Two-dimensional satellite-transition MAS NMR of quadrupolar nuclei: shifted echoes, high-spin nuclei and resolution

21 September 2001

Chemical Physics Letters 345 (2001) 400±408

www.elsevier.com/locate/cplett

Two-dimensional satellite-transition MAS NMR of quadrupolar nuclei: shifted echoes, high-spin nuclei and resolution Kevin J. Pike 1, Sharon E. Ashbrook, Stephen Wimperis * School of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD, UK Received 6 June 2001; in ®nal form 2 August 2001

Abstract Modi®cations to the satellite-transition magic angle spinning (STMAS) experiment for obtaining high-resolution NMR spectra of quadrupolar nuclei are discussed. A phase-modulated `shifted-echo' STMAS experiment that yields pure absorptive lineshapes is presented and shown to be compatible with the `split-t1 ' technique used in multiplequantum (MQ) MAS NMR to reduce the duration of t2 acquisition and avoid shearing the ®nal two-dimensional spectrum. The application of STMAS to nuclei with spin greater than I ˆ 3/2 is also considered, the dispersion of isotropic shifts achieved by STMAS and MQMAS are compared, and the e€ects of anisotropic `cross-term' broadening mechanisms on linewidths in `isotropic' STMAS spectra are discussed. Ó 2001 Elsevier Science B.V. All rights reserved.

1. Introduction In 2000, Gan [1] introduced the two-dimensional satellite-transition magic angle spinning (STMAS) NMR experiment. As with the doublerotation (DOR) [2], dynamic angle spinning (DAS) [3], and multiple-quantum magic angle spinning (MQMAS) experiments [4], this new experiment allows NMR spectra of half-integer spin quadrupolar nuclei in powdered solids to be obtained that are free of ®rst- and second-order quadrupolar broadening. The high-resolution or isotropic NMR spectra of nuclei such as 11 B, 23 Na *

Corresponding author. Fax: +44-1392-263-434. E-mail address: [email protected] (S. Wimperis). 1 Present address: Department of Physics, University of Warwick, Coventry CV4 7AL, UK.

(spin quantum number I ˆ 3/2), 17 O, 27 Al (I ˆ 5/2), 59 Co (I ˆ 7/2) and 93 Nb (I ˆ 9/2) provided by these techniques have proved invaluable to studies of glasses, minerals, microporous solids and other inorganic materials. The STMAS experiment correlates the singlequantum central transitions (CT) (mI ˆ )1/2 M +1/ 2) and satellite transitions (ST) (mI ˆ ‹1/2 M ‹3/ 2, mI ˆ ‹3/2 M ‹5/2, etc.) of a half-integer spin I quadrupolar nucleus in a two-dimensional NMR experiment performed under MAS conditions. The CT is inherently free of ®rst-order quadrupolar broadening while a combination of MAS and rotor-synchronized acquisition removes the ®rst-order broadening of the ST. Under MAS conditions, the residual second-order quadrupolar broadening of the CT di€ers from that of the ST by a simple scaling factor and, therefore, it is possible, using

