Overhauser Effects in Non-Conducting Solids at 1.2 K

Overhauser effects in non-conducting solids at 1.2 K

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Citation Ji, X., et al., "Overhauser effects in non-conducting solids at 1.2K." Journal of Magnetic Resonance 286 (Jan. 2018): p. 138-42 doi10.1016/J.JMR.2017.11.017 ©2018 Author(s)

As Published 10.1016/J.JMR.2017.11.017

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Overhauser Effects in Non-Conducting Solids at 1.2 K

X. Jia,b,c, T.V. Cand,e, F. Mentink-Vigierf, A. Borneta,g, J. Milania,g, B. Vuichouda,g, M. A. Caporinih, R. G. Griffind,e,*, S. Jannina,g, M. Goldmani, and G. Bodenhausenb,c

aInstitut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland bDepartement de Chimie, Ecole Normale Superieure, PSL Research University, UPMC Univ Paris 06, CNRS, Laboratoire des Biomolecules (LBM), 24 rue Lhomond, 75005 Paris, France cSorbonne Universites, UPMC Univ Paris 06, Ecole Normale Superieure, CNRS, Laboratoire des Biomolecules (LBM), Paris, France dDepartment of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA eFrancis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA fNational High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, USA gInstitut des Sciences Analytiques, CRMN CNRS-ENS Lyon-UCBL, Université de Lyon, 69100 Villeurbanne, France hAmgen Inc., 360 Binney Street Cambridge, Massachusetts 02142, USA i2 Allée Geneviève Anthonioz de Gaulle, 93260 Les Lilas, France

Abstract

Recently, it was observed that protons in non-conducting solids doped with 1,3-bisdiphenylene-2-

phenylallyl (BDPA) or its sulfonated derivatives (SA-BDPA) can be polarized through the

Overhauser effects via resonant microwave irradiation. The effects were present in magic angle

spinning spectra and magnetic fields between 5 and 18.8 T at temperatures near 100 K. This

communication reports similar effects in static samples at 6.7 T and, more importantly, at

temperatures as low as 1.2 K, in a different dynamic regime than in the previous study. Our results

provide new information towards understanding the mechanism of the Overhauser effect in non-

conducting solids. We discuss possible origins of the fluctuations that can give rise to an

Overhauser effect at such low temperatures.

Graphical Abstract

*Corresponding author: [email protected].

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HHS Public AccessAuthor manuscriptJ Magn Reson. Author manuscript; available in PMC 2019 January 01.

Published in final edited form as:J Magn Reson. 2018 January ; 286: 138–142. doi:10.1016/j.jmr.2017.11.017.

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Keywords

Dynamic Nuclear Polarization; Solid Effect; Overhauser Effect

INTRODUCTION

The Overhauser effect (OE) was the original mechanism predicted and observed to lead to

dynamic polarization of nuclei in solids. It was initially predicted to be present in conductors

[1] and was observed in Li metal [2–4]. The enhancement from the OE can be either positive

or negative depending on whether the mechanism is dominated by zero- or double-quantum

cross-relaxation rates Γ1,ZQ or Γ1,DQ across the ZQ and DQ transitions of the electron/

nuclear spin pair, respectively. These rates are governed by fluctuations of the electron-

nuclear spin couplings due to the translational motion of electrons in conducting solids [3,

5–8]. Nuclear Overhauser effects (NOEs) are also observed in liquid state NMR spectra and

are an essential tool in determining protein structures. In these solution NMR experiments

nuclear dipole couplings are modulated by molecular tumbling and lead to NOEs that permit

internuclear distance measurements [9, 10].

In contrast to these well established manifestations of OEs are recent observations by Maly

et al.[11], Haze et al. [12] and Can et al. [13] that surprisingly revealed the presence of OEs

in non-conducting solids in magic angle spinning (MAS) experiments boosted by dynamic

nuclear polarization (DNP). For example, the study by Can et al. [13] was performed on two

samples: (i) a 99% deuterated polystyrene (PS) matrix doped with 1,3-bisdiphenylene-2-

phenylallyl (BDPA) using a film casting method, and (ii) a water-soluble sulfonated variant

of the same polarizing agent (SA-BDPA) in a glass-forming solvent mixture (d8-

glycerol:D2O:H2O, v:v:v = 60:30:10.) The proton NMR signals of samples spinning at

ωr/2π = 8 kHz were observed at ~100 K in magnetic fields of 9.4 T (400 MHz/263 GHz)

and 14.1 T (600 MHz/395 GHz) and at 18.8 T (800 MHz/527 GHz). The positive absorption

signals in the middle of Zeeman field profile result from on-resonance saturation of the EPR

line of the BDPA radicals. They appear midway between the absorptive and emissive lines

that are characteristic of the solid effect (SE) in an electron/nuclear spin pair. These lines

