Ž .Solid State Nuclear Magnetic Resonance 11 1998 211–214
Field sweep broadline NMR spectroscopy
I.J.F. Poplett, M.E. Smit...
( )I.J.F. Poplett, M.E. SmithrSolid State Nuclear Magnetic Resonance 11 1998 211–214212
w xtaining ceramics 6 . ZrO polymo...
( )I.J.F. Poplett, M.E. SmithrSolid State Nuclear Magnetic Resonance 11 1998 211–214 213
such experiments are very far rem...
( )I.J.F. Poplett, M.E. SmithrSolid State Nuclear Magnetic Resonance 11 1998 211–214214
linear current-field relationship....
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Transcripts - PoplettMESmithSSNMR1998v11p211FieldSweepNMR

  • 1. Ž .Solid State Nuclear Magnetic Resonance 11 1998 211–214 Field sweep broadline NMR spectroscopy I.J.F. Poplett, M.E. Smith ) School of Physical Sciences, UniÕersity of Kent, Canterbury, Kent, CT2 7NR, UK Received 20 October 1997; accepted 2 December 1997 Abstract A novel NMR spectrometer is described that is uniquely versatile in its ability to accurately record broad lines by sweeping the superconducting magnetic field and to perform standard high resolution solid state NMR experiments. Broadline observation is illustrated by 27 Al spectra from static samples. Such an instrument opens up many nuclei for serious study by NMR in the solid state for the first time. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Broadline NMR; Aluminium-27; Magnetic field sweep; NMR spectrometer 1. Introduction Today, NMR spectroscopy largely means a pulsed Ž .Fourier transform FT approach using high field w xsuperconducting magnets 1 . The relatively broad frequency range simultaneously excited by a short Ž .radiofrequency rf pulse brought great improve- ments in time efficiency compared to the original Ž .continuous wave CW approach of sweeping the w xfield or frequency 1 . The pulsed FT approach has been extremely successful for high resolution solu- w xtion state experiments 2 and in high resolution experiments of solids, such as magic angle spinning 29 w x 13 especially for spin-1r2 nuclei, e.g., Si 3 and C w x4 . However, pulsed NMR has a weakness in that the bandwidths of pulse excitation, probe and the w xspectrometer itself 1 makes accurate recording of broad lines very difficult, so that NMR of broad lines ) Corresponding author. Fax: q44-01227-827558; e-mail: m.e.smith@ukc.ac.uk. now at Department of Physics, University of Warwick, Coventry, CV4 7AL, UK. has become a largely neglected field. There are also effects of deadtime as the system recovers from the rf pulse, even in the best engineered systems that lead to observable distortions in broad NMR lines, although these can be partially alleviated by using w xecho techniques 1 . Recording broad lines has many potential advantages for ascertaining NMR interac- tions, such as first-order quadrupole and chemical shift anisotropy. 2. Discussion The neglect of broad line NMR spectroscopy has meant that determining, e.g., the quadrupole interac- tion for some nuclei can be generally difficult, and those that experience significant quadrupole broaden- Ž 25 45 47,49 59 63,65 67 ing e.g., Mg, Sc, Ti, Co, Cu, Zn, 91 93 135,137 139 .Zr, Nb, Ba and La are only considered for NMR of solids in special cases. There is no doubt that broadline information can be useful, with exam- ples of relevance to materials science including 91 Zr w x 139 of ZrO polymorphs 5 and La in lanthanum-con-2 0926-2040r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. Ž .PII S0926-2040 97 00110-0
  • 2. ( )I.J.F. Poplett, M.E. SmithrSolid State Nuclear Magnetic Resonance 11 1998 211–214212 w xtaining ceramics 6 . ZrO polymorphs could be2 readily distinguished in static NMR spectra and mix- tures of these polymorphs could be quantified by the w xspectra 5 . The data from these materials were accu- mulated by stepping the frequency but such experi- ments are somewhat tedious, time-consuming and require accurate retuning of the electronics at each step, and very few examples of automation of such an approach exist. Alternatively, to record such spectra, the magnetic field could be swept and high field superconducting magnets have been used in this mode to examine w xinternal fields of magnetic materials 7 and quan- w xtum-tunnelling level-crossing effects 8 . Sweeping of superconducting magnetic fields is also widely used in physics to examine, e.g., de Haas–van Alphen, magnetoresistance and magneto–optical ef- w xfects 9 . Often, the superconducting magnets used in Ž . 27 Fig. 1. a Field sweep Al NMR spectrum of a-Al O at an observation frequency of 79.1355 MHz with 10 G steps between successive2 3 Ž . Ž .spin-echoes which have been converted to a frequency, b the one pulse spectrum using a 0.5 ms pulse, and c a spin echo using an Ž .interpulse spacing of 100 ms. The singularities of the quadrupolar pattern are marked Al 1 .1,2,3
