tunneling-induced spin alignment at low and zero field

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Tunneling-induced spin alignment at low and zero field M. Tomaselli, U. Meier, and B. H. Meier Citation: The Journal of Chemical Physics 120, 4051 (2004); doi: 10.1063/1.1649315 View online: http://dx.doi.org/10.1063/1.1649315 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/120/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Spin-symmetry conversion in methyl rotors induced by tunnel resonance at low temperature J. Chem. Phys. 140, 084302 (2014); 10.1063/1.4865835 Methyl rotational tunneling dynamics of p-xylene confined in a crystalline zeolite host J. Chem. Phys. 121, 4810 (2004); 10.1063/1.1781119 Determination of the proton tunneling splitting of the vinyl radical in the ground state by millimeter-wave spectroscopy combined with supersonic jet expansion and ultraviolet photolysis J. Chem. Phys. 120, 3604 (2004); 10.1063/1.1642583 Detection of the tunneling-rotation transitions of malonaldehyde in the submillimeter-wave region J. Chem. Phys. 110, 4131 (1999); 10.1063/1.478296 Microwave rotation-tunneling spectroscopy of the water–methanol dimer: Direct structural proof for the strongest bound conformation J. Chem. Phys. 107, 3782 (1997); 10.1063/1.474736 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 155.247.166.234 On: Sun, 23 Nov 2014 01:15:22

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Page 1: Tunneling-induced spin alignment at low and zero field

Tunneling-induced spin alignment at low and zero fieldM. Tomaselli, U. Meier, and B. H. Meier Citation: The Journal of Chemical Physics 120, 4051 (2004); doi: 10.1063/1.1649315 View online: http://dx.doi.org/10.1063/1.1649315 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/120/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Spin-symmetry conversion in methyl rotors induced by tunnel resonance at low temperature J. Chem. Phys. 140, 084302 (2014); 10.1063/1.4865835 Methyl rotational tunneling dynamics of p-xylene confined in a crystalline zeolite host J. Chem. Phys. 121, 4810 (2004); 10.1063/1.1781119 Determination of the proton tunneling splitting of the vinyl radical in the ground state by millimeter-wavespectroscopy combined with supersonic jet expansion and ultraviolet photolysis J. Chem. Phys. 120, 3604 (2004); 10.1063/1.1642583 Detection of the tunneling-rotation transitions of malonaldehyde in the submillimeter-wave region J. Chem. Phys. 110, 4131 (1999); 10.1063/1.478296 Microwave rotation-tunneling spectroscopy of the water–methanol dimer: Direct structural proof for the strongestbound conformation J. Chem. Phys. 107, 3782 (1997); 10.1063/1.474736

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Page 2: Tunneling-induced spin alignment at low and zero field

COMMUNICATIONS

Tunneling-induced spin alignment at low and zero fieldM. Tomaselli,a) U. Meier, and B. H. MeierPhysical Chemistry, ETH-Zu¨rich, CH-8093 Zu¨rich, Switzerland

~Received 17 December 2003; accepted 30 December 2003!

The transfer of rotational to spin angular momentum of CH3 groups according to the Haupt effectis shown to be independent of magnetic field strength, including zero field. Haupt enhanced pulsednuclear resonance signals ofg-picoline have been observed at fields below 50 mT with a sensitivityenhancement of more than 3 orders of magnitude over thermally polarized experiments. ©2004American Institute of Physics.@DOI: 10.1063/1.1649315#

The spin eigenfunctions of CH3 groups are classified ac-cording to A and E symmetry with a total proton nuclear spinof I 53/2 and I 51/2, respectively. According to the Pauliprinciples, functions of symmetry A and E combine withrotational states of the same symmetry tunnel-split byD t .Fast methyl rovibrational relaxation within each manifold ofrotational states~A or E! is usually observed in the solid andcaused by efficient phonon-driven modulations of the rota-tional potential. The situation is, however, dramatically dif-ferent for cross-relaxation among A and E spin species. Sym-metry conversion involves a flip of a proton spinsimultaneously with the change in tunneling state and hastherefore a considerably lower probability than A-A or E-Erelaxation.1 Accordingly, the populations of the ground CH3

