Carbon-13 zero-field NMR in high field

JOURNAL

OF MAGNETIC

RESONANCE

89,205-209

( 1990)

Carbon-13 Zero-Field NMR in High Field ROBERTTYCKOANDGARYDABBAGH AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, New Jerse,v 07974 AND

JAMESC.DUCHAMPANDKURTW.ZILM Department of Chemistry, Yale University, New Haven. Connecticut 06511 Received

April

5, 1990

The dependence of nuclear spin interactions in high field on molecular or crystallite orientation results in inhomogeneously broadened, “powder pattern” lineshapes in NMR spectra of polycrystalline and noncrystalline solids. This orientation dependence frequently prevents or complicates the determination of molecular structures in solids by NMR. Recently, the technique of zero-field NMR in high-field (zfhf NMR) has been introduced as a means of obtaining high-field NMR spectra of solids that are free of inhomogeneous broadening, yet retain the structural information inherent in nuclear spin couplings ( 1-3). Zfhf spectra of small groups of spins coupled by magnetic dipole-dipole interactions show sharp lines with resolved splittings that depend only on internuclear distances. Zfhf spectra are obtained by combining rapid sample rotation with the synchronous application of resonant RF pulse sequences. Proper combinations of sample rotations and RF irradiations have the effect of averaging the orientationdependent, high field dipole-dipole couplings-and, in principle, quadrupole couphngs-to the form of scalar, zero-field couplings. High-field NMR spectra with the appearance of zero-field spectra are thereby obtained with the full sensitivity and isotopic selectivity of high-field NMR. Previous experimental demonstrations of zfhf NMR have focused on spectra of small groups of protons obtained by diluting selectively deuterated molecules in perdeuterated host matrices (2,3). In this Communication, we present the first examples of i3C zfhf NMR spectra. These examples are significant because it is frequently simpler to synthesize i3C-labeled organic molecules and polymers than it is to synthesize molecules that are selectively and sufficiently highly deuterated. In addition, it may prove possible to apply the techniques described below to natural abundance 13C NMR. The 13C zfhf spectra presented here also represent a clear demonstration of the isotopic selectivity of the technique. A true zero-field NMR spectrum of a typical ‘3C-labeled organic material, obtained for example by field cycling techniques (49), would consist of a very large number of transitions from a coupled spin system that includes many protons as well as the carbon nuclei. Individual transitions would generally not be 205

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resolved. This is because no viable schemes for actively decoupling one isotope from another in zero field have been developed. In contrast, heteronuclear decoupling can be incorporated into a zfhf experiment, since the experiment takes place entirely in high field where different isotopes have well-separated resonant frequencies. The zfhf spectra described below therefore show only transitions arising from the 13C nuclei. Preliminary 13C zfhf experiments were carried out on a homebuilt double resonance spectrometer at Yale University. Spectra reported here were obtained on a Chemagnetics CMX spectrometer, operating at a 13C carrier frequency of 25.3 MHz, with a homebuilt double resonance probe that incorporates a Doty Scientific 7 mm highspeed spinner assembly and a homebuilt spinning rate stabilization system. Tune-up procedures were followed to calibrate the RF fields and to eliminate RF phase transients (3). Pulse sequences used to obtain zfhf spectra are shown in Fig. 2. Zfhf spectra are obtained in two-dimensional experiments as described previously (2, 3)) but with the addition of standard cross-polarization and proton decoupling. The pulse sequence in the tl dimension of Fig. 2a is constructed from the subcycles shown in Fig. 2b or 2c, as explained in detail in Ref. (3). The subcycle in’Fig. 2b ideally consists of four contiguous 76” pulses with amplitude u1 = 4.20/ rn, in frequency units, where rR is the sample rotation period. Five subcycles, with overall RF phase shifts in multiples of 72”, are concatenated to give a sequence with length rn. In order to average out anisotropic chemical shifts (CSA) and resonance offsets, 7r pulses are applied at the end of each ra period and the sign of all RF phases is reversed during alternate periods. In practice, 1.O ~CLS delays between pulses are required for phase-switching, and the r pulse length is 7.0 I.LS.To maintain the pulse sequence period equal to rR, the 76” pulses are reduced in length and increased in amplitude. In Fig. 2c, two subcycles, which differ only in the overall RF phase, are used to construct a sequence with period 2rR. As discussed in Ref. (3)) this sequence averages out the CSA over a period rn and Creates scalar average dipole-dipole couplings over a period 27a. In Fig. 2c, Ideally, the delay lengths in the subcycles are r = (21/36O)~R, T’ Vl = 18.2/~~. = ra/ 180, and 7” = (25/36O)rR. In practice, residual CSA effects must be removed by applying a pulses at the end of each 27s period and reversing the sign of all RF phases during alternate periods; 7’ must then be reduced slightly to maintain the pulse sequence period equal to 2rR. The sequence in Fig. 2c is preferred for systems in which the CSA is comparable to or larger than the dipole-dipole couplings. In such systems, it is advantageous to average out the CSA over a period that is shorter than the period over which scalar average dipole-dipole couplings are created. The sequence in Fig. 2b is preferred for systems in which the dipole-dipole couplings are larger than the CSA. In such systems, it is advantageous to create scalar average dipole-dipole couplings over the shorter period. Spectra of two double-labeled polycrystalline compounds are reported. ( CH3)2C( OH)SO,Na (bisulfite adduct of acetone, or BSA), labeled at both methyl carbons, was prepared by reaction of a mixture of double-labeled and unlabeled acetone with NaHS03(,+ followed by precipitation with ethanol. NH&O2 ( CH2 )$Oz NH4 (diammonium succinate, or DAS), labeled at both methylene carbons, was prepared

