The necessity for synchronization in DANTE experiments with rotating samples. Reclamation of the asynchronous DANTE pulse train by combination with the TOSS sequence

JOURNAL

OF MAGNETIC

RESONANCE

90,426-432

( 1990)

The Necessity for Synchronization in DANTE Experiments with Rotating Samples. Reclamation of the Asynchronous DANTE Pulse Train by Combination with the TOSS Sequence TOSHIHITO

NAKAI

AND

CHARLES

A.

MCDOWELL

Depurtment cf Chemistry, The University of British Cdumhiu, 2036 ,&fain Mull. Vancouver, British Columbia, Cunudu V6T 1 Y6 Received

July 2, 1990

In recent NMR spectroscopic studies, frequency-selective excitation techniques have become increasingly important. One of their roles is the simplification of spectra by eliminating unwanted resonance lines (1, 2). Another is extracting specific slices of full multidimensional spectra such as two-dimensional exchange/spin diffusion spectra (3), 2D separation spectra (4), and imaging (5). Among such techniques, the DANTE pulse train (6) prevails widely because one can compose the pulse sequence with hard pulses which are more stable in amplitude, and more easily available, than soft pulses, and, moreover, one can readily adjust the effective field strength by altering the pulse spacing. In solid-state NMR, the DANTE pulse train is usually applied to powdered samples under magic-angle spinning (MAS) as well as to single crystals, so that it can operate on specific well-resolved lines ( I). In the MAS experiment, each pulse of the DANTE train should be irradiated synchronously with the spinning period: all the transverse magnetizations due to crystallites are individually modulated by MAS and only become in-phase after every spinning period (7). However, not only is this synchronization difficult in practice but also there are some disadvantages which ensue following a long pulse interval fixed at the spinning period. typically 250400 ps: such a long interval often renders the selective band narrower than the linewidth. Also, the magnetizations decay, in compounds with short T2 values, during the pulse train. Sample heating may occur in the cases where a heteronuclear decoupling field is applied simultaneously. Caravatti et al. ( I ) showed experimentally that synchronization is not necessary for selective saturation with the DANTE-90” pulse train. In addition, Bork and Schaefer (8) recently pointed out that the asynchronous DANTE-180” pulse train inverts an entire spinning sideband family and attempted to explain this observation qualitatively. In fact, we can confirm that the asynchronous DANTE pulse train works properly as long as the spinning frequency, UR, is large compared with chemical-shift anisotropy, AU. Figures 1a- 1c show the ‘%I spectra for polycrystalline glycine observed at vR = 4.0 kHz using the CP (cross-polarization)MAS-DANTE saturation/inversion sequence which consists of the pulse train CP-90”-DANTE-delay-90°-acquisition. ln Figs. 1a- 1C, one can see that the asynchronous pulse train almost completely saturates 0022.2364/90 $3.00 Copyright ‘c 19YOby Acadcmlr Press. Inc. All rights ofrcproductmn I” any form reserved.

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LL

(a)

(d)

(e)

(b)

A.

(f)

(c)

II

’ 10

I 0

’

I

1

1

-10

I 10

I

I 0

I

’

I

-10

kHz FIG. 1. “C asynchronous DANTE saturation/ inversion spectra for polycrystalline glycine under MAS at (a-c) vR = 4.0 kHz and (d-f) 1.5 kHr. The carrier is set to the center peak of the carboxylic carbon at 0 kHz. The DANTE pulse train is composed ofpairs ofsuccessive pulses. (27.5”),-(25.7”);: the individual pairs are spaced by 20 us. The numbers of pairs, N, are (a, d) 0, (b. e) 50 (DANTE-90”). and (c, f) 100 (DANTE180” ). respectively. The magnetizations are enhanced using the Hartman-Hahn CP with a contact time of 1 ms: the RF field for both ‘H and “C was 77 kHz. The ‘H decoupling field is applied during the DANTE pulse train and the acquisition period, while. during the delay of 10 ms between them, the RF field is turned off. The recycle time was 3 s. For the spectra of (a-c) and (d-f), 16 and 64 FIDs were accumulated, respectively.

and inverts the resonance line ascribed to the carboxylic carbon together with its small spinning sidebands. Also, the numbers of small-flip-angle pulses. 50 and 100, necessary for saturation and inversion, respectively, coincide with those expected by calculation: we used ( 1.8O)$ pulses consisting of two successive long antiphase pulses, (27.5”),-( 25.7”);, thereby overcoming the nonideality ascribed to pulse transients.

