MAS NMR spectroscopy of solids

Volume 208, number 5,6

CHEMICAL PHYSICS LETTERS

18 June 1993

Distorted powder lineshapes in 27Al CP/MAS NMR spectroscopy of solids Patrick J. Barrie Department of Chemisiry, UniversityCollege London, 20 Cordon Street, London

WClH OAJ, UK

Received 5 April 1993

*‘AI CP/MAS NMR spectra of aluminium acetylacetonate show distinct lineshapes depending on the experimental conditions as the different orientations within the powder cross-polarize with differmg efficiencies. Selective polarization transfer to the l/2++- l/2 transition occurs at rfpower levels corresponding to 3w, = c+, while non-selective polarization transfer can occur at higher 27A1rfpowers. Two-dimensional quadrupole nutation spectra show that the cross-polarization lineshapes arise principally because the different orientations have different 27Alnutation frequencies. The results obtained highlight the caution needed in the interpretation of CP/MAS NMR spectra of quadrupolar nuclei.

1. Introduction Cross-polarization (CP) with magic-angle spinning (MAS) has become routinely used for recording solid-state NMR spectra of spin I= l/2 nuclei in the vicinity of hydrogen nuclei [ 1,2 1. This is because CP provides a signal enhancement and usually allows faster recycle delays between scans than would be needed for observing the nucleus under investigation by single pulse excitation. NMR studies of half-integral quadrupolar nuclei by CP are by contrast rare. This is partly because relaxation times of quadrupolar nuclei tend to be small, allowing faster recycle delays in single pulse excitation experiments than would be appropriate for CP, and also because the T,, relaxation times of the quadrupolar nucleus under investigation may be too short for efficient CP to occur. This latter problem is compounded by the observation that the effective T,, of half-integral quadrupolar nuclei can be reduced by MAS [ 3 1, and this effect on the spin dynamics of cross-polarization from I= l/2 to S= 3/2 nuclei has been considered theoretically [ 41. Despite these difficulties CP spectra (either static or with MAS) have been reported on half-integral quadrupolar nuclei either to give a signal enhancementinthecaseof43Ca [5],“Mo [6] and”0 [7], or to aid spectral assignment based on proximity to 486

hydrogen nuclei in the case of I70 [ 7 1, ’'B [ 8 1, 23Na [ 91 and 27A1[ 10-141. However, interpretation of CP/MAS spectra requires a knowledge of the powder lineshapes that are observed under the conditions of the experiment. While some workers have found that fairly reliable second-order quadrupolar lineshapes are observed using CP [ 7,8 1, Hayashi and Hayamizu [ 151 recently reported distorted lineshapes in CP/MAS NMR spectra of 23Na. The lineshapes were ascribed to the effect of different orientations within the powder having different spinlock efficiencies. It has also been pointed out that orientations close to the magic-angle will cross-polarize at slower rates [ 71. In this Letter, 1 present *‘Al CP/MAS NMR spectra of aluminium acetylacetonate, a compound which shows an unusually well-defined quadrupolar lineshape. As well as polarization transfer selectively to the l/2-l/2 transition at rf power levels corresponding to 30,~=%+ the existence of a non-selective polarization transfer maximum at higher aluminium rf powers is demonstrated. The spectra show distinct lineshapes which depend on the experimental conditions. Variable contact time measurements indicate that in this case T,, relaxation effects on the lineshape are negligible. In contrast to previous work the results are interpreted as being due to the different orientations within the powder having differ-

0009-2614/93/$ 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.

Volume 208, number 5,6

18 June 1993

CHEMICAL PHYSICS LETTERS

em 27Alnutation frequencies, and this suggestion is supported by two-dimensional quadrupole nutation spectroscopy. (a)

2. Experimental 27Al solid-state NMR spectra were recorded at a frequency of 78.2 MHz on a Bruker MSL-300 spectrometer using a double-bearing probe to spin 4 mm rotors. CP spectra were obtained under a range of experimental conditions, both static and with MAS at different spinning speeds. As well as power level settings corresponding to the modified HartmannHahn condition for selective polarization transfer to the l/2* - l/2 transition of 3y,B,= Y&, [ 161, the possibility of polarization transfer at higher 27Al powers was also explored. High-power proton decoupling was used during the acquisition time, though it was found that MAS alone at speeds greater than ~3 kHz was sufficient to narrow the lines completely. Proton decoupling during the acquisition period did, however, significantly narrow the static spectra obtained. Two-dimensional quadrupole nutation spectra were recorded by increasing the pulse length in I ps increments from 1 to 64 us. The spectra were transformed in magnitude mode using a sinebell filter in the t, dimension. The spinning speed for the MAS nutation experiments was set at 3.3 kHz; this ensured that the maximum pulse length used was only a small fraction of the rotor period. Chemical shifts are given relative to 1 M aluminium nitrate aqueous solution.

