ARTICLE IN PRESS

0926-2040/$ - se

doi:10.1016/j.ss

�CorrespondE-mail addr

Solid State Nuclear Magnetic Resonance 30 (2006) 55–59

www.elsevier.com/locate/ssnmr

Hardware modification of a 7mm MAS NMR probe toa single-crystal goniometer

Gabor Kovacs, Janos Rohonczy�

Department of General and Inorganic Chemistry, Institute of Chemistry, Eotvos Lorand University, Budapest, Hungary

Received 26 October 2005; received in revised form 31 January 2006

Abstract

Tensorial terms of the Hamiltonian can be measured by solid-state single-crystal nuclear magnetic resonance (NMR) spectroscopy

which requires a goniometer NMR probehead. Goniometer probes; however, are not standard parts of solid NMR spectrometers and are

available only at a much higher price than magic-angle spinning (MAS) probeheads widely used in research. Due to requirements of

MAS experiments, modern probeheads are designed for small ceramic rotors, which are 1–4mm in diameter, to reach very high angular

frequencies, so there are several older 7mm MAS probeheads used rarely todays in NMR laboratories. In this paper, a simple method is

presented how to rebuild step-by-step a 7mm Bruker MAS probehead to be suitable for single-crystal spectroscopy. In the second part31P chemical shift tensors of Na4P2O7 � 10H2O are determined to demonstrate the functionality of the rebuilt probehead.

r 2006 Elsevier Inc. All rights reserved.

Keywords: NMR; Single crystal; MAS probe; ASICS; Goniometer; Sodium pyrophosphate; Home-built probehead; Nuclear magnetic resonance; 31-P

NMR; Crystal orientation

1. Introduction

Several magnetic interactions are known in the nuclearmagnetic resonance (NMR) spectroscopy, thus manytensorial terms appear in the Hamiltonian. Most importantones are chemical shielding, dipolar and quadrupolarcoupling. In solid state, the intermolecular distances andorientations are fixed and NMR spectra become sensitiveof crystal orientation. In single crystals, macroscopicorientation correlates directly with molecular orientation.This makes it possible to determine not only the isotropicaverage but all components of tensors. Knowing thesevalues may give further details on microscopic structure,bonds in solid state, etc. Theoretical calculations alsoprovide primarily tensorial components and they areaveraged mathematically later in NMR spectra simula-tions. In testing the accuracy of mathematical methodscomparing of tensorial elements directly is a better waythan comparing simulated isotropic spectra.

e front matter r 2006 Elsevier Inc. All rights reserved.

nmr.2006.02.002

ing author. Fax: +361 372 2909.

ess: rohonczy@para.chem.elte.hu (J. Rohonczy).

As these NMR experiments require large single crystalsthat in many cases hardly available, other methods havebeen developed to determine anisotropy components.Principal values of chemical shielding and partial informa-tion on orientation can be obtained analyzing rotationalsidebands in magic-angle spinning (MAS) experiments orusing two-dimensional methods [1–4]. All techniques usingpowder samples; however, can supply only relativeinformation on tensor orientations, i.e. orientation betweenshielding tensors of different nuclei. To determine orienta-tion relative to the crystal lattice the most straightforwardway is still to use solid-state single-crystal spectroscopy. Insuch an experiment, several NMR spectra need to berecorded of one single crystal, knowing the direction of thecrystal axes in the laboratory frame for each spectrum. Theexperiment requires a goniometer NMR probehead forexact orientation of the crystal. Goniometer probes;however, are not standard parts of solid NMR spectro-meters and are available only at a much higher price thanMAS probeheads widely used in research today or theymust be constructed at home [5–7]. In this technical paper,a simple method to rebuild a 7mm Bruker MAS probeheadto be suitable for single-crystal spectroscopy is presented.

ARTICLE IN PRESS

Fig. 1. Probehead rebuilding kit: (1) top PCB plate, (1a) hold-down-clip,

(1b) signal contact, (1c) cord leader, (1d) signal contact wire, (2) bridge, (3)

copper pins (1.50mm in diameter), (4) board to keep pins in line, (5) RF

contacts, (6) ratchet pawl (signal contact), (7) ratchet wheel (sample

holder), (8) screws to hold the pawl and (9–10) signal connector and fixing

plate.

