and methine carbon signals simultaneously by cross depolarization/cross repolarization. The last figure shows the depolarization and dipolar dephasing techniques applied to a coal, in this case a sample obtained from the Argonne Premium Coal Sample Bank (Pocahontas) [ 121, to suppress the signals due to proton-bearing carbons. The normal CP/MAS spectrum is shown in fig. 5a, the depolarized spectrum in fig. 5c, and the dephased spectrum in fig. 5b. The two techniques show essentially identical results, with the aromatic carbon divided into quaternary and proton-bearing fractions in the ratio of 3 : 2. Again, the cross-depolarization technique does not require the large linear phase correction necessary in dipolar dephased spectra. Compared to solution NMR there are relatively

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few techniques for subspectral editing in solids; notably dipolar dephasing, separated local field methods [ 13 1, and techniques that are applicable to carbon attached to quadrupolar nuclei such as 14Nand ‘H [ 141. Cross depolarization/cross repolarization may prove to be a simpler alternative to the recently developed asynchronous magic angle spinning separated local field (MASSLF) method for distinguishing methine from methylene carbons [ 13 1, and may be useful addition to the many NMR techniques now in use in coal analysis [ 15 1.

Acknowledgement We thank Brock University for providing sabbatical leave (to JSH ), the National Research Council of Canada for providing financial support under the Visiting Scientists Program, and Professor H.L. Holland for the loan of estrone and related estrogens.

References [ 11M. Alla and E. Lippmaa, Chem. Phys. Letters 37 ( 1976)

(b)

(a)

I

Fig. 5. (a) “C CP/MAS spectra of Pocabontas lvb coal; (b) spectrum obtained with 60 ps dipolar dephasing time; (c) SW trum obtained with cross depolarization/cross repolarization.

260; S.J. Opella and M.H. Frei, J. Am. Chem. Sot. 101 (1979) 5854. [ 21 L.B. Alemany, D.M. Grant, T.D. Alger and R.J. Pugmire, J. Am. Chem. Sot. 105 (1983) 6697. [ 3] A. Soderquist, D.J. Burton, R.J. Pugmire, A.J. Beeler, D.M. Grant, B. Durrand andA.Y. Huk, Energy Fuel 1 ( 1987) 50, and references therein. [ 41 N. Zumbulyadis, J. Chem. Phys. 86 (1987) 1162. [ 51 S. Kaplan, E.M. Conwell, A.F. Richter and A.G. MacDiarmid, J. Am. Chem. Sot. 110 (1988) 7647. [ 61 D.G. Gory, Chem. Phys. Letters 152 (1988) 431. [7] M.T. Melchior, Poster B-29, 22nd Experimental NMR Conference, Asilormar, California (April 1981) . [ 8] L.B. Alemany, D.M. Grant, R.J. Pugmire, T.D. Alger and ICW. Zilm, J. AmChem. Sot. 105 (1983) 2133. [ 91 D.E. Axelson, Solid state nuclear magnetic resonance of fossil’ fuels: an experimental approach (Multiscience Publications Limited; Canadian Government Publishing Centre, Supply and Services Canada, Ottawa, 1985). [ 10 ] B. Busetta, C. Courseille and M. Hospital, Atia Cry&. B 29 (1973) 298; A. Cooper, D.A. Norton and H. Hauptman, Acta Cryst. B 25 (1969) 814; B. Busetta, Y. Barrans, G. Precigoux and M. Hospital, Acta Cryst. B 32 ( 1976) 1290. [ 111J.W. Blund and J.B. Stothers, Org. Magn. Reson. 9 ( 1977) 439.

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[ 121 KS. Vorres, Users Handbook for the Argonne Premium Coal Sample Program, 0%~ of Basic Energy Sciences, Division of Chemical Sciences, United States Department of Energy (1989). [ 131 G.G. Webb and K.W. Zilm, J. Am. Chem. Sot. 11 (1989) 2455.

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[ 141J.C. Hexem, M.H. Freyand S.J. Gpella, J. Am. Chem. Sot. 103 (1981) 467; P. Barker, N.E. Burlinson, B.A. Dunell and J.A. Ripmeester, J. Magn. Reson. 60 ( 1984) 486. [ 15]N.K.Sethi,R.J.Pugmire,J.C.FacelliandD.M.Grant,Anal. Chem. 60 ( 1988) 1574, and references therein.