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FIG. 1. (a) High-field “C powder pattern spectrum of polycrystalline bisulfite adduct of acetone. Five percent of the molecules are ‘3C-labeled at both methyl carbons. (b) 13Czfhf spectrum of the same material, obtained with the pulse sequence in Fig. 2c ( TV = 400.0 ps, 48 t, points, 100 shots per t, point). Sharp Iines with a dipolar splitting determined only by the distance between methyl carbons are seen. ( c) Powder pattern spectrum of diammonium succinate. Thirteen percent of the molecules are labeled at both methylene carbons. (d) Zfhf spectrum of the same material, obtained with the pulse sequence in Fig. 2a ( rR = 167.0 prs,48 t, points, 32 shots per t, point).

by bubbling NH3(,, through an acetone solution of a mixture of double-labeled and unlabeled succinic acid. Figures la and lb are high-field powder pattern and zfhf 13C NMR spectra of BSA in which 5% of the molecules are double-labeled. The powder pattern lineshape is a convolution of CSA and dipolar patterns. No dipolar splitting is apparent. The zfbf spectrum, obtained with the sequence in Fig. 2c with 7~ = 400.0 PS,consists of three equally spaced lines with an outer splitting of 110 Hz. Taking account of the dip&u scaling factor of 0.076 for this pulse sequence (3), the observed splitting implies a carbon-carbon distance of 2.50 A. This distance should be compared with the 2.5 1 8, distance between methyl carbons calculated for a tetrahedral geometry and 1.54 A carbon-carbon single-bond distances. The true zero-field spectrum of a system of randomly oriented pairs of coupled nuclei consists of three lines of equal intensity. The extra intensity of the center line in Fig. lb is largely attributable to single 13C nuclei at natural abundance.

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FIG. 2. (a) Two-dimensional experiment for obtaining 13Czfhf spectra with cross-polarization and proton decoupling. (b, c) Pulse sequences for the dimension of the experiment. When applied in synchrony with rapid sample rotation, these sequences average the high-field dipole-dipole couplings to a scalar, zero-field form. The angle between the sample rotation axis and the dc magnetic field is 64” in b and 66” in c. The notation 19~represents an RF pulse with flip angle 0 and phase 4, in degrees.

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Figures lc and 1d are spectra of DAS in which 13% of the molecules are doublelabeled. Again, the powder pattern spectrum is the result of a convolution of CSA and dipolar patterns. Three well-resolved lines are seen in the zfhf spectrum, which was obtained with the sequence in Fig. 2b, for which the dipolar scaling factor is 0.082 (3), with 7a = 167.0 ps. The outer splitting of 497 Hz implies a distance between methylene carbons of 1.55 A. The high resolution apparent in Fig. 1b, with 12 Hz linewidths, results from several experimental considerations. First, carrying out the experiments at the comparatively low field of 2.36 T reduces line broadening and distortion of the zfhf spectra due to the CSA. Second, proton decoupling fields of 160 kHz (37.6 G) are used during the t, dimension in Fig. 2a. Very strong decoupling fields are required to efficiently decouple protons from carbons while a pulse train is being applied to the carbons. Reducing the decoupling field to 120 kHz (28.2 G) results in an additional line broadening of 10 Hz. Third, the sample volume is restricted to a 3.2 X 6.5 mm cylinder to reduce RF inhomogeneity to less than +4%.

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in conclusion, we have demonstrated that it is possible to obtain 13C zfhf spectra of labeled molecules with high resolution and high sensitivity. The dipolar splittings observed in the spectra give accurate measurements of specific internuclear distances. The resolution achieved on model compounds-if it can be generalized to other systems of interest-indicates that carbon-carbon distances as large as 4 i\ can be measured with the zfhf technique. The technique may therefore prove useful for determining molecular structures or conformational probabilities in polycrystalline and noncrystalline solids. REFERENCES I. R. TYCKO, 2. R. TYCKO, 3. R. TYCKO,

J. Magn. Reson. 75, 193 (1987). Phys. Rev. Lett. 60,2734 (1988). J. Chem. Phys. 92,57X (1990). 4. N. F. RAMSEY AND R. V. POUND, Phys. Rev. 81,278 5. D.

P. WEITEKAMP,

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(1983). 6. D. B. ZAX, A. BIELECKI, K. W. ZILM, A. PINES, AND D. P. WEITEKAMP, J. Chem. Phy.s. 83, 487: (1985). 7. J. M. MILLAR, A. M. THAYER, A. BIELECKI, D. B. ZAX, AND A. PINES, J. Chem. Ph.w, 83,934 (1985). 8. R. KREIS, D. SUTER, AND R. R. ERNST, Chem. Phys. Lett. 118, 120 ( 1985 ). Y R. KREIS, A. THOMAS, W. STUDER, AND R. R. ERNST. J. Chem. Phys. 89.6623 ( 1988).