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cannot originate from the cross effect (CE) or from thermal mixing (TM) since these

mechanisms would result in drastically different Zeeman field profiles [14]. The central

absorption signal has been attributed by to an Overhauser effect (OE), which was not

expected in non-conducting samples. Furthermore, Overhauser effects in solution

experiments are known to decrease at higher fields. This lead to the interesting, and at the

time correct, prediction that heteronuclear 13C solution NMR experiments on proteins that

rely on OEs to boost the signal intensity should not be conducted for proton frequencies

above ~100 MHz [15]. The subsequent development of polarization transfer methods based

on pulse experiments circumvented this problem [16].

The fact that the absorption OE signals are positive in the studies by Haze, et al. [12, 17] and

Can et al. [13] indicates that zero-quantum rates must be dominant, i.e., that Γ1,ZQ > Γ1,DQ.

Nevertheless, unlike the situation in solutions and conducting solids, the motions responsible

for the OE in non-conducting solids have yet to be characterized. Thus, additional

experimental data obtained under different experimental conditions could be useful in

understanding the origins of these effects. Here we report the observation of an OE for

BDPA in polystyrene in a magnetic field of 6.7 T at low temperatures of 1.2 and 4.2 K that

are typically used for dissolution DNP [18, 19].

RESULTS

Figure 1 illustrates the MAS Zeeman field profiles recorded at 14.1 T (ω0H/2π = 600

MHz/ω0S/2π = 395 GHz) from samples of ~2% BDPA doped into a mixture of d8 (95 mole

%) and d5 (5 mole %)-polystyrene (PS) and d14 (95 mole %)-and h14 (5 mole%)-ortho-

terphenyl (o-TP). The field profiles exhibit the expected SE enhancements at ω0S ± ω0I,

where S and I denote the electron and nuclear spin species respectively. In addition, midway

between the SE enhancements, we observe a positive enhancement due to the OE. In the

case of o-TP we observed ε = 75 and for PS ε = 18. These results are consistent with the

presence of efficient ZQ relaxation that drives the positive OE. We also note that o-TP

appears to be an excellent host for BDPA [20] as larger OE enhancements have been

observed in other labs, and at 18.8 T (800 MHz/527 GHz) they are larger than at 14.1 T [21].

Thus, the unusual trend towards increased enhancements at higher fields is confirmed by

these data.

Figure 2 shows the enhancement for the same sample of BDPA/PS as a function of the

microwave frequency (DNP frequency profile). In this case the microwave power Pμw = 87.5

mW and T = 4.2 K, without MAS. Since the polystyrene matrix contains some dissolved

molecular oxygen, we degassed the sample by pumping at 1 mbar over silica desiccant for

one week at room temperature. Nevertheless, the same general frequency profile is observed

after several freezing and thawing cycles. The central positive OE peak at 187.74 GHz must

be due to a dominant zero-quantum transition probability Γ1,ZQ > Γ1,DQ, while the peaks on

the left- and right-hand sides are due to the SE mechanism.

A comparison with the signals observed in thermal equilibrium shows that the proton

polarization levels achieved by the OE at 6.7 T are P(1H) = 5.15% at 1.2 K and 0.584% at

4.2 K prior to degassing and about 10% and 1% after degassing. In Figure 3 we show the

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build-up curves of the proton polarization P(1H) arising from the OE and the build-up time

constants Tbup are given in Table I.

Finally, similar results were observed for 40 mM BDPA radicals in a different glass forming

solution -- d8-toluene/d8-THF/h8-THF. Figure 4 shows the build-up behavior of the proton

polarization P(1H) after presaturation at 1.2 K in a field of 6.7 T, either without microwaves

to monitor the saturation recovery (SR), or with the microwave frequency set at the positive

and negative lobes of the solid effect (SE+ and SE−), or at the position of the (positive) OE.

The three curves in Figure 4 that refer to SE and OE have similar time constants (Table 2)

revealing the importance of leakage to the proton reservoir.

The rates R1(1H) due to BDPA should be proportional to

where P(e) is the electron polarization. If the electrons are saturated, we have P(e) = 0, while

P(e) = P(e)eq in absence of microwave irradiation. If BDPA was the main source of

relaxation, different time constants should be observed when microwave irradiation is on or

off. Hence we conclude that additional relaxation sources must be present, and paramagnetic

oxygen should be one of the sources of nuclear relaxation, since degassing tends to increase

the OE.