G. Kovacs, J. Rohonczy / Solid State Nuclear Magnetic Resonance 30 (2006) 55–5956

2. Hardware construction

To get high-resolution solid-state spectra similar to theones recorded in liquid phase, MAS has been developed[8,9]. Under MAS conditions the sample is filled in a rotorand it is rotated about an axis inclined at 54:7� to theexternal magnetic field. Typical rotating frequencies are inthe region of 5–27 kHz. As a side-effect there are rotationalsidebands under MAS conditions. In order to decrease thedisturbing effect of sidebands for observing isotropic lines,MAS NMR probeheads are designed to reach higher andhigher spinning frequencies [10]. Probably, there are older7mm MAS probeheads used rarely todays in several NMRlaboratories as modern MAS probes are designed forrotors 1–4mm.

It was our basic principle not to degrade the originalfunctionality of the probehead so the rebuilding wascarried out in a completely reversible way. All extra partswere fixed by screws, pins or soldering. No extra hole wasdrilled in the original parts. Some other aspects had to beconsidered during the rebuilding process.

�

Fig. 2. Modifying the flipping angle of the stator. Left: original state of

the stator. (A) top ring, (B) stator, (C) pin, (D) metal ring, (E) RF

contacts, (F) air flow pipe for sample eject and (G) flipping rod. Right:

inserting the bridge and the stiffening plate and fixing them by three pins

indicated by vertical arrows.

No ferromagnetic or strong paramagnetic materialscould be used. It would have been a security risk as verystrong attractive forces inside the cryomagnet can eventear out small parts. These flying parts can damageelectronics around the probe and it is extremely difficultto remove them from the cryomagnet bore. Plexi-glassproved to be an ideal material for our extra parts. It canbe shaped easily.

� Bruker narrow bore magnets widely used all over theworld have a bore of 54mm. There is very small room inthe probehead cylinder for our extra parts. In thesespectrometers using a worm for positioning the crystalwould require more space or disassembling the probe-head in an irreversible way. In present case, instead ofcontinuous positioning we used discrete steps. A ratchetwheel was designed to orientate the sample in themagnet. The sample was glued directly onto the wheel.This solution has some advantages: the same precisioncan be reached as if a worm and a stepper motor wasapplied, can be operated with a single cord, cheap, anumber of identical gears may be built and store thesamples glued for several experiments. On the otherhand, there is no need to have more than 10–15 points tofit sine curves. � Keeping the size of metallic parts as small as possible isimportant because all metallic parts influence theoscillator impedance adding extra capacity must becompensated by tuning the probe. They reduce thefrequency range in which the probehead operatesproperly. Therefore, thin wires were used and unneces-sary copper from printed circuit board (PCBs) wasremoved.

Respecting these considerations, 10 new parts wereconstructed. This ‘rebuilding kit’ can be seen in Fig. 1.

At first, the metal ring (denoted by D in Fig. 2) fixing theair flow pipe (F) was moved away with ca. 8mm along thepipe from the bottom of ceramic stator (B). After detachingof the pipe, the metal ring was removed from the pipe andkept in a safe place. Unfortunately, the probehead isdesigned originally to operate only in a small regionaround the magic angle. To change the flipping angle of thestator to be perpendicular to the external field requiresconstructing a plexi-glass ‘bridge’ (2 in Fig. 1) and astiffening plate of PCB (4 in Fig. 1). The original copperpin (C) between the stator (B) and the flipping rod (G) wasgently knocked out by a metal pin having smaller diameter.Such a reversible way all pins can be carefully removed andreinserted several times. The bridge is inserted between thestator and the flipping rod and fixed by pins. The stiffeningplate (4) is fixed by the same two pins to the bridge (2) anda shorter one to the rod (G) to avoid extra freedom ofmovement and make adjustment possible both in push andpull directions by the micrometer screw. Details can be seenin Fig. 2 on the right. Calibration and fine adjustment ofthe flipping angle will be done by micrometer screw atbottom.The gold-plated electric contacts (E) that connect the coil

in the stator to the amplifier circuit are short, too. A newpair of copper contacts (5) (with 1mm in diam. gold-platedcopper pins) were assembled to the original contacts to

ARTICLE IN PRESS

Fig. 3. Drawing of the bridge (2) and the ratchet pawl (1b,6). Dimensions are given in mm. Holes in the bridge are 1.55mm in diam. Thickness of PCB of

the pawl is 0.7mm.