tunneling states~characterized by a tunneling temperatureTt

such thatpE /pA5e2D t /kTt) are often slow to respond tochanges in the lattice temperature and may significantly de-viate from the Boltzmann equilibrium. This effect has strik-ing consequences in the nuclear magnetic resonance~NMR!of compounds containing CH3 groups with large tunnel fre-quency and accordingly low rotational potential barrier, i.e.,n t5D t /h@un0u, wheren052gHB0/2p denotes the protonLarmor frequency at the applied magnetic field strengthB0 .It has been shown by Haupt and by others2 that below;60K a large dynamic proton dipolar polarization can be inducedafter a sudden change in temperature of the solid~often re-ferred to as the ‘‘Haupt effect’’!. Invoking spin thermody-namic arguments the phenomenon may be understood as acoupling between the dipolar and the rotational tunnelingreservoir, the latter one possessing a much larger heat capac-ity and being driven out of thermal equilibrium by rapidtemperature steps.2 We have found previously that forg-picoline (n t.130 GHz) up to a;70 fold enhancement ofthe tunneling-induced1H signal compared to the thermalZeeman signal is obtained at a field of 5.17 T, and haveproposed this as a possible method of polarizing rare nuclearspins by double-resonance techniques and utilizing a suitable‘‘Haupt matrix’’.3 Since the tunnel polarization is indepen-dent of magnetic field strength, the Haupt effect could be a

potential approach of sensitive nuclear resonance detection atlow and zero field, thereby benefiting from the intrinsic highresolution of pure electric quadrupole and dipole-dipole in-teraction spectroscopy4–7 as well as reducing the require-ments of a high-field magnet to produce reasonable polariza-tions ~i.e.,hn0/2kT.631023 at 23.5 T and 4 K!. Moreover,the conditionn t@un0u is easier to fulfill at low fields and itcan be surmised that a larger class of methyl compounds ispotentially useful for Haupt enhancement. We have verifiedthe feasibility of Haupt experiments at low and zero field byobserving the enhancement of the nuclear resonance ing-picoline produced by a sudden change in sample tempera-ture. The experiments shown in Fig. 1 were performed in astatic magnetic field of 23.5 mT provided by a small resistive90 mm bore diameter magnet~4800-turn solenoid; 1.6 mH,22.9 V! driven by a bipolar operational power supply inconstant-current mode~BOP 72-6M, Kepco, Inc.!. Thesample containing 0.4 cm3 g-picoline ~Aldrich, 99%! wasplaced in a 8 mm-diam solenoid~22 turns,;2 mH! of a tankcircuit operating at 1 MHz, the1H Larmor frequency at 23.5mT. The nuclear resonance was detected using a modifiedInfinity-plus spectrometer~Varian, Inc.!, the 1 MHz signalbeing preamplified~AM-1299, Miteq! and converted to 42MHz before fed into the quadrature receiver. Typicalp/2pulse lengths of 10ms were adjusted at 0.5<n0<2 MHzusing a modified AR-100LMB power amplifier. Low tem-perature operation was achieved by fitting the probe assem-bly into a continuous flow helium cryostat CF1200~Oxford,Inc.!. The sensitivity-enhanced dipolar1H spectra, each ob-tained in a single shot using a solid-echo excitation sequencewith 10° tip-angle pulses are shown in Figs. 1~a! and 1~b!.The transients were acquired 140 s after a 9→55 K ‘‘up-ward’’ temperature jump@including 40 min of thermal equili-bration at 9 K; Fig. 1~a!# and 400 s after a 9←55 K ‘‘down-ward’’ temperature jump@Fig. 1~b!#, respectively. Theinversion of the NMR signal depending on the direction ofthe temperature step indicates a reversal in sign of the dipolarspin temperature caused by the different initial populationimbalance among the tunnel-split A and E levels.2,3 Figure1~c! shows the appearance of the ordinaryg-picoline protonresonance at 7.5 K~156 transients!. For comparison, the pro-a!Electronic address: [email protected]