DISCUSSION

Under a variety of experimental conditions, BDPA and its derivatives mediate DNP by the

OE. This enhancement is strongest at relatively high fields [13, 21]. The analysis is based on

a two-spin sub-system comprising the unpaired BDPA electron S and a proton I that benefits

from the enhancement, which can be one of the 16 strongly coupled protons of BDPA (half

of which have a hyperfine coupling of ~1 MHz, and the others a coupling of ~5.5 MHz.) In

the two-spin sub-system, the zero-and double-quantum cross-relaxation rates Γl,ZQ or Γl,DQ

are determined by the magnitude of the interactions that connect the relevant states and by

the spectral density J(ω) that describes the fluctuations of these interactions. The hyperfine

interactions have both scalar and anisotropic dipolar components. The fluctuations of the

isotropic part Aiso[IzSz + ½ (I+S− + I−S+)] can only contribute to zero-quantum relaxation

rates. On the other hand, the anisotropic dipolar couplings comprise the entire “dipolar

alphabet”, including I+S+ and I−S− terms that allow double-quantum relaxation processes.

The two spectral densities J(ω0S − ω0I) and J(ω0S + ω0I) are nearly equal, i.e., J(ω0S − ω0I)

≈ J(ω0S + ω0I) ≈ J(ω0S), since the electron Larmor frequency is much higher than the proton

Larmor frequency ω0S = 660 · ω0I. Consequently, the ratio Γ1,ZQ/Γ1,DQ of the rates must be

determined primarily by the relative strengths of the interactions, i.e., by the ratio between

the scalar hyperfine couplings that can only contribute to the ZQ transitions, and the dipolar

hyperfine couplings that are predominant for the DQ transitions.

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The discussion in the previous paragraph provides a phenomenological description to the OE

in non-conducting solids based on the spectral density J(ω0S). Even though the origins

(motions/dynamics) of such a spectral density are largely unknown, it is worth noting that

J(ω0S) also contributes to the electron spin-lattice relaxation T1e, and there is a sharp

difference in the underlying mechanism depending on the temperature [22]. In general, at

high temperatures there exist significant vibrational modes at frequencies that are much

higher than the ESR Larmor frequency ω0S, and two phonons whose frequency difference

equals to ω0S can be responsible for J(ω0S). In this regime, the T1e relaxation is

predominantly due to a two-phonon process such as a Raman or Orbach process. At lower

temperatures, such high frequency modes are suppressed and therefore the dominating

relaxation mechanism must be a direct, one-phonon process corresponding to lattice

vibrations near the ω0S in the range of 100 GHz (sub-THz.) In a sample containing 15 mM

BDPA in a glassy sulfolane:DMSO matrix, it was shown that T1e at 3.35 T was mediated by

Raman process above 60 K and by direct, one-phonon process below 40 K [23]. Thus, it is

likely that the OE in BDPA at 100 K (Figure 1) and above (data not shown) is due to some

Raman process, whereas at 1.2 K or 4.2 K it is likely due to direct, one-phonon process.

The one-phonon mechanism was recently discussed by Pylaeva, et al. [24]. Using MD

simulations, the authors attributed the fluctuation of the hyperfine couplings in BDPA to the

rearrangement of the double and single bonds connecting the central carbon to either of the

two fluorene moieties in BDPA. The corresponding spectral density is predicted to exhibit a

maximum near 650 GHz. This prediction is yet to be confirmed experimentally. If

confirmed, such stochastic motions might be relevant to experiments at very low

temperatures (below 10 K, Figure 2) but not at higher temperatures (above 80 K, Figure 1)

as discussed above. In any case, the question of the nature of the motions that could be

responsible for the non-vanishing spectral density J(ω0S) that explains the existence of the

zero-quantum cross-relaxation rate Γl,ZQ and hence the positive OE, is still largely open. We

shall consider three different hypotheses:

i. First, as a result of annealing during repeated cooling and warming up to room

temperature, it is possible that the BDPA radicals may have diffused in the

polystyrene matrix and may have formed clusters. The couplings between the

free electrons of such clusters could lead to band structures, not unlike what is

observed in conductors. This hypothesis might in principle be verified by ESR or

ENDOR spectroscopy. The fact that the samples have repeatedly been frozen and

melted without any significant effects does not speak in favor such an annealing

hypothesis.

ii. Second, the unpaired electrons of the BDPA radical may hop between two

localized states by thermal activation, rather than by tunnelling through the

barrier. The barrier height could be estimated by DFT calculations. It might be

possible to verify such effects by recording ENDOR spectra of the same samples

at 4.2 or 1.2 K, since localization might give rise to more complex hyperfine

couplings in BDPA than expected for a symmetrically delocalized unpaired

electron. The resulting splittings would likely be masked by g-anisotropy in ESR

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spectra at high fields, but might be resolved in ENDOR spectra. However, high

frequency ENDOR spectra do not support this hypothesis [25, 26].

iii. Third, we note that the strongest OE in non-conducting solids is observed with

the BDPA radical, which has large hyperfine couplings (~ 5 MHz) between the

electron and the protons [27]. Such a coupling requires the unpaired electron to

be delocalized within the BDPA molecule. In a classical representation, this

would mean that the unpaired electron is in constant motion within the molecule.