Fig. 4. Drawing of the top PCB plate (1) with the hold-down-clip (1a) viewed from above and the ratchet-wheel as sample holder (7). One step of the wheel

equals to 13:85� (26 cogs on the wheel). The wheel has two small glass mirrors indicated at the outline of the wheel to help to determine initial offset angles.

On the right-hand side shape of a dummy sample holder can be seen with a mirror on top. It is for calibration of flipping angle of the stator. Dimensions

are given in mm.

1This test device should be connected only when clicking the gear and

no acquisition is running. Otherwise it causes severe noise in spectra.

G. Kovacs, J. Rohonczy / Solid State Nuclear Magnetic Resonance 30 (2006) 55–59 57

enlarge them by genuine plastic screws of the stator(contacts and screws are shown in Fig. 5). As the originalRF contacts are made also by hand, exact geometry andsizes cannot be given for these extra parts. A PCB (1) wasassembled to the top of the probehead that is used forsoldering the cord leader (1c) and a signal contact inappropriate positions. Details can be seen in Figs. 2 and 3.

As it was already mentioned, ratchet wheels wereconstructed of plexi-glass as sample holder. It is essentialto know the exact rotational angle of the crystal during theexperiment. For this purpose two small mirrors (made ofglass cover slip used with microscope slides) were cyano-acrylate glued to the sample holder. The ratchet pawl hasdouble functionality. It is a signal contact. It ensures thatexactly one cog had stepped by clicking the wheel to thenext position. On the other hand, the pawl allows to clickthe wheel to one direction only and blocks the other whichmakes possible to have a well defined orientation for eachcog. This pawl is fixed by a small copper piece (8) havingtwo perpendicular holes for screws. One of them has sleeve-nut to hold signal contact (6). It is fastened with a screw inan original hole of the frame of the probe (can be seen inFig. 5). The other contact of this switch was assembled tothe top PCB (1b,d) plate. The signals of this switch is ledout of the probe by a thin shielded wire to a signalconnector (9,10) assembled at bottom by screws of the BBRF connector. The shielded wire is led through the probealong the wire of the optical sensor of MAS rotor. To fix

the laying surface of the gear onto the stator a hold-down-clip (1a) was assembled to the top plate, too. Drawings ofthe pawl, top plate and wheel can be seen in Figs. 3 and 4.A single non-elastic cord can be applied to turn the

crystal from one position into another. The cord is placedin the probe as it is shown in Fig. 5 on the left. It is slippedin the probe through the hole of the top PCB plate (1), ledacross the cord leader (1c) and around the gear in its slot. Itis convenient to put a thin silicone ring in the slot of thegear to avoid slithering of the cord. At inserting the probeinto the magnet the cord is passed through up the magnetbore and can be fastened outside. It is proved to be the bestto drive the ratchet manually at the top of the magnet asone can ‘feel’ as the gear clicks. It can be turned by pullingthe proper end of the cord while keeping it gently tight bythe other. After clicking the gear to the next position it isimportant to be pulled back by the other end of the corduntil the ratchet pawl blocks it in order to get a well-definedposition. It is easy to check that exactly one cog has beenstepped by conductivity test sound of a multimeterconnected at the bottom of the probe (9).1

There is another cord. It is tied to the hold-down-clipand led through up the magnet bore in the same way.Tightening it with an aluminum block (weight of approx.

ARTICLE IN PRESS

Fig. 5. Rebuilt 7mm Bruker MAS probehead. On the left-hand side the

pulling and tightening cords (denoted by p.c., t.c., respectively) can be seen

slipped in the probe. The tightening cord is tied to the hold-down-clip (1a)

at the point marked by arrow. On the right-hand side the hold-down-clip

is in its working position. Plastic screws (p.s.) fixing the new contacts to

the original ones can be seen, too. Other parts are denoted by labels

mentioned before.

Fig. 6. Left: optical layout for calibrating flipping angle to 90�. The

double arrow shows the fine adjustment of the flipping angle of the stator.

Right: optical layout for determining initial offset angle ðtan a ¼ x=2mÞ

using the outer mirror of the sample holder. B represents the direction of

the magnetic field relative to the probe when the probe is inserted into the

magnet. Other parts are denoted by labels mentioned before.

G. Kovacs, J. Rohonczy / Solid State Nuclear Magnetic Resonance 30 (2006) 55–5958

0.4 kg) outside the magnet, the ratchet wheel has a stablelaying on the surface of the stator.

The rebuilt probehead with the pulling and tighteningcords slipped in it can be seen in Fig. 5. After having thecovering cylinder mounted, it is ready to be inserted intothe magnet.