JOURNAL OF CHEMICAL PHYSICS VOLUME 120, NUMBER 9 1 MARCH 2004

40510021-9606/2004/120(9)/4051/4/$22.00 © 2004 American Institute of Physics

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ton line of water~also at 1 MHz with 1024 transients! isdisplayed in Fig. 1~d!. In all four cases the spectrometer gainsettings were identical. The water resonance, however, wasproduced by a sample of two times the number of nuclei asthe g-picoline and the quality factor of the 1 MHz tank cir-cuit was;10 times higher than the one used to obtain thespectra in Figs. 1~a!–1~c!. On the basis of comparison withthe weakg-picoline resonance in Fig. 1~c!, we estimate thenuclear population difference to be increased by about 4 or-ders of magnitude in our experiment, conforming to a protonpolarization of;3%. The NMR signals obtained were suffi-ciently intense (S:N.100) so that the observation of thetransient dynamic polarization and measurement of the longpolarization and depolarization times could be made withsuccessive almost nondestructive excitation pulses by reduc-ing the rf power to a low level. Figure 2 shows the build-upand decay of the1H magnetization following a temperaturejump from 9 to 55 K. Two types of experiments were per-formed: In ~a! dipolar alignment was induced at zero field

and adiabatically transferred to Zeeman order by cyclingB0

to 23.5 mT for signal detection purposes only. In~b! thetransient dipolar signal@p/2 phase shifted with respect to theZeeman signal in~a!# was also sampled every 20 s withsmall tip-angle echo excitation withB0 constant at 23.5 mT(n051 MHz) or 47 mT (n052 MHz). The insets in Fig. 2show the experimental temperature—B0—and rf-pulsetiming diagrams as well as typical complex 1 MHz protonspectra obtained at the maximum of the transient Hauptresponse. The thin solid curves in Fig. 2 are fits to theexperimental data according to the formMa,b(t)56M@e2t/Td2e2t/Tc#,2 whereTc denotes the A–E inter-conversion time andTd the dipolar relaxation time, both atthe final temperature.M depends on the amplitude of thetemperature jump as well as on the initial tunnel polarization.We obtain for all three experimentsTc5120(10) s andTd

565(10) s at 55 K, consistent with earlier measurements atmuch higher field.2,3 Similar experiments were also per-formed for a 9←55 K temperature jump and we extractTc

51200(100) s andTd5110(10) s at 9 K, again in agreementwith the high-field values. Adiabatic field-cycling schemescan also be used to directly detect the nuclear quadrupoleresonance~NQR!, e.g., of the nitrogen-14 spins (I 51) at

FIG. 1. 1H NMR spectra recorded at 23.5 mT~1 MHz Larmor frequency!.~a! and ~b! show tunneling-enhanced dipolar1H spectra ofg-picoline fol-lowing an upward~9→55 K within 20 s! and downward~9←55 K within 2min! temperature jump, respectively. Both spectra were obtained in a singleshot using abx-td-by-td-det solid-echo withtd525ms andb5p/18 tip-angle pulses.~c! Thermal1H spectrum ofg-picoline at 7.5 K obtained by asolid-echo excitation withtd525ms andb5p/2; 156 acquisitions with 400s repetition delay. All other spectrometer parameters were identical as in~a!.~d! 1H resonance of water at 300 K; 1024 acquisitions, 2 s repetition delayusing ap/2-td-p-td-det Hahn-echo~Ref. 8! with td5200ms. The;50 Hzresolution of our present low-field setup is dominated by current instabilitiesof the B0 power supply. We expect that with improved stability and shimlinewidths below 1 Hz can be obtained.

FIG. 2. Proton magnetization intensity traces ing-picoline at 55 K follow-ing a temperature jump~9→55 K within 20 s! after 40 min of thermalequilibration at 9 K. The transient signal was sampled every 20 s with asolid-echo using;1° tip-angle pulses (td525ms). In ~a! the dipolar orderwas produced at zero field and adiabatically converted to Zeeman order~circles! by field cycling~within 0.5 s! for echo detection at 23.5 mT. Con-versely, in~b! dipolar order was produced at a constant field of 23.5 mT~squares! or 47 mT~triangles! and is proportional to the negative intensity ofthe imaginary component of the complex signal~the maximum of the 47 mTtrace is normalized to the maximum of the 23.5 mT trace!.

4052 J. Chem. Phys., Vol. 120, No. 9, 1 March 2004 Tomaselli, Meier, and Meier

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zero field. The experimental temperature,B0 , and the rf-pulse timing diagrams are shown in Fig. 3~a!. Following atemperature step from 7.5 to 55 K, proton dipolar order wasproduced at zero field and converted to Zeeman order byadiabatic onset ofB0 . Thermal contact among the proton andnitrogen-14 reservoirs through mutual spin flips was accom-plished by adjustingB0 to a 1H–14N level matchingcondition4 during tm @see level diagram in Fig. 3~a!#. The14N spin order was then converted to pure quadrupole orderby demagnetization to zero field and detected by a spin-echopulse sequence. Figure 3~b! displays the tunneling-amplified14N NQR spectrum ofg-picoline including the energy dia-gram of theI 51 three-level system at zero field. Each tran-sition n0 , n2 , andn1 was detected in a single shot using areceiver bandwidth of 100 kHz and level matching at 17.7mT duringtm @i.e.,gHB0/2p.nz(

