19 December 1997

Ž .Chemical Physics Letters 281 1997 44–48

Pulsed field gradient multiple-quantum MAS NMR spectroscopyof half-integer spin quadrupolar nuclei

C.A. Fyfe ), J. Skibsted 1, H. Grondey, H. Meyer zu Altenschildesche 2

Department of Chemistry, UniÕersity of British Columbia, 2036 Main Mall, VancouÕer B.C., Canada V6T 1Z1

Received 1 October 1997; in final form 27 October 1997

Abstract

Ž . Ž .Pulsed field gradients PFGs have been applied to select coherence transfer pathways in multiple-quantum MQ MAS27 Ž .NMR spectra of half-integer spin quadrupolar nuclei in rigid solids. Al triple-quantum 3Q MAS NMR spectra of the

Ž . Ž . Ž .aluminophosphate molecular sieves VPI-5 and AlPO -18 have been used to demonstrate the selection of the 0 ™ 3 ™ y14

coherence transfer pathway using PFGs and no phase cycling. Compared to MQMAS experiments that employ phase cyclingschemes, the main advantage of the PFG-MQMAS technique is its simplicity, which should facilitate the combination ofMQMAS with other pulse sequences. q 1997 Elsevier Science B.V.

1. Introduction

In recent years solid-state magic-angle spinningŽ . ŽMAS NMR spectroscopy of half-integer nuclei e.g.11 17 23 27 .B, O, Na, Al has emerged as a powerful toolin structural studies of a wide range of importantmaterials. For these nuclei single-pulse MAS NMRspectra of the central transition are often consider-ably broadened by the residual second-orderquadrupolar interaction. This prevents a straightfor-ward identification of resonances from structurallydifferent sites. A significant improvement in resolu-tion has been previously obtained by the double

) Corresponding author.1 Present address: Instrument Center for Solid-State NMR

Spectroscopy, Department of Chemistry, University of Aarhus,DK-8000 Aarhus C, Denmark.

2 Present address: Laboratory for Technical Chemistry, ETHZentrum, CH-8037 Zurich, Switzerland.

Ž . w xrotation DOR 1,2 and dynamic-angle spinningŽ . w xDAS 2,3 NMR techniques, which remove thesecond-order quadrupolar broadening by spinningthe sample around two different axes. More recently,

Ž .the multiple-quantum MQ MAS experiment hasw xbeen introduced 4,5 , which by correlating

multiple-quantum and single-quantum coherencesunder MAS gives a two-dimensional spectrum whereall anisotropies are removed in one dimension. Com-pared to DOR and DAS NMR the MQMAS experi-ment has the great advantage of mechanical simplic-ity in only requiring sample rotation around onefixed axis.

Since the introduction of the MQMAS techniquea number of studies have been presented, which

w xinvestigated the sensitivity 5–7 , MQ excitation andw xconversion efficiencies 8–10 and quantitative relia-

w xbility 11 of the experiment. Several studies alsofocused on methods for acquiring phase sensitivespectra from which additional information about theanisotropic part of the second-order quadrupolar in-

0009-2614r97r$17.00 q 1997 Elsevier Science B.V. All rights reserved.Ž .PII S0009-2614 97 01235-9

( )C.A. Fyfe et al.rChemical Physics Letters 281 1997 44–48 45

teraction can be extracted from the two-dimensionalw xlineshapes 12–15 . Most recently, it has been shown

that the MQMAS experiment can be combined withcross polarization to or from a spin-1r2 nucleus,resulting in either a 2D heteronuclear correlation

w xNMR spectrum 16 or a MQMAS spectrum thatshows only resonances from sites, which are coupled

w xto the spin-1r2 nucleus 17 . A common feature ofthe MQMAS experiments reported to date is that thecoherence selection is performed by phase cycling ofthe rf pulses and the receiver, requiring pulse-phaseshifts which differ from the common 0, pr2, p,

w x3pr2 phases 18 and resulting in extensive phasecycling schemes. The latter requirement becomesespecially important in extensions of the MQMASexperiment to include a magnetization transfer stepto a heteronucleus.

In this work we demonstrate the selection ofcoherences in the MQMAS experiment by the use ofpulsed magnetic field gradients. Pulsed field gradi-

Ž .ents PFGs are currently employed for coherenceselection in liquid-state NMR with the main advan-tages of removing artifacts and reducing the mini-mum number of scans in 2D experiments of abun-

w xdant spins 19,20 . PFGs have also been used incombination with MAS in magnetic-resonance imag-

w xing of polymers 21–24 and for obtaining high-reso-lution heteronuclear correlation NMR spectra ofpolymeric materials with substantial molecular mo-

w xbility 25 . The pulsed field gradient multiple-quan-Ž .tum MAS PFG-MQMAS NMR spectra presented

in this work represent the first examples of coher-ence selection using field gradients in studies ofhalf-integer quadrupolar nuclei in rigid solids.