0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 9 1 2 - 5

K.J. Pike et al. / Chemical Physics Letters 345 (2001) 400±408

two-dimensional correlation, to refocus the second-order broadenings while retaining resolution due to isotropic shifts. Gan originally suggested two pulse sequences for STMAS correlation: a phase-modulated twopulse sequence and an amplitude-modulated z-®ltered three-pulse sequence. The ®rst yields twodimensional lineshapes that are not purely absorptive, while the second exhibits lower sensitivity and yields relatively intense `diagonal' peaks that are likely to interfere with the information-bearing `cross' peaks. In this Letter we demonstrate a phase-modulated three-pulse `shifted-echo' or `whole-echo' STMAS experiment that yields pure absorptive lineshapes and show that the `split-t1 ' approach, widely used in MQMAS to reduce the duration of the t2 acquisition window and avoid the need for shearing the ®nal two-dimensional spectrum, is easily incorporated into the new experiment. We also consider the application of STMAS to nuclei with spin quantum number higher than I ˆ 3/2, compare the dispersion of isotropic shifts achieved by STMAS and MQMAS, and discuss the e€ect of certain anisotropic `cross-term' broadening mechanisms on linewidths in `isotropic' STMAS spectra. 2. Echoes and antiechoes Fig. 1a shows the pulse sequence and coherence transfer pathways for two versions of the phasemodulated two-pulse STMAS experiment proposed by Gan. Phase cycling [5,6] may be used to select either the p ˆ +1 pathway during the t1 evolution period (dotted pathway) or the p ˆ )1 pathway (solid pathway). Figs. 2a and b show computer simulations (performed in the time-domain) of the two-dimensional NMR spectra that would result from the application of these two experiments to a spin I ˆ 3/2 nucleus. The two peaks in each frequency dimension are the CT (mI ˆ )1/2 M +1/2) and ST (mI ˆ ‹1/2 M ‹3/2), shifted apart from one another by their isotropic second-order quadrupolar shifts and both broadened by the second-order quadrupolar broadening. There is only a single satellite peak in each dimension because, following the approach de-

401

scribed by Gan, we have assumed in our simulations that rotor-synchronized acquisition has been performed in both t1 and t2 time domains and that the very large number of satellite spinning sidebands have been `aliased' or `folded' to appear on top of one another. The two-dimensional spectrum in Fig. 2a, acquired with the p ˆ )1 ® )1 sequence in Fig. 1a, exhibits two `ridge-like' diagonal peaks, lying precisely along the F1 ˆ F2 diagonal and arising from CT ® CT and ST ® ST coherence transfer processes, and two ridge-like cross peaks, arising from CT ® ST and ST ® CT coherence transfer processes. The two diagonal peaks arise from timedomain antiecho signals (second-order quadrupolar broadening refocusing condition t2 ˆ )t1 ) and hence have very poor phase properties. In contrast, the two cross peaks arise from echo signals (refocusing conditions t2 ˆ 9/8t1 for CT ® ST and t2 ˆ 8/9t1 for ST ® CT) and have much better phase properties. The ridge gradient of the CT ® ST cross peak is )9/8 and of the ST ® CT cross peak is )8/9. In the two-dimensional spectrum in Fig. 2b, obtained with the p ˆ +1 ® )1 sequence in Fig. 1a, the diagonal peaks now have the better phase as they arise from echo signals (refocusing condition t2 ˆ t1 ), while the cross peaks arise from antiecho signals (refocusing conditions t2 ˆ )9/8t1 and t2 ˆ )8/9t1 ). The echo signals in Figs. 2a and b do not yield pure absorption-mode lineshapes because they are truncated asymmetrically when t1 is small. The antiecho signals yield lineshapes with even worse phase properties because not only are they truncated when t1 is short but, as t1 increases, they move backwards out of the t2 acquisition window. These phase problems can be overcome using the three-pulse shifted-echo [7,8] STMAS pulse sequence shown in Fig. 1b. In this experiment the use of the p ˆ +1 pathway and a ®xed free precession interval, s, result in both echo and antiecho signals being shifted forward in t2 so that they are not truncated when t1 is short. As t1 increases in duration, the echo signals move further forward in t2 while the antiecho signals move backward. If the s interval is suciently long, the antiecho signals will not be truncated even when t1 reaches its maximum length and pure-absorption mode line-