  • 3. ( )I.J.F. Poplett, M.E. SmithrSolid State Nuclear Magnetic Resonance 11 1998 211–214 213 such experiments are very far removed from those used in chemical NMR spectroscopy having low Ž 3.magnetic field homogeneity e.g., 0.1% over 1 cm Ž .and often cold i.e., -10 K access temperatures. This contrasts markedly with the necessary high Ž y5 3.homogeneity e.g., -10 % over 1 cm and room temperature access magnets of high field NMR spec- troscopy. Field-swept pulsed NMR spectra has been re- w x w xported, e.g., from titanium metal 10,11 , NbSe 122 w x w xZn 13 and most recently alumina 14 . All these measurements were carried out in magnetic fields with resolution far below that required for normal Ž .spectroscopy and mostly with cold e.g., -10 K access. Hence they are essentially magnets dedicated to this experiment and they often also have high Ž .helium consumptions e.g., 30 lrday . The need for a more flexible NMR spectrometer was highlighted nearly a decade ago by the comment of Turner et al. w x15 ‘‘ . . . a high field continuous wave approach would appear most attractive for studies of extremely broad lines in ceramic materials, but unfortunately such an approach is not commercially available.’’ This approach has always been essentially available but only on noncommercial, low resolution instru- ments, as described above. For commercial exploita- tion there have been two main perceived drawbacks: Ž .1 the prohibitively expensive helium costs because of increased consumption on sweeping the field, and Ž .2 the need to have a specialised, dedicated instru- ment since such a system would not have sufficient performance for conventional spectroscopy. Neither of these perceptions are necessarily correct. A novel 7 T magnet from Magnex Scientific delivered to the University of Kent has, in addition to the main superconducting solenoid, a supercon- ducting sweep coil that can alter the magnetic field Ž .by 1 T. The boil-off rate is slightly increased ;10% whilst sweeping and the sweep operation in no way detracts from the performance of the system in high resolution mode when it is not being swept. The magnet has excellent resolution, being entirely equivalent in all tests, due to a dedicated high resolu- tion solid state NMR spectrometers, producing a 13 C MAS decoupled linewidth on admanatane of 2.5 Hz before and after a field sweep run using the same room temperature shim file. Hence, this extremely versatile NMR spectrometer provides both broad line field sweep and conventional high resolution opera- tion in a single instrument. The novel wideline field sweep operation of this system is illustrated by 27 Al for which the system has a nominal resonance frequency of 79 MHz. Spectra were accumulated using spin-echoes on a Chemag- netics CMX 300 Infinity spectrometer with low power 908 pulses of 45 ms duration. The additional field is controlled by the current from a Lakeshore 610 that is set directly from the Spinsight software via an RS-232 link. The field can be varied by "0.5 T with a settability of 0.0001 T and exhibits a highly Fig. 2. 27 Al field sweep NMR spectrum of yttrium aluminum garnet using a frequency of 78.457 MHz with 4.5 G steps between successive Ž .spin-echoes with 80 transients averaged at each point using a recycle time of 5 s. The singularities from the two sites are marked Al 1 1,2 Ž . Ž .and Al 2 . Note that the frequency axis is broken for convenience .1,2