2. Experimental

The PFG-MQMAS spectra were recorded using ahome-built MAS probe with a single set of gradientcoils and a commercially available 5 mm spinner

Ž .system Doty Scientific Inc. . The two coils weremounted cylindrically around the spinner axis, withan equal distance to the center of the rotor and with atotal separation of the coils of 17.5 mm. The innerand outer diameters of the coils were 16 mm and 20mm, respectively, each containing 28 turns. A Brukermicro-imaging unit, interfaced to a Bruker MSL-400

spectrometer, was used to feed the coils with pulsedDC currents, resulting in an effective B field that0

increased linearly along the spinner axis. The maxi-mum gradient strength was approximately 60 Grcm.The experiments used an rf field strength of 60 kHzat 104. 26 MHz and it was observed that the intro-duction of the gradient coils did not affect the rfperformance of the probe. A more detailed descrip-tion of the construction and performance of the

w xPFG-MAS probe will be given elsewhere 26 .

3. Results and discussion

The basic two-pulse sequence for a PFG-MQMASexperiment, which correlates the ps3 and psy1coherences for a half-integer spin nucleus, is shownin Fig. 1. Excitation and conversion of the triple-

Ž .quantum 3Q coherence is performed by the two rfpulses and, for simplicity, the t -acquisition begins2

w xat the top of the echo 10 . The coherence transferpathway is selected by applying two gradient pulses,

Ž . Ž .Fig. 1. Radio frequency rf and gradient g pulse schemes for thePFG-MQMAS experiment selecting the coherence transfer path-

Ž .way shown below. In addition to the duration of the gradient tg

the fixed time D includes a gradient ring down delay. For theexperiments in Figs. 2 and 3 this delay was Dyt s50 ms. Theg

value of k depends on the nuclear spin and the coherence transferŽ . w xpathway here ks19r12 4,10 .

( )C.A. Fyfe et al.rChemical Physics Letters 281 1997 44–4846

w xwhich fulfill the condition 3G t sG t 19 , where1 1 2 2

the G are the gradient amplitudes and the t de-i i

scribe their respective durations. In order to mini-mize fixed delays, which are necessary for the use offield gradients, we have chosen to use two gradients

Ž .of identical lengths i.e. t st st but with rela-g 1 2Ž .tive strengths of 1:3 i.e. G s3G . During the2 1

optimization of the PFG-MQMAS experiment it wasobserved that the sensitivity and coherence selectionwere not very sensitive to slight missettings of therelative gradient strengths. For gradient strengths ofG s17 Grcm and G s51 Grcm the best results1 2

were obtained using gradient ring down delays of 50ms and gradient lengths of t s210 ms. Further-g

more, it was observed that the minimum gradienttime, which completely supresses signals from anyother coherence transfer pathway, depends primarilyon the absolute gradient strengths. This is expectedsince the dephasing experienced by a particular co-herence p depends on the product pt G .i i

27 Ž .Fig. 2. Contour plot of the Al PFG-3QMAS spectrum 9.4 T ofhydrated VPI-5 obtained using the pulse scheme in Fig. 1 withb s10.9 ms, b s3.2 ms, G s17 Grcm, G s51 Grcm, and1 2 1 2

t s210 ms. The spectrum was recorded using a spinning speedg

n s14.3 kHz, a t increment of 33.3 ms and a total of 96 tr 1 1

increments. 200 scans were acquired with a repetition delay of 0.2s, resulting in a total experiment time of 1 h and 9 min. Theone-dimensional trace in the F dimension is a summation while1

the trace in F corresponds to a skyline projection. The spectrum2Ž .is referenced to a 1.0 M aqueous solution of Al NO in both3 3

dimensions.

27 Ž .Fig. 3. Contour plot of the Al 3QMAS spectrum 9.4 T ofas-synthesized AlPO -18 obtained using a RIACT rf pulse se-4

w xquence 11 with pulsed field gradients. The sequence differs fromthe one depicted in Fig. 1 in that the pulse b is substituted by a1

908 solid pulse followed by a spin-lock of duration t and bSL 2Ž .by another t spin lock pulse t f1r4n s19 ms . TheSL SL r

spectrum was recorded and processed using the same parametersas given in Fig. 2 except for n s14.2 kHz, 452 scans for each tr 1

increment, and a total experiment time of 2.5 h.