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Fig. 1. Pulse sequences and coherence transfer pathways for STMAS. (a) Two-pulse sequence as described by Gan. Phase cycling is used to select either the p ˆ +1 or )1 coherence pathway during t1 . (b) Three-pulse shifted-echo sequence. (c) Split-t1 version of the sequence in (b). For spin I ˆ 3/2: k ˆ 9/17, k0 ˆ 8/17, k00 ˆ 0. For spin I ˆ 5/2: k ˆ 24/31, k0 ˆ 0, k00 ˆ 7/31. For spin I ˆ 7/2: k ˆ 45/73, k0 ˆ 0, k00 ˆ 28/73. For spin I ˆ 9/2: k ˆ 72/127, k0 ˆ 0, k00 ˆ 55/127. A complex (i.e., not real or hypercomplex) Fourier transform should be used to process the phase-modulated data produced by these sequences.

shapes will be obtained for both diagonal (echo) peaks and cross (antiecho) peaks, as shown in the simulation in Fig. 2c. Fig. 2d shows the computer-simulated spectrum resulting from applying the three-pulse shiftedecho STMAS pulse sequence to a spin I ˆ 5/2 nucleus. (Gan et al. [9] have also recently and independently suggested using a shifted-echo STMAS sequence for spin I ˆ 5/2.) For I ˆ 5/2, there are two sets of ST: inner ST (mI ˆ ‹1/2 M ‹3/2) and outer ST (mI ˆ ‹3/2 M ‹5/2), which we will abbreviate to ST1 and ST2 , respectively. As a result, there are three diagonal peaks in Fig. 2d (CT ® CT, ST1 ® ST1 and ST2 ® ST2 ) and three pairs of cross peaks

(CT M ST1 , CT M ST2 and ST1 M ST2 ). Using the coherence transfer pathway shown in Fig. 1b, the peaks with positive ridge gradients (the three diagonal peaks and the CT M ST1 cross peaks) are echo signals while those with negative ridge gradients (the CT M ST2 and ST1 M ST2 cross peaks) are antiecho signals. The simulated shifted-echo spectra in Figs. 2c and d were recorded assuming that all three radiofrequency pulses had essentially in®nite amplitude (x1 /2p ˆ )cI B1 /2p ˆ 10 MHz). In practice, as used in shifted-echo MQMAS experiments [8], the third pulse should have a much reduced amplitude and act as a selective inversion pulse across the

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Fig. 2. Computer-simulated (in the time domain) two-dimensional STMAS spectra. (a) Spin I ˆ 3/2 spectrum produced by sequence in Fig. 1a with selection of p ˆ )1 pathway during t1 . (b) Spin I ˆ 3/2 spectrum produced by sequence in Fig. 1a with selection of p ˆ +1 pathway during t1 . (c) Spin I ˆ 3/2 spectrum produced by sequence in Fig. 1b with `hard' third pulse. (d) Spin I ˆ 5/2 spectrum produced by sequence in Fig. 1b with hard third pulse. (e) Spin I ˆ 3/2 spectrum produced by sequence in Fig. 1b with `soft' third pulse. (f) Spin I ˆ 3/2 spectrum produced by split-t1 sequence in Fig. 1c. Simulation parameters: Larmor frequencies, m0 ˆ 105.8 MHz, except in (d), where m0 ˆ 104.3 MHz; quadrupolar coupling constant, CQ ˆ 2.60 MHz, except in (d), where CQ ˆ 3.33 MHz; asymmetries, g ˆ 0; radiofrequency ®eld strengths, m1 ˆ 10 MHz, except for soft third pulse in (e) and (f), where m1 ˆ 10 kHz; spectral widths, SW1 ˆ SW2 ˆ 16 kHz, except in (d), where SW1 ˆ SW2 ˆ 10 kHz and (f), where SW1 ˆ 8.47 kHz. The dashed line marks the F1 ˆ F2 diagonal.