14N), corresponding to theZeeman and dipole broadenedn0 transition at 757 kHz#. TheNQR linewidth of 150–200 Hz is most probably dominatedby insufficient shielding of the earth magnetic field. By in-

spection of the resonance position of the three sharp peaks@note thatn01n25n1 consistent with the level diagram inFig. 3~b!# we extract a14N quadrupole coupling constantue2qQ/hu54.4183(2) MHz and an asymmetry parameterh50.3428~1! in agreement with the literature values.9 Basedon a comparison of then0 signal in Fig. 3~b! with the 757kHz 1H signal of a water sample at 300 K, and taking intoaccount that the maximum14N powder signal obtainablewith linearly polarized rf is 43% of the total,10 we extrapo-late a tunneling-enhancement factor of;23103 with re-spect to the thermal14N signal at 7.5 K. In a second experi-mental setup we also investigated the amplification of thenatural abundance carbon-13 signal at 46.6 mT using adouble-resonance crossed-coil configuration. The13C signalwas detected with a 32-turn solenoid~8 mm diam,;4 mH!of a 500 kHz tank circuit while1H irradiation was produced

FIG. 3. ~a! Experimental temperature jump and adiabatic field-cyclingscheme for14N detection at zero field~see text!. The inset emphasizes thelevel-matching condition duringtm . ~b! Tunneling-amplified14N NQRspectrum ofg-picoline~0.4 cm3! obtained with the timing diagram shown in~a! andb5g14NB1t5p/4 wheret520 ms is the duration of the rf pulse~40min of thermal equilibration at 7.5 K!. The three resonant peaks~obtained inseparate experiments with one transient! are due to the transitions betweenenergy levels~shown inset! of 14N nuclei in the presence of an electric fieldgradient in g-picoline. In the inset,u0&5u10& and u6&5@u11&6u121&#/&,whereuIm& is the eigenstate of the14N nucleus with^I z&5m.

FIG. 4. ~a! Tunneling-amplified13C spectrum ofg-picoline~0.4 cm3! at 46.6mT recorded in a single shot 120 s after a 4.5→55 K temperature jump~50min thermal equilibration at 4.5 K!. The cross-polarization pulse-timing dia-gram is shown as an inset. Half-Gaussian1H ARRF and ADRF11 ramps~0.5ms length withsarrf5sadrf50.15 ms) were used to toggle between dipolarand Zeeman order. A13C ARRF ramp ~half Gaussian,tcp58 ms, sarrf

52.6 ms) was used for polarization transfer. The1H decoupling and13Clocking field had an amplitude of 36 and 20 kHz, respectively.~b! 1H reso-nance of water~0.6 cm3! at 300 K and 11.7 mT; 2048 acquisitions with arepetition delay of 4 sec using ap/2-td-p-td-det Hahn-echo pulse se-quence withtd5400ms and additional 1.98 MHz irradiation during detec-tion. ~c! Same as~b! without 1.98 MHz irradiation. The Bloch-Siegert shiftof Dnbs.180 Hz between trace~b! and ~c! corresponds to a 1.89 MHzrf-field amplitude of 36 kHz@Dnbs /n05n1

2/(n22n02)#.

4053J. Chem. Phys., Vol. 120, No. 9, 1 March 2004 Spin alignment at low and zero field