3. Application notes

In this section, some application notes are provided howto carry out single-crystal solid-state experiments. A testexperiment is described determining CSA of 31P inNa4P2O7 � 10H2O. It is proved to be an ideal test material.

�

2

It has single crystals in cm size.

� It is easy to determine crystal axes by macroscopicgeometry, because of well-grown surfaces. It is non-toxic, inexpensive, commercially available. � Refined crystal structure is available in the literature. Itis monoclinic with cell dimensions a ¼ 17:93 A, b ¼

6:96 A, c ¼ 14:85 A and b ¼ 118�310. The space group isI2/c. It has two chemically equivalent 31P nuclei joinedby symmetry operations [11–13].

� 31P has 1

2spin and it is easy to detect. One scan is

acceptable in each crystal position.

� The crystal has acceptable low air sensitivity. (High-power 1H decoupling is applied during acquisition. Tokeep the crystal thermostated a constant air-flow isnecessary. In 24 h it becomes slightly opaque, in someweeks the crystal is completely degraded.)

The crystal surfaces of Na4P2O7 � 10H2O was indexedwith an optical goniometer. A spindle-stage, a commoninstrument in mineralogy, was used for fixing the crystaloriented on the gear. The spindle-stage has two handlesand a microscope to orientate, then hold the gear and thecrystal simultaneously until the glue hardens. The crystalwas fixed by solvent-free glue.2 The plexi gear has two glassmirrors to help fixing the crystal oriented to the gear. Theinner mirror of the plexi gear is used as a reference point.After fixing the crystal offset angles should be measured;

It was similar to well-known brands such as Henkel’s Pattex Powermix.

one between the inner mirror and a well-defined crystalsurface; the other between the outer and the inner mirror.At this point crystal orientation is known relative to theouter mirror.Next a dummy sample holder can be inserted in the

probehead having a mirror on top (shown in Fig. 4). Afterinserting this dummy sample holder, an optical layoutshould be set up to calibrate the flipping angle of the statorby red laser pointer beam. It is convenient to make a righttriangle with sides of 1.5, 2 and 2.5m of hardly elasticcopper wire to help placing the probehead, screen and laserpointer in proper geometry. The flipping angle should befine adjusted by the micrometer screw until equals exactlyto 90� (left-hand side in Fig. 6).After having calibrated the flipping angle of the stator,

the sample holder gear with the crystal glued on it can beinserted. It is convenient to mark one cog of the wheel as‘origin’ and turn this cog to the pawl. Then the laserpointer can be used in a similar layout to measure distancex and to calculate initial offset angle belongs to this cog byreflection on the outer mirror on the gear. At this point alldata is known to calculate crystal orientation relative to theexternal magnetic field knowing the ‘cog number’ as theratchet steps. Then the pulling and tightening cords shouldbe slipped in the probe. This optical layout can be used toestimate angular precision of the ratchet as well by clickingthe gear into this origin position by the cords and measureoffset angle by the laser pointer several times. Angularprecision was found to be �1�. Finally the coveringcylinder is to be mounted. It is ready to be installed intothe magnet. By passing a thread with a copper ring on itsend from top through the magnet, the pulling andtightening cords can be lifted through the magnet usingthe ring as a threader as the probehead is being insertedfrom bottom. It is recommended to invoke somebody’shelp. One can lift the cords by the threader at the top whilethe other one inserts and mounts the probe at the bottom.At last, the tightening cord can be weighted.Magnetic field homogeneity was shimmed using an

original zirconium-dioxide MAS rotor filled with D2O

ARTICLE IN PRESS

Table 1

Chemical shift tensors of two 31P nuclei in Na4P2O7 � 10H2O

Nuc. siso (ppm) Principal values (ppm) Orientation Ref.

1. �1 80 �48 �31 a ¼ 192� b ¼ 47� g ¼ 28� Our data

2. �2 79 �48 �31 a ¼ 8� b ¼ 49� g ¼ 329� Our data

1./2. �1 76.3 �47.9 �31.4 Not published [17]

1. �1.7 76 �49 �32 Not published [18]

2. �2.0 78 �58 �26 Not published [18]

Estimated errors are �2 ppm, �2�. Note that the two nuclei are chemically equivalent.