CT. Such a `reduced-power' pulse is too weak and has the wrong ¯ip angle to invert the ST and so the only peaks that survive the phase cycling are those that correspond to the CT during t2 , i.e., for I ˆ 3/ 2, the CT ® CT and ST ® CT peaks and, for

I ˆ 5/2, the CT ® CT, ST1 ® CT and ST2 ® CT peaks (and note that the intensity of this last cross peak is often too low to be observed in practice). This results in a dramatic simpli®cation of the twodimensional STMAS spectrum, as shown for I ˆ 3/

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2 in Fig. 2e, and, furthermore, has important consequences for how the experiment is performed (see Section 3). Since only those cross peaks that correspond to CT coherence during t2 are detected in a shiftedecho STMAS experiment, it is also possible to incorporate a split-t1 evolution period, as widely used in MQMAS [10±14]. By splitting the t1 evolution period into two, with the second part coming either before (for I ˆ 3/2) or after the ®nal pulse (for I ˆ 5/2) as shown in Fig. 1c, and by choosing the relative durations of the two parts to be in proportion to the second-order quadrupolar broadenings of the CT and (inner) ST, the ST…1† ® CT cross peak will appear with its ridge gradient parallel to F2 , thereby avoiding the need for shearing the ®nal two-dimensional spectrum. Such a STMAS spectrum is simulated for I ˆ 3/2, in Fig. 2f. The time-domain (anti)echo signal corresponding to the ST ® CT cross peak is stationary in the t2 acquisition window, with the result that this window can be kept as short as possible, thereby minimizing the acquisition of noise [12]. In practice, this also means that the CT ® CT echo signal moves rapidly outside the acquisition window as t1 is increased, reducing the amplitude of this unwanted diagonal peak. 3. Experimental implementation and results Experiments were performed at a magnetic ®eld strength of B0 ˆ 9.4 T on a Bruker MSL 400 spectrometer. Both the second-order broadening and second-order isotropic shift are proportional to B0 1 and hence the resolution of a CT relative to its associated ST is independent of B0 . Therefore, as with MQMAS, the bene®ts of performing STMAS experiments at magnetic ®elds higher than this would merely be the normal (but important) ones of increased sensitivity and chemical shift dispersion and decreased second-order broadening. Conventional Bruker 4 mm MAS probeheads were used and powdered samples were packed in 4 mm MAS rotors. The magic angle was set to an estimated accuracy of 54.736°‹0.005°, ®rst by optimizing the number and amplitude of rotary echoes in a normal FID and then by max-

imizing the height of the ST…1† ® CT echo signal observed in a one-dimensional three-pulse shiftedecho STMAS experiment performed with a ®nite t1 value. Rotor-synchronized acquisition in the t2 time domain can be dicult to optimize and perform correctly owing to phase changes in the higher frequency satellite spinning sidebands and baseline distortions caused by the audiofrequency ®lters in the spectrometer receiver. One of the advantages of the shifted-echo STMAS pulse sequences shown in Figs. 1b and c is that it is not necessary to rotor synchronize in t2 . As shown in Figs. 2e and f only the CT signal is detected in the t2 /F2 domain and, with the MAS rates normally used, this exhibits either no spinning sidebands or, at worst, very weak ones that contain little intensity. In contrast, rotor-synchronized acquisition is essential in the t1 domain of an STMAS experiment as ST spinning sidebands are detected in this dimension. However, the absence of any audiofrequency ®ltration in t1 means that this presents few problems. It is important to note that the splitt1 approach used in the STMAS sequence in Fig. 1c is fully compatible with rotor-synchronized acquisition: it is simply a case of synchronizing the ®rst part of the evolution period, kt1 (where the ST are evolving), with the rotary echoes. Generally, we have chosen to use an increment of kt1 that is equal to half of a rotor period, thereby yielding an F1 (isotropic) spectral width of 2kmMAS , where mMAS is the MAS rate. This means that two sets of ST…1† ® CT cross peaks will be observed, a centreband and one spinning sideband, separated by a frequency of kmMAS in the isotropic F1 dimension. Compared with full rotor synchronization, this has the disadvantage that the signal-to-noise ratio is immediately halved, but has the advantage that at least one of the two sets of ST…1† ® CT cross peaks will be well resolved from the CT ® CT diagonal peaks, as the latter will not normally exhibit any sidebands in F1 . The resolution of a CT relative to its inner ST decreases rapidly as the spin quantum number I, increases (compare, for example, Figs. 2c and d) and hence the advantage of this `halfsynchronization' is particularly signi®cant for I ˆ 5/2, 7/2 and 9/2 nuclei.