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However, the asynchronous DANTE pulse train is not effective in the case where the spinning frequency is not large in comparison with the chemical-shift anisotropy. One may often encounter such a situation with a commercial superconductivity magnet operating at 200-500 MHz for ‘H and the sample spinner rotating at l-4 kHz. Figures 1d- 1f show the spectra for glycine with experimental parameters the same as those for Figs. la-lc other than the spinning frequency, which is now uR = 1.5 kHz. In this case, 50 and 100 small-flip pulses are, respectively, not sufficient for saturation and inversion of the center peak. Furthermore, the DANTE pulses do not have uniform influence on the entire sideband family although Caravatti rt al. ( I ) asserted that the pulse train can saturate all the spinning sidebands. Figure 2 shows the relation between the number of small-flip pulses, N, and the total intensity of the spinning sidebands, I, for uR = 1.5 and 4.0 kHz. At a glance. the asynchronous DANTE pulse train under slow spinning conditions proves not to invert the resonance line perfectly no matter how many pulses may be applied. It follows that 1D exchange/spin diffusion experiments will suffer from serious sensitivity losses if the magnetization is not inverted completely in the preparation period. The undesirable phenomena mentioned above can be understood by considering the magnetization paths ( 9). The directions of the magnetizations due to individual crystallites differ, even if on-resonance, because the chemical-shift interactions are modulated by MAS. and consequently execute inherent paths: their breadths are about L/vn (rad). and their positions specified by the centers of gravity also deviate from the initial direction of the magnetizations by approximately &J/v~ at the maximum

FIG 2. The relation between the numbers of small-flip pulses. h’, and the total intensity I, of the carboxylic carbon in glycine in the DANTE saturation/inversion experiments = 4.0 kHz (filled circles) and I .5 kHz (filled squares). The intensity is normalized with the case where N = 0. The experimental parameters are the same as those in Fig. I.

of the sidebands, performed at uR respect to that in

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FIG. 3. A pictorial representation of magnetizations in the rotating frame due to certain crystallites. The carrier frequency is assumed to coincide with the Larmor frequency. The magnetizations can precess only because of the chemical-shift anisotropy modulated by MAS as indicated in (b) and (c). The DANTE pulses along the ~1’ axis are designed to flip the magnetizations initially prepared along the +z axis to the -z axis through the +x axis direction as depicted in (a). In practice. a DANTE pulse may flip back the magnetization toward the +z axis direction as in (b). or have no effect on the magnetization as shown in (c). depending on the particular crystallites.

( 9). Both the breadths and the positions of the paths yield a distribution of the directions of the instantaneous magnetizations since they vary according to the orientations of the crystallites. If a small-flip pulse is applied during the spinning period. it rarely rotates the magnetizations properly as depicted in Fig. 3a, but may flip backward (Fig. 3b), or have no effect (Fig. 3c), according to the particular crystallites affected. Obviously, these phenomena are all the more remarkable when the distribution of the magnetizations is widely spread. The asynchronous DANTE pulse train. therefore, loses its effectiveness under the condition that the spinning frequency does not exceed the chemical-shift anisotropy, namely L/vR > 1; it takes more pulses than expected to saturate. or invert; and it cannot completely invert all the magnetizations of the

-l-r 2 hetero.

‘H

decoupl

N CP

TTlTlTlT

-lT 2

13 C

Toss

Dante

FIG 4. A pulse sequence for the TOSS-DANTE excitation measurements. The transverse magnetizations prepared with CP are aligned by the TOSS sequence so that the DANTE pulse train which follows can effectively store them along the +z or PI axis direction without spinner synchronization. The ‘H decoupling held is applied during the TOSS and DANTE sequences and the acquisition period.