The application of PFGs in 3QMAS experimentsis illustrated in Figs. 2 and 3 by PFG-MQMASspectra of the aluminophosphate molecular sieveshydrated VPI-5 and as-synthesized AlPO -18. Ac-4

cording to powder X-ray diffraction studies, both ofthese samples contain three Al sites of which twohave tetrahedral coordination, while the third site has

Ž . w xan octahedral VPI-5 27 or penta-coordinatedŽ . w xAlPO -18 28 environment. The spectrum in Fig. 24

employs the pulse sequence of Fig. 1, while Fig. 3shows the result of combining PFGs with the rota-

Ž .tion-induced adiabatic coherence transfer RIACTw xpulse sequence 11 . No phase cycling was applied to

the rf pulses or the receiver in both of these spectra.Similar MQMAS spectra were recorded without gra-dients using conventional phase cycling schemes but

Ž .otherwise identical conditions not shown . A com-parison of the PFG-MQMAS spectra with those ob-tained using phase cycling shows virtually no differ-ences in absolute signal intensity or signal-to-noiseratio.

( )C.A. Fyfe et al.rChemical Physics Letters 281 1997 44–48 47

Ž .The 3QMAS spectrum of VPI-5 Fig. 2 illus-trates the improvement in resolution by MQMASNMR, since the two tetrahedral Al sites cannot beresolved in a single-pulse 27Al MAS spectrum at 9.4T. Furthermore, the projection in the isotropic di-mension of Fig. 2 displays relative intensities of thethree resonances which are close to the expected

w x1:1:1 ratio 27 . This ratio is also expected forw xAlPO -18 28 and deconvolution of the summation4

in the F dimension of Fig. 3 gives relative intensi-1

ties of 1.0:1.11:0.89, when the intensities of theŽ .first-order spinning sidebands not shown are taken

into account.The 3QMAS spectrum in Fig. 3 is the first appli-

cation of the RIACT excitation scheme in a 3QMASexperiment for an Is5r2 nucleus. The RIACTsequence was introduced in MQMAS NMR for Is

w x3r2 nuclei 11 and it utilizes the occurence ofadiabatic coherence transfers between central transi-

Ž . Ž .tion 1Q and triple-quantum 3Q coherences duringthe spin-lock period as a result of avoided crossingsof eigenstates induced by the sample rotation. Theconditions for 1Ql3Q transfers in spin-lock exper-iments of quadrupolar nuclei were first explored byVega using a qualitative theoretical description for

w xIs3r2 nuclei 29 . This work also included calcu-lated results for a spin-5r2 nucleus, which indicatethat an adiabatic passage in this case leaves the"3r2 eigenstates unaffected. At a first sight thequalitative calculations thereby suggest that no RI-ACT effect should be observed in a 3QMAS experi-ment for a Is5r2 spin system. However, in asubsequent study by Grey and Vega of transfer of

Ž .populations in double resonance TRAPDOR NMRw x 1 2730 , their experimental results on a H– Al spinsystem led to the conclusion that the simple theoreti-cal model is flawed, since it underestimates thedegree of mixing of all of the eigenstates when thepassages are faster than adiabatic, i.e. 1Ql3Qtransfers may occur in this case. For the spin systemand the experimental conditions used for the PFG-3QMAS spectrum shown in Fig. 3 the passages arenot strictly adiabatic which may explain the observa-tion of a significant RIACT effect. However, at themoment we cannot completely exclude that the ob-served 1Ql3Q transfers could be partly a result ofa nutation excitationrconversion. A more detailedstudy of the application of the RIACT excitation

scheme in 3QMAS experiments on Is5r2 nuclei isw xcurrently in progress 31 .

4. Conclusions

A simple design of a gradient MAS probe with asingle gradient along the direction of the spinner axishas allowed the first demonstration of coherenceselection by pulsed field gradients in MQMAS NMRof quadrupolar nuclei in rigid solids. Without anynoticeable reduction in sensitivity compared to stan-dard phase cycling schemes, the PFG-MQMAS ex-periment has the main advantage of not requiringany phase cycling of the rf pulses. Although themethod has been demonstrated for the 3QMAS ex-periment for a spin-5r2 nucleus, it is equally appli-cable to the selection of other coherence transferpathways and other half-integer spin nuclei. PFGsmay also be useful in other types of solid-state NMRexperiments such as homo- and heteronuclear corre-lation MAS NMR for spin-1r2 nuclei. Furthermore,we expect that the reduced phase cycle will signifi-cantly facilitate current work on the incorporation ofthe MQMAS technique in double resonance experi-

w x w xments, such as the INEPT 32 and TEDOR 33pulse sequences, which include a magnetization

w xtransfer step to a spin-1r2 nucleus 31 . These com-binations would give two-dimensional heteronuclearcorrelation spectra where both dimensions displayisotropic shifts.

Acknowledgements

We acknowledge the financial assistance of theNSERC of Canada in the form of operating and

Ž .equipment grants CAF . JS thanks the Danish Natu-Ž .ral Research Council J. No. 9600396 for financial

support. We are indebted to Mr. T. Markus and Mr.O. Greiner for assistance with the construction of theprobe.