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Presaturation of the CT was not found to be necessary in our STMAS experiments [1]. In part, this is because the split-t1 method of acquisition reduces the amplitude of the CT ® CT diagonal peaks. More importantly, however, it is possible to optimize the ®rst two pulse durations such that the intensities of the CT ® CT diagonal peaks are minimized without signi®cantly compromising the ST…1† ® CT cross-peak intensities. As with setting the magic angle, this optimization is easily done by observing the relevant echo signals in a one-dimensional three-pulse shifted-echo STMAS experiment performed with a ®nite t1 value. We have also found true presaturation very dicult to achieve experimentally. This last observation is possibly a consequence of the rotor-driven interconversion of the rotating-frame eigenstates (i.e., in the presence of a radiofrequency ®eld) which means that any saturation of the CT will be transferred adiabatically into the ST. Fig. 3a shows a two-dimensional 23 Na STMAS spectrum of a mixture of sodium sulphate Na2 SO4 , and sodium oxalate Na2 C2 O4 , obtained using the I ˆ 3/2 version of the split-t1 shifted-echo pulse sequence in Fig. 1c. This spectrum was recorded as described above: the t2 acquisition was not rotorsynchronized, incrementation of kt1 was half-rotor-synchronized, while presaturation was not employed. As a result of the use of a split-t1 evolution period, the two ridge-like ST ® CT cross peaks (one from Na2 SO4 and one from Na2 C2 O4 ) appear parallel to F2 , with the F1 dimension containing the isotropic frequencies. The two (unresolved) CT ® CT diagonal peaks have a steep, non-zero ridge gradient and appear near the centre of the two-dimensional spectrum. Note that, in this case, it is the ST ® CT centreband peaks that are well resolved from the CT ® CT diagonal peaks, while the ST ® CT sideband peaks (marked *) overlap slightly with these. Fig. 3b shows a two-dimensional 17 O STMAS spectrum of synthetic forsterite, Mg2 SiO4 , incorporating 35% 17 O. This spectrum was recorded using the I ˆ 5/2 version of the split-t1 shifted-echo pulse sequence in Fig. 1c. In this example, the three ST1 ® CT centreband peaks appear very close to the CT ® CT diagonal peaks (because of the decreased resolution between CT and ST1

405

Fig. 3. Two-dimensional (a) 23 Na (I ˆ 3/2) and (b) 17 O (I ˆ 5/2) STMAS NMR spectra of (a) a mixture of Na2 SO4 and Na2 C2 O4 and (b) Mg2 SiO4 (35% 17 O enriched). The split-t1 pulse sequence in Fig. 1c was used. The 16-step phase cycle was: 1st pulse, 0°; 2nd pulse, 0° 90° 180° 270°; 3rd pulse, 4(0°) 4(90°) 4(180°) 4(270°); receiver, 4(0°) 4(180°). Ridge peaks other than the desired ST…1† ®CT ones (which are parallel to F2 ) are labelled. Spinning sidebands are marked with *. Note that the CT®CT diagonal peaks exhibit a gradient equal to the modulus of the chemical shift scaling factor, 1.000 in (a) and 0.548 in (b). Experimental parameters: 2 s relaxation interval; 8 kHz MAS rate; 16 transients acquired for each of (a) 192 and (b) 256 t1 increments of duration (a) 118.05 ls and (b) 80.73 ls; pulse durations (a) 1.7, 1.7, 34 ls and (b) 1.5, 1.5, 40 ls.