  • 4. ( )I.J.F. Poplett, M.E. SmithrSolid State Nuclear Magnetic Resonance 11 1998 211–214214 linear current-field relationship. The current is set to Ž .a particular value, allowed to settle ;1 min and a series of spin-echoes accumulated using extended w xphase cycling 16 to give sufficient signal-to-noise before stepping to the next field setting and repeating the experiment, eventually stepping through the com- plete lineshape. All of this is done automatically from within the pulse program and the results from a-Al O are shown in Fig. 1a. A classical first-order2 3 quadrupolar powder pattern is seen with three singu- Ž .larities from which the quadrupole coupling C ofQ Ž .2.3 MHz and asymmetry parameter h of 0 can be deduced, in good agreement with single crystal work w x17 . The field sweep spectrum can be compared to a Ž .conventional one pulse spectrum Fig. 1b where the central transition is strongly recorded with only the first two singularities of the broad outer transitions Ž .showing up as weak features. An echo Fig. 1c still records only the first two singularities, but better records the overall outer transition intensity com- pared to a single pulse. Both single pulse and echo spectra do not have sufficient bandwidth to record the complete lineshape and distort the intensity of the outer transitions that is recorded. A second illustration is given by the 27 Al spectrum Ž .of yttrium aluminium garnet YAG that has octahe- drally and tetrahedrally coordinated aluminium sites whose C s differ by a factor of 10, and have beenQ extensively characterised by NMR of single crystals w x w x18 and powder MAS 19,20 . A highly detailed distortion-free field sweep NMR spectrum is ob- Ž . Ž .tained Fig. 2 and gives C s of 0.6 MHz AlOQ 6 Ž .and 6.0 MHz AlO both with hs0, in good4 agreement with previous work. In both cases, it should be noted that the field sweep NMR spectra are as they come from the spectrometer. There has been no correction of the baseline nor need to phase the spectrum. 27 Al NMR broadline spectra obtained by sweeping a superconducting solenoid allow much information to be extracted. These experiments were performed on a spectrometer that additionally has all the capa- bilities of a conventional high resolution solid state NMR spectrometer. This greatly enhances the range of nuclei that can be meaningfully and routinely studied. There are other new experiments that could be envisaged such as one-dimensional stray field imaging of solids without moving the sample. These possibilities mean that such an NMR instrument could be a standard commercial configuration and have a widespread application in solid state chem- istry, mineralogy, materials and biological sciences. Acknowledgements Ž .The authors thank Mr. R.S. Thomas Kent and Ž .Dr. P. Jonsen Chemagnetics for their help in imple- Žmenting the field sweep, Dr. T.J. Bastow CSIRO, .Melbourne for his helpful comments on the manuscript and the University of Kent for its support of NMR. References w x1 E. Fukushima, S.B.W. Roeder, Experimental Pulse NMR, Addison-Wesley, Reading, MA, 1981. w x2 A.E. Derome, Modern NMR Techniques for Chemistry Re- search, Pergamon, Oxford, 1987. w x3 G. Engelhardt, D. Michel, High Resolution Solid-State NMR of Silicates and Zeolites, Wiley, Chichester, 1987. w x4 K. Schmidt-Rohr, H.W. Spiess, Multidimensional Solid-State NMR and Polymers, Academic Press, London, 1994. w x Ž .5 T.J. Bastow, M.E. Smith, Solid State NMR 1 1992 165. w x Ž .6 T.J. Bastow, Solid State NMR 3 1994 17. w x Ž .7 Y. Furukawa, S. Wada, J. Phys. Condensed Matter 6 1994 8023. w x8 S. Clough, A.J. Horsewill, P.J. McDonald, F.O. Zelaya, Ž .Phys. Rev. Lett. 55 1985 1794. w x9 O. Madelung, Introduction to Solid-State Theory, Springer- Verlag, Berlin, 1981. w x Ž .10 A. Narath, Phys. Rev. 162 1967 320. w x Ž .11 H. Ebert, J. Abart, J. Voitlander, J. Phys. F 16 1986 1287. w x12 E. Ehrenfreund, A.C. Gossard, F.R. Gamble, T.H. Geballe, J. Ž .Appl. Phys. 42 1971 1491. w x13 J. Abart, E. Pelangie, W. Socher, J. Voitlander, J. Chem. Ž .Phys. 78 1983 5468. w x14 X. Wu, E.A. Juban, L.G. Butler, Chem. Phys. Lett. 221 Ž .1994 65. w x15 G.L. Turner, R.J. Kirkpatrick, S.H. Risbud, E. Oldfield, Am. Ž .Ceram. Soc. Bull. 66 1987 653. w x16 A.C. Kunwar, G.L. Turner, E. Oldfield, J. Magn. Reson. 69 Ž .1986 124. w x Ž .17 R.V. Pound, Phys. Rev. 79 1950 685. w x Ž .18 K.C. Brog, W.H. Jones, C.M. Verker, Phys. Lett. 20 1966 258. w x19 R. Dupree, M.H. Lewis, M.E. Smith, J. Appl. Cryst. 21 Ž .1988 109. w x20 D. Massiot, C. Bessada, J.P. Coutures, F. Taulelle, J. Magn. Ž .Reson. 90 1990 231.

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