3. Results and discussion The structure of aluminium acetylacetonate is known from a single crystal X-ray structure determination to contain a single aluminium site whose coordination is close to perfect octahedral symmetry [ 171. Fig. lc shows the normal spectrum of aluminium acetylacetonate recorded using single pulse excitation. This shows a well-defined second-order quadrupolar lineshape with two maxima and a “foot”; the spectrum can be simulated using the quadrupole coupling parameters e2qQ/h= 3.0 MHz and q=O. 15. This gives the value of the isotropic

(bl

t

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-20 0 ppm

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Fig. 1. 27AlMAS spectra of aluminium acetylacetonate at spinning speeds of 4.0-4.3 kHz. (a) Non-selective CP (vu= 70 kHz, ~,=94 kHz, 8 ms contact time); (b) selectiveCP (vu=30 kHz, uH= 94 kHz, 8 ms contact time ); (c ) single pulse excitation.

chemical shift to be 0.0 ppm after correction for the second-order quadrupole shift. Spinning sidebands from the satellite transitions are also observed, with the centreband for the l/2*3/2 transition occurring at 2.2 ppm. The power settings for recording the CP spectra were determined by varying the 27Alrf power keeping the ‘H power constant. Two polarization transfer maxima were observed, one corresponding to a nonselective polarization transfer (fig. la), and the other to a selective polarization transfer (fig. lb) At the ‘H power level used the first maximum gave a stronger signal than the first. Spinning sidebands from the satellite transitions are observed in the case of the non-selective polarization transfer. The different lineshapes obtained in the CP spectra indicate that the different orientations present within the powder sample must cross-polarize with vastly different efficiencies. In the case of the selective trans487

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fer CP spectrum, the right-hand maximum is substantially weaker than the left-hand one (it corresponds to orientations with cos tl close to 0) while the “foot” is completely absent (it corresponds to orientations with cos B close to 1). The opposite is the case in the non-selective transfer CP spectrum. These spectra immediately indicate that caution is needed in the interpretation of CP/MA!S NMR spectra of quadrupolar nuclei, as the lineshapes may be very different from the expected powder pattern, and different lineshapes are obtained depending on the exact conditions of polarization transfer. Variable contact times experiments were then performed at both the polarization transfer maximum positions (see fig. 2). It was found that there was little variation in lineshape with contact time, and that the CP signal strength continued increasing with contact time up to the longest contact time studied

18 June 1993

(20 ms). The resulting curves of peak area against contact time for both sets of CP data (fig. 3) could be fitted assuming two rates of CP growth, with negligible T,p effects ( T,,> 100ms), The non-selective CP transfer position was best fitted with 16% of aluminium observed having TM_,growth rate of 0.79 ms, and 83% having TA,_Hof 17.3 ms, while the selective CP transfer position was fitted by 17% of aluminium observed having T,,_, of 0.69 ms and 84Oh having TAI_H of 18.5 ms. Thus the CP transfer rates are virtually identical for the two transfer processes. The effect on the CP spectra of varying the spinning speed between 1.8 kHz and 10 kHz was also investigated (see fig. 4). The general features of the CP spectra remain the same, though there are some differences in shape particularly at the fastest spinning speed. It was noted that there was some variation in the optimum 27Alrf power level for the nonselective CP position with spinning speed; it is known

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h

IO

Contact time I ms

0.4 ms

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-20 PPm

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Fig. 2. *‘Al CP/MAS spectra of aluminium acetylacetonate obtained using contact times of 20, 8, 2 and 0.4 ms (spinning speed=4270 Hz). (a) Non-selective CP (vU=70 kHz, v,=94 kHz); (b) selective CP (U,+,=30kHz, v,=94 kHz).

488

0-

Contact time I ms Fig. 3. Results of variable contact time CP/MAS experiments. (a) Non-selective CP (v,=70 kHz, vH= 94 kHz); (b) selective CP ( V~U=30 kHz, I+,= 94 kHz). The curves give the best fit for a two-component rate of polarization transfer.

CHEMICAL PHYSICS LETTERS

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lective excitation. In the general case, when neither of the two extreme conditions apply, a single site will show a range of nutation frequencies [ 22-251, and it needs to be recognized that these nutation frequencies also depend on orientation. Fig. 5 shows the two-dimensional quadrupole nutation spectrum with MAS recorded with a relatively high 27Alrf power level (v, = 90 kHz ) . It can be seen that there are peaks corresponding to nutation frequencies of 120 and 260 kHz. Significantly the spectrum shows that the nutation frequencies are orientation dependent. Thus orientations corresponding to the right-hand maximum and the “foot” of the powder spectrum precess more at the lower frequency than the left-hand maximum. This explains why the non-selective CP/MAS spectrum of aluminium acetylacetonate shows stronger signals for the right-hand maximum and “foot”. At lower 27A1rf power settings the nutation maximum correspond-

(al

1

20

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-20 PPm

-40

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I

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-20

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-40

PPm

Fig. 4. 27AlCP/MAS spectra of aluminium acetylacetonate at spinning speeds of 10,4.3 and 1.9 Wz ( 8 ms contact time). (a) Non-selectiveCP (vu=70 kHz, v,=94 kHZ); (b) selective CP (vM=30kHz, v,,=94 kHz).