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signals for higher spin quantum numbers) while the three ST1 ® CT sideband peaks are well resolved. The ST2 ® CT cross peaks can also be seen in Fig. 3b, albeit with very low intensity. 4. Isotropic frequency dispersion When measured in absolute frequency units, the isotropic chemical shifts and second-order shifts observed in an isotropic STMAS spectrum are scaled relative to those found in a normal MAS spectrum. Adapting the formalism of [14] for the STMAS case, the isotropic chemical shift scaling factor is given by xCS …I; p; q† ˆ

p ‡ R…I; p; q† ; 1 ‡ jR…I; p; q†j

…1†

where p is the ST coherence order, q is a label for the ST (mI ˆ ‹(q)1) M ‹q) used, and R(I,p,q) is the ridge gradient of the ST ® CT cross peak: R…I; p; q† ˆ

A4 …I; p; q† : A4 …I; 1; 1=2†

…2†

As in MQMAS, the scaling factor of the isotropic second-order quadrupolar shift, xQS (I,p,q), is related to that of the chemical shift by xQS …I; p; q† ˆ xCS …I; p; q†

10 : 17

…3†

The STMAS scaling factors xCS (I,p,q) are given in Table 1 for I ˆ 3/2, 5/2, 7/2 and 9/2, where they are

compared with the MQMAS scaling factors xCS (I,p). For I ˆ 5/2, 7/2 and 9/2, the isotropic frequency dispersion between ST1 (q ˆ 3/2) ® CT cross peaks is the same as the frequency dispersion obtained in a triple-quantum MAS experiment. However, the maximum STMAS scaling factor found in Table 1 is only 1.000 (compared with 4.760 for MQMAS) while, for I ˆ 3/2, the chemical shift scaling factor obtained with STMAS is 2.125 times smaller than that observed in a triple-quantum MAS spectrum. Isotropic projections of 17 O STMAS and triplequantum MAS NMR spectra of the synthetic forsterite sample described above are compared in Fig. 4. As predicted by the I ˆ 5/2 scaling factors in Table 1, the frequency dispersion (as measured in Hz) is the same in both isotropic spectra. As discussed in [14], an increase or decrease in the chemical shift scaling factor will translate directly into an increase or decrease in the isotropic resolution if the isotropic linewidth is unchanged. All other things being equal, therefore, the resolution in the two isotropic spectra in Fig. 4 should be the same as they have the same scaling factor (|xCS (I,p,q)| ˆ |xCS (I,p)| ˆ 0.548) and were recorded with the same maximum value of t1 (i.e., the same experimental limiting resolution). However, the linewidth is clearly greater, and hence the resolution poorer, in the STMAS spectrum in Fig. 4b. The most likely explanations for this increased STMAS linewidth are that it is due either to a small deviation (of less than 0.005°) of the exper-

Table 1 Absolute chemical shift scaling factors, xCS (I,p,q) and xCS (I,p), in STMAS and MQMAS isotropic spectra as a function of spin quantum number I, coherence order p, and (for STMAS) transition label q STMAS

MQMAS

I

p

q

|xCS (I,p,q)|

I

p

|xCS (I,p)|

3/2

)1

3/2

1.000

3/2

)3

2.125

5/2 5/2

+1 )1

3/2 5/2

0.548 1.000

5/2 5/2

+3 )5

0.548 2.297

7/2 7/2 7/2

+1 )1 )1

3/2 5/2 7/2

0.233 1.000 1.000

7/2 7/2 7/2

+3 +5 )7

0.233 1.700 2.311

9/2 9/2 9/2 9/2

+1 +1 )1 )1

3/2 5/2 7/2 9/2

0.134 0.895 1.000 1.000

9/2 9/2 9/2 9/2

+3 +5 +7 )9

0.134 0.649 4.760 2.297

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order interactions can broaden isotropic STMAS spectra and may be responsible for some of the additional linewidth seen in Fig. 4b. 5. Conclusions