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N 0

32

64

96

126

FIG 5. The relation between the numbers of small-flip pulses, A’, and the total intensity of the spinning sidebands, I, of the carboxylic carbon in glycine in the simple DANTE and the combined TOSS-DANTE (Fig. 4) excitation experiments: the former is performed at v R = 4.0 kHz (filled circles) and 1.5 kHz (filled squares), whereas the latter is done at 1,R = 1.5 kHa alone (open squares). The intensity is normalized with respect to that in the CPMAS spectra at the corresponding spinning frequencies. The DANTE pulse train is composed of pairs of successive pulses, (29. l”)m-(27.7”);; the individual pairs are spaced by 100 ps. The RF field for both ‘H and “C is 71 kHz. The other experimental parameters are the same as those adapted for Fig. 1.

crystallites, since the pulses do not effect them coherently. Also, the repetition of the precession by modulated chemical-shift interactions and the small-angle flipping by RF pulses disturb the paths of individual magnetizations in a complicated way so that the entire spinning sideband family cannot be influenced in-phase. Some manipulations to reduce the distribution of the magnetization directions are required to recover the efficiency of the asynchronous DANTE sequence in the case of slow spinning, or large chemical-shift anisotropy. Unfortunately, the inherent magnetization path breadths cannot be changed if another pulse train is not applied simultaneously with the DANTE pulses. Instead of such complicated manipulations, we propose a method which greatly reduces the deviations of magnetization directions by aligning the centers of gravity of the paths before the DANTE pulse train is irradiated. Griffin and co-workers (9, IO) found that the TOSS (fatal sideband suppression) sequence (I I) has that desired effect. We combine the TOSS sequence with the DANTE excitation sequence which does not include the first storing 90” pulse, since the former sequence has adequate influences only on the transverse magnetizations. Figure 4 shows the combined TOSS-DANTE pulse sequence. The DANTE pulse train should start at the normal TOSS acquisition point when the centers of gravity of all the magnetization paths point in the same direction. Figure 5 shows the relation between the number of DANTE small-flip pulses and

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(b)

I

,

I

10

I 0

I

I

’

-10

kHz FIG. 6. ‘jC asynchronous (a) DANTE-90” and (b) TOSS-DANTE-90” excitation spectra for glycine under MAS at vn = 1.5 kHz: the number of DANTE small-flip pulses is set to 64 for both spectra. The carrier is adjusted to the center peak of the carboxylic carbon at 0 kHz. The experimental conditions are described in the legend to Fig. 5.

the total intensity of the spinning sidebands in the DANTE excitation and TOSSDANTE experiments. Each small-flip pulse ( 1.4” )@is composed of successive ( 29.1 o )$ and (27.7”); pulses. The intensity is normalized with respect to that in the CPMAS spectra. Again, one can see imperfect excitation at vR = 1.5 kHz, in contrast to the almost ideal excitation observed at vR = 4.0 kHz in the simple DANTE experiments: even in the latter experiments the intensity of DANTE-90” (N = 64) decreases by 20% compared with that in the CPMAS spectrum. This reduction is considered to be mainly attributable to decay during the DANTE pulse train and not to the inefficiency of the DANTE pulse sequence. On the contrary, the TOSS-DANTE sequence employed at vR = 1.5 kHz excites a signal of almost the same intensity as that in the DANTE experiment at vR = 4.0 kH z. Figure 6 shows the whole spectra observed in the DANTE-90” and the TOSS-DANTE-90” experiments at 1.5 kHz. The total intensity of the latter is enhanced 1.7 times that of the former. All the experiments were carried out on a Bruker MSL200 spectrometer with operating frequencies of 50.332 MHz for 13C and using a Doty Scientific MAS probe. Commercial polycrystalline glycine was utilized without additional purification. ACKNOWLEDGMENTS The

authors

thank

the Natural

Sciences

and Engineering

Research

Council

of Canada

for grants.

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