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by a 40-turn Helmholtz pair~25 mm diam,;30 mH! tuned at1.98 MHz. Both rf coils and theB0 coil were oriented mu-tually perpendicular. Typicalp/2 pulse lengths of 12 and 7ms for the13C and1H channel, respectively, were adjustedwith this setup. Since the detection system could be easilyjammed by the 1.98 MHz proton rf, a home-built 700 kHzlow-pass filter~Chebyshev, 0.1 dB ripple: 1.2 and 57 dBattenuation at 0.5 and 1.98 MHz, respectively! was placedbefore the wide-band preamplifier protected by a 500 kHzl/4-equivalent with crossed diodes to ground. Figure 4 sum-marizes the results. Trace~a! shows the tunneling-amplifiedg-picoline 13C nuclear resonance obtained in a single shot at46.6 mT with the cross-polarization11 pulse sequence dis-played as an inset. The proton decoupling field strength of 36kHz was calibrated with the1H line in water@trace~b! and~c!# by measuring the Bloch-Siegert shift12 at n05500 kHzwith additional 1.98 MHz irradiation during detection@2048acquisitions using a Hahn-echo excitation and identical gainsettings as in~a!#. The expected chemical-shift dispersion ofthe 13C line ~;200 Hz at 46.6 mT!3 is partially obscured byB0 inhomogeneities of the current low field setup. Based ona comparison of traces~a! and ~b! we extrapolate atunneling-enhancement factor of;104 with respect to thethermal13C signal at 55 K and an enhancement of;103 withrespect to the thermal13C signal at 4.5 K. These figures aresomewhat smaller than the maximum amplification predictedaccording to an ~entropy-conserving! adiabatic transferprocess,11 indicating that the13C ARRF was not fully effi-cient and/orT1r nuclear relaxation processes short-circuit thepolarization transfer partially. A similar enhancement wasalso observed with alternative double-resonance schemes us-ing a Hartmann-Hahn type transfer.13

The experiments show that the tunneling-induced spinpolarization is quite independent of magnetic field strengthand confirm the feasibility of Haupt experiments at low and

zero field using Faraday detection. A proton signal amplifi-cation of ;104 is observed (n051 MHz) and enables thesensitive detection of low-g 14N and rare13C spins by meansof adiabatic field cycling and/or double-resonance techniquesat frequencies as low as 500 kHz. Direct14N NQR detectionis particularly attractive due to the intrinsic high resolution atzero field4–7 and large dispersion of quadrupole couplingconstants. We expect that sample spinning at zero field willnot affect the dynamic polarization enhancement. It is evenconceivable that the induced dipolar spin alignment will bespontaneously converted to Zeeman order along the rotationaxis according to the Barnett effect.14,15 The possibility ofrare-spin NMR detection at low fields by means of adiabaticfield cycling in combination with cross polarization andmagic-angle spinning to further improve the sensitivity andresolution is quite evident.

1A. J. Horsewill, Prog. Nucl. Magn. Reson. Spectrosc.35, 359 ~1999!.2J. Haupt, Phys. Lett.38A, 389~1972!; J. Haupt, Z. Naturforsch. A28A, 98~1973!; P. Beckmann, S. Clough, J. W. Hennel, and J. R. Hill, J. Phys. C10, 729~1977!; P. S. Allen, Faraday Symp. Chem. Soc.13, 133~1978!; E.Crits, L. Van Gerven, and S. Emid, Physica B150, 329 ~1988!; M. Mur-phy and D. White, J. Chem. Phys.91, 4504~1989!; S. Emid and J. Smidt,Chem. Phys. Lett.77, 318 ~1981!.

3M. Tomaselli, C. Degen, and B. H. Meier, J. Chem. Phys.118, 8559~2003!.

4R. E. Slusher and E. L. Hahn, Phys. Rev. Lett.12, 246 ~1964!.5G. W. Leppelmeier and E. L. Hahn, Phys. Rev.141, 724 ~1966!.6D. P. Weitekamp, A. Bielecki, D. Zax, K. Zilm, and A. Pines, Phys. Rev.Lett. 50, 1807~1983!.

7M. D. Hurlimann, C. H. Pennington, N. Q. Fan, J. Clarke, A. Pines, and E.L. Hahn, Phys. Rev. Lett.69, 684 ~1992!.

8E. L. Hahn, Phys. Rev.80, 580 ~1950!.9A. Peneau, M. Gourdji, and L. Guibe´, J. Mol. Struct.111, 227 ~1983!.

10Y. K. Lee, Concepts Magn. Reson.14, 155 ~2002!.11A. Pines, M. G. Gibby, and J. S. Waugh, J. Chem. Phys.59, 569 ~1973!.12F. Bloch and A. Siegert, Phys. Rev.57, 522 ~1940!.13S. R. Hartmann and E. L. Hahn, Phys. Rev.128, 2042~1962!.14E. L. Hahn, Proc. Specialized Colloque Ampe`re, Portorozˆ, p. 4 ~2003!.15S. J. Barnett, Phys. Rev.6, 239 ~1915!.

4054 J. Chem. Phys., Vol. 120, No. 9, 1 March 2004 Tomaselli, Meier, and Meier

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