Fig. 4. Isotropic projections (not `skyline projections') of (a) triple-quantum MAS and (b) STMAS spectra of Mg2 SiO4 (35% 17 O enriched). The projection in (b) is derived from the spinning sideband peaks (marked *) in Fig. 3b. The correspondence of the peak positions con®rms that the two projections have the same chemical shift scaling factor (0.548). Despite the fact that the two spectra were recorded with the same maximum value of t1 (20.7 ms), the STMAS projection has broader linewidths and, hence, poorer resolution.

imental sample rotation axis from the true, theoretical magic angle or to instability of the MAS rate during the experiment. However, other, more fundamental, explanations are possible. It has recently been shown that certain second-order quadrupolar±dipolar cross-term interactions can broaden isotropic MQMAS spectra (these interactions are anisotropic, revealing `isotropic spectrum' to be a misnomer) [15±18]. In MQMAS the dipolar interaction must occur between two quadrupolar nuclei for broadening to occur. In STMAS, in contrast, second-order cross-term broadening will also occur if there is a dipolar interaction between the observed quadrupolar nucleus and surrounding spin I ˆ 1/2 nuclei, if the latter are not decoupled. Furthermore, there is a second-order quadrupolarCSA cross-term interaction that can broaden isotropic STMAS spectra. Therefore, compared with MQMAS, a much wider range of second-

The STMAS experiment is a promising alternative to the DOR, DAS and MQMAS experiments for obtaining high-resolution NMR spectra of quadrupolar nuclei in powdered solids. Drawing upon our experience with MQMAS, we have demonstrated novel split-t1 shifted-echo STMAS pulse sequences by acquiring 23 Na (I ˆ 3/2) and 17 O (I ˆ 5/2) STMAS spectra that exhibit pure absorption-mode lineshapes. We have also discussed the practical implementation of these experiments. The isotropic frequency dispersion achieved by STMAS is, in general, the same or worse than that achieved by MQMAS and, in view of the additional sources of linebroadening identi®ed in isotropic STMAS spectra, it is likely that the MQMAS experiment will generally yield higher resolution. However, the STMAS experiment has excellent sensitivity: the 17 O STMAS experiment in Fig. 4b was recorded in 3.5 h, while the MQMAS experiment in Fig. 4a was recorded in 11.2 h.

Acknowledgements We are grateful to EPSRC for generous support (Grant Nos. GR/M12209 and GR/N07622) and to Mr. Jamie McManus for useful discussions about cross-term broadening in STMAS. References [1] Z. Gan, J. Am. Chem. Soc. 122 (2000) 3242. [2] A. Samoson, E. Lippmaa, A. Pines, Mol. Phys. 65 (1988) 1013. [3] K.T. Mueller, B.Q. Sun, G.C. Chingas, J.W. Zwanziger, T. Terao, A. Pines, J. Magn. Reson. 86 (1990) 470. [4] L. Frydman, J.S. Harwood, J. Am. Chem. Soc. 117 (1995) 5367. [5] G. Bodenhausen, H. Kogler, R.R. Ernst, J. Magn. Reson. 58 (1984) 370.

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[6] P.J. Hore, J.A. Jones, S. Wimperis, NMR: The Toolkit, Oxford Chemistry Primers (92), Oxford University Press, Oxford, 2000 (Chapter 6). [7] P.J. Grandinetti, J.H. Baltisberger, A. Llor, Y.K. Lee, U. Werner, M.A. Eastman, A. Pines, J. Magn. Reson. A 103 (1993) 72. [8] D. Massiot, B. Touzo, D. Trumeau, J.P. Coutures, J. Virlet, P. Florian, P.J. Grandinetti, Solid State NMR 6 (1996) 73. [9] Z. Gan, D. Massiot, L. Alemany, F. Taulelle, S. Steuernagel, F. Engelke, in: Poster M/T 086, 42nd Experimental NMR Conference, Orlando, Florida, March 2001. [10] S.P. Brown, S.J. Heyes, S. Wimperis, J. Magn. Reson. A 119 (1996) 280.

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