References

w x Ž .1 A. Samoson, E. Lippmaa, A. Pines, Mol. Phys. 65 19881013.

w x Ž .2 A. Llor, J. Virlet, Chem. Phys. Lett. 152 1988 248.

( )C.A. Fyfe et al.rChemical Physics Letters 281 1997 44–4848

w x3 K.T. Mueller, B.Q. Sun, G.C. Chingas, J.W. Zwanziger, T.Ž .Terao, A. Pines, J. Magn. Reson. 86 1990 470.

w x Ž .4 L. Frydman, J.S. Harwood, J. Am. Chem. Soc. 117 19955367.

w x5 A. Medek, J.S. Harwood, L. Frydman, J. Am. Chem. Soc.Ž .117 1995 12779.

w x6 D. Massiot, B. Touzo, D. Trumeau, J.P. Coutures, J. Virlet,P. Florian, P.J. Grandinetti, Solid State Nucl. Magn. Reson. 6Ž .1996 73.

w x Ž .7 D. Massiot, J. Magn. Reson. A. 122 1996 240.w x Ž .8 S. Ding, C.A. McDowell, Chem. Phys. Lett. 270 1997 81.w x9 J.P. Amoureux, C. Fernandez, L. Frydman, Chem. Phys.

Ž .Lett. 259 1996 347.w x10 G. Wu, D. Rovnyak, B. Sun, R.G. Griffin, Chem. Phys. Lett.

Ž .249 1996 210.w x11 G. Wu, D. Rovnyak, R.G. Griffin, J. Am. Chem. Soc. 118

Ž .1996 9326.w x12 C. Fernandez, J.P. Amoureux, Solid State Nucl. Magn. Re-

Ž .son. 5 1996 31.w x13 J.H. Baltisberger, Z. Xu, J.F. Stebbins, S.H. Wang, A. Pines,

Ž .J. Am. Chem. Soc. 118 1996 7209.w x14 S.P. Brown, S.J. Heyes, S. Wimperis, J. Magn. Reson. A 119

Ž .1996 280.w x15 J.P. Amoureux, C. Fernandez, S. Steuernagel, J. Magn.

Ž .Reson. A. 123 1996 116.w x16 S.H. Wang, S.M. De Paul, L.M. Bull, J. Magn. Reson. 125

Ž .1997 364.w x17 M. Pruski, D.P. Lang, C. Fernandez, J.P. Amoureux, Solid

Ž .State Nucl. Magn. Reson. 7 1997 327.

w x Ž .18 A. Wokaun, R.R. Ernst, Chem. Phys. Lett. 52 1977 407.w x19 A. Bax, P.G. De Jong, A.F. Mehlkopf, J. Smidt, Chem. Phys.

Ž .Lett. 69 1980 567.w x Ž .20 P. Barker, R. Freeman, J. Magn. Reson. 64 1985 334.w x21 D.G. Cory, J.W.M. van Os, W.S. Veeman, J. Magn. Reson.

Ž .76 1988 543.w x22 G. Schauss, B. Blumich, H.W. Spiess, J. Magn. Reson. 95¨

Ž .1991 437.w x Ž .23 M. Buszko, G.E. Maciel, J. Magn. Reson. A. 107 1994

151.w x24 P. Blumler, B. Blumich, in: P. Diehl, E. Fluck, H. Gunther,¨ ¨ ¨

Ž .R. Kosfeld, J. Seelig Eds. , NMR Basic Principles andProgress, vol. 30, Springer, Berlin, 1993, pp. 209–277.

w x25 W.E. Maas, F.H. Laukien, D.G. Cory, J. Am. Chem. Soc.Ž .118 1996 13085.

w x26 C.A. Fyfe, H. Meyer zu Altenschildesche, H. Grondey, J.Skibsted, in preparation.

w x27 L.B. McCusker, Ch. Baerlocher, E. Jahn, M. Bulow, Zeo-¨Ž .lites, 11 1991 308.

w x28 A. Simmen, L.B. McCusker, Ch. Baerlocher, W.M. Meier,Ž .Zeolites 11 1991 654.

w x Ž .29 A.J. Vega, J. Magn. Reson. 96 1992 50.w x Ž .30 C.P. Grey, A.J. Vega, J. Am. Chem. Soc. 117 1995 8232.w x31 C.A. Fyfe, J. Bretherton, H. Grondey, H. Meyer zu Al-

tenschildesche, J. Skibsted, work currently in progress.w x32 C.A. Fyfe, K.C. Wong-Moon, Y. Huang, H. Grondey, J. Am.

Ž .Chem. Soc. 117 1995 10397.w x33 C.A. Fyfe, K.T. Mueller, H. Grondey, K.C. Wong-Moon, J.

Ž .Phys. Chem. 97 1993 13484.