G. Kovacs, J. Rohonczy / Solid State Nuclear Magnetic Resonance 30 (2006) 55–59 59

and standard 2H lock channel. 31P spectrum was referencedto 85% H3PO4 solution. In our test experiment chemicalshift tensors of Na4P2O7 � 10H2O was determined. T1

relaxation time was measured and delay time was set to15min between two consecutive scan. Spectra wererecorded using one scan. During acquisition high-power1H decoupling was applied. (TD ¼ 8192, SW ¼ 100 kHz,SFO1 ¼ 202:4639542MHz, P1 ¼ 14:6 ms, PL1 ¼ 0 dB,D1 ¼ 15 min, Bruker DRX 500 spectrometer, MAS 500SB-BL7 probe).

To determine the chemical shift tensors a computerprogram called ASICS was used. It is freely available fromthe internet and has been developed to evaluate single-crystal NMR spectra [14]. ASICS is capable to determinenot only chemical shift tensors but quadrupolar couplingtensors as well. ASICS was originally developed for Varianspectrometer data format. A small program was written byus (available on request) to convert Bruker-type data setsfor ASICS readable format. For further details on ASICS,see Ref. [15].

The Tenon frame was defined that two axes coincidewith crystallographic axes a, b. The third axis denoted by c�

is perpendicular to a and b. About each axis in 13 stepswere 31P spectra recorded (a rotation of 180�).

The principal values and orientation of chemical shifttensors of the two 31P can be found in Table 1. Euler anglesare defined following the convention can be found in Ref.[16]. In Refs. [17,18], MAS spectra were used to determinethe components of the tensor. Principal values given bypresent method are in accordance with them.

4. Conclusion

A simple method has been presented step-by-step how torebuild a 7mm Bruker MAS probehead for single-crystalspectroscopy. Using this hardware chemical shift, dipolarand quadrupolar coupling tensors can be determined.

Chemical shift tensors of 31P in sodium pyrophosphatedecahydrate have been determined to demonstrate thefunctionality.

Acknowledgments

The use of instruments at the Department of Miner-alogy, Eotvos Lorand University, Budapest is acknowl-edged. We thank T. Vosegaard for submitting sources ofASICS. This project was supported by OTKA ResearchFoundation of Hungary (T 037658).

References

[1] M.M. Maricq, J.S. Waugh, J. Chem. Phys. 70 (1979) 3300.

[2] J. Herzfeld, A.E. Berger, J. Chem. Phys. 73 (1980) 6021.

[3] M. Linder, A. Hohener, R.R. Ernst, J. Chem. Phys. 73 (1980) 4959.

[4] J.S. Shore, S.H. Wang, R.E. Taylor, A.T. Bell, A. Pines, J. Chem.

Phys. 105 (1996) 9412.

[5] T. Vosegaard, V. Langer, P. Daugaard, E. Hald, H. Bildsøe, H.J.

Jakobsen, Rev. Sci. Instrum. 67 (1996) 2130.

[6] W.P. Power, S. Mooibroek, R.E. Wasylishen, T.S. Cameron, J. Phys.

Chem. 98 (1994) 1552.

[7] R.S. Honkonen, F.D. Doty, P.D. Ellis, J. Am. Chem. Soc. 105 (1983)

4163.

[8] E.R. Andrew, A. Bradbury, R.G. Eades, Nature 182 (1958) 1659.

[9] I.J. Lowe, Phys. Rev. Lett. 2 285 (1959) 2285.

[10] M. Mehring, Principles of High Resolution NMR in Solids, second

ed., Springer, Berlin, 1983.

[11] D.M. MacArthur, C.A. Beevers, Acta Crystallogr. 10 (1957) 428.

[12] W.S. McDonald, D.W.J. Cruickshank, Acta Crystallogr. 22 (1967)

43.

[13] D.W.J. Cruickshank, Acta Crystallogr. 17 (1964) 672.

[14] http://nmr.imsb.au.dk/bionmr/software/asics.php

[15] T. Vosegaard, E. Hald, V. Langer, H.J. Skov, P. Daugaard,

H. Bildsøe, H.J. Jakobsen, J. Magn. Reson. 135 (1998) 126.

[16] K. Schmidt-Rohr, H.W. Spiess, Multidimensional Solid-state NMR

and Polymers, Academic Press, San Diego, 1994.

[17] A. Kubo, C.A. McDowell, J. Chem. Phys. 92 (1990) 7156.

[18] S. Un, M.P. Klein, J. Am. Chem. Soc. 111 (1989) 5119.