,J

-L_--I

1

,

-20

0

20

-40

Pp(n

that the optimum power setting for cross-polarization of spin I= l/2 nuclei is also affected by spinning speed [ 181. The general condition for cross-polarization to occur is that the precession frequencies of the two coupled spins must be the same [ 191. In the case of quadrupolar nuclei in solids, it is known that the precession (or nutation) frequency depends on the relative magnitudes of the rf power (We) and the quadrupole coupling constant ( oQ). In the event that then we have non-selective excitation in which all transitions are excited and the effective 90” pulse (the pulse length for maximum signal strength), tp, is given by w&,= ~12, while if w& then we have selective excitation (in which only the l/2 *- l/2 transition is excited) and the effective 90” pulse is given by w,+(Z+ 1/2)t,=lr/2 [20,21]. This means that the nutation frequency in the case of selective excitation is (I+ l/2) times that for non-se-

I

A

b i!b

wdscoQ

1200

- 300

k-5-d

mQ

I

’400

Fig. 5. “Al MAS quadrupolar nutation spectrum of aluminium acetylacetonate (vu=90 kHz, spinning speed=3.3 kHz). The single pulse excitation MAS spectrum is shown above on the same scale.

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ing to 3vAIcan match the ‘H precession frequency, and selective CP is observed which shows a strong signal for the left-hand maximum and absence of the “foot”. The fact that the nutation frequencies depend on orientation has been largely overlooked in the literature, and it is clear that this can significantly affect CP lineshapes of quadrupolar nuclei in solids (both static and with MAS). It is worth noting that CP experiments at varying 27Al rf powers can give information about the “Al nutation spectrum. Finally it should be pointed out that the lineshapes arise because MAS does not remove the anisotropic part of the second-order quadrupole interaction, and that CP spectra obtained with double rotation (DOR) will not suffer the same difficulties of interpretation

rJ61.

4. Conclusions The nutation frequencies of quadrupolar nuclei within a sample are orientation dependent, and this accounts for the CP/MAS lineshapes observed. This work highlights the caution needed in the interpretation of nutation and CP/MAS spectra of quadrupolar nuclei in materials of unknown structure.

Acknowledgement I thank Dr. Chris Groombridge for his initial suggestions for this work. The NMR spectra were recorded on the ULIRS solid-state NMR facility at University College London.

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References [ 1] A. Pines, M.G. Gibby and J.S. Waugh, J. Chem. Phys. 59 (1973) 569. [2] J. Schaefer and E.O. Stejskal, J. Am. Chem. Sot. 98 ( 1976) 1031. [3] A.J. Vega, J. Magn. Resort. 96 (1992) 50. [4] A.J. Vega, Solid State Nucl. Magn. Resort. 1 (1992) 17. [5] R.G. Bryant, S. Ganapathy and SD. Kennedy, J. Magn. Reson. 72 (1987) 376. [ 61 J.C. Edwardsand P.D. Ellis, Magn. Reson. Chem. 28 ( 1990) s59. [ 71 T.H. Walter, G.L. Turner and E. Oldfield, J. Magn. Reson. 76 (1988) 106. [ 8 ] D.E. Woessner,Z. Physik. Chem. I52 ( 1987) 5 1. [9] R.K. Harris and G.J. Nesbitt, J. Magn. Reson. 78 (1988) 245. [IO] H.D. Morris and P.D. Ellis, J. Am. Chem. Sot. 111 (1989) 6045. [ 111H.D. Morris, S. Bank and P.D. Ellis, J. Phys. Chem. 94 (1990) 3121. [ 121 J. Rocha and J. Klinowski, J. Chem. Sot. Chem. Commun. (1991) 1121. [ 131 L. Kellberg, M. Linsten and H.J. Jakobsen, Chem. Phys. Letters 182 (1991) 120. [ 141J. Rocha, S.W. Carr and J. Klinowski, Chem. Phys. Letters 187(1991)401. [ 151S. Hayashi and K. Hayamizu, Chem. Phys. Letters 203 (1993) 319. [16]S.Vega,Phys.Rev.A23 (1981) 3152. [ 171P.K. HonandCE. Pfluger, J. Coord. Chem. 3 (1973) 67. [18]A.J. Vega, J.Am. Chem. Sot. 110 (1988) 1049. [ 191 C.P. Slichter, Principles of magnetic resonance (Springer, Berlin, 1990). [20] A. Samoson and E. Lippmaa, Phys. Rev. B 28 ( 1983) 6567. [2 1 ] P.P. Man, J. Klinowski, A. Trokiner, H. Zanni and P. Papon, Chem. Phys. Letters 151 (1988) 143. [22] A. Samoson and E. Lippmaa, Chem. Phys. Letters 100 (1983) 205. [23]A.P.M.Kentgens,J.J.M. Lemmens,F.M.M.GcmtsandW.S. Veeman, J. Magn. Reson. 71 ( 1987) 62. [24] A. Samoson and E. Lippmaa, J. Magn. Reson. 79 ( 1988) 255. [25] N.C. Nielsen, H. Bildsee and H.J. Jakobsen, J. Magn. Reson. 97 (1992) 149. [26] Y. Wu, D. Lewis, J.S. Frye, A.R. Palmer and R.A. Wind, J. Magn. Reson. 100 (1992) 425.