The measured hyperfine coupling, for example in ENDOR, would then result

from its time average. If this is the case, one needs to take into account the time

dependence of the hyperfine coupling induced by molecular vibrations. The

modulation of the hyperfine coupling would then lead to enhance values of Γ1,ZQ

as the Zeeman field increases and ω0S approaches vibrational frequencies. At

present this appears to be the most plausible mechanism for the origin of the OE

in non-conducting solids.

EXPERIMENTAL

Sample preparation

Polystyrene doped with 2% BDPA was prepared by a film casting method: 2.4 mg of BDPA

in complex with benzene and 95 mg of PS-d8 together with 5 mg of PS-d5 were dissolved in

2 ml of chloroform. The solution was then spread on a glass surface. A thin film of PS doped

with 2% BDPA was collected and ground thoroughly after evaporation of the solvent. The

residual solvent was then removed under vacuum for at least 12 h. (Can et al. [13])

DNP experiments

DNP was performed at T = 1.2 or 4.2 K and B0 = 6.7 T in a home-built polarizer by

applying microwave irradiation in a range between 187.5 and 188.5 GHz, without frequency

modulation, without microwave gating, without cross-polarization from protons to

carbon-13, and without dissolution. An ELVA (VCOM 10/94/400) microwave source

operating at 94 GHz ± 250 MHz with 350 mW power was coupled to a VDI doubler (D200)

with ca. 25% efficiency. The proton NMR signals were observed using a doubly resonant

NMR coil with a Helmholtz configuration with an inner volume of about 1 cm3 (13C and 1H

resonating at 71.73 and 285.23 MHz). (Bornet et al. [18, 19])

CONCLUSIONS

Our observations highlight the ability for BDPA in different media to mediate DNP by OE at

high magnetic fields, but raises questions about the origin of this effect. We have found that

OEs can also be observed in PS, o-TP and glassy toluene/tetrahydrofuran mixtures doped

with BDPA. By optimizing the sample formulation, one might be able to take advantage of

OEs for dissolution DNP on molecular glasses containing various molecules of interest.

Possible sources of the OE mechanism could be motions of the BDPA radical or molecular

vibrations modulating the hyperfine coupling that would be more effective at higher Zeeman

fields.

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Acknowledgments

This work was supported by the Swiss National Science Foundation (SNSF), the Ecole Polytechnique Fédérale de Lausanne (EPFL), the French CNRS, the European Research Council (ERC contract ‘Dilute para-water’) and by the US National Institute of Biomedical Imaging and Bioengineering (EB-002804 and EB-002026).

ABBREVIATIONS

BDPA 1,3-bisdiphenylene-2-phenylallyl

SA-BDPA sulfonated BDPA

PS polystyrene

DNP dynamic nuclear polarization

OE Overhauser Effect

MAS magic angle spinning

References

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23. Lumata L, Kovacs Z, Sherry AD, Malloy C, Hill S, van Tol J, Yu L, Song L, Merritt ME. Electron spin resonance studies of trityl OX063 at a concentration optimal for DNP. Phys Chem Chem Phys. 2013; 15:9800–9807. [PubMed: 23676994]

24. Pylaeva S, Ivanov KL, Baldus M, Sebastiani D, Elgabarty H. Molecular Mechanism of Overhauser Dynamic Nuclear Polarization in Insulating Solids. Journal of Physical Chemistry Letters. 2017; 8:2137–2142. [PubMed: 28445055]

25. Bennati M, Farrar CT, Bryant JA, Inati SJ, Weis V, Gerfen GJ, Riggs-Gelasco P, Stubbe J, Griffin RG. Pulsed electron-nuclear double resonance (ENDOR) at 140 GHz. J Magn Reson. 1999; 138:232–243. [PubMed: 10341127]

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Highlight JMR17-244

Unexpected Overhauser effects are observed in samples of BDPA doped into

polystyrene at 1.2 K

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Figure 1. Zeeman field profiles illustrating the DNP enhancement of protons in (open circles)

polystyrene (PS) and (red dots) ortho-terphenyl (o-TP) doped with BDPA. The profiles were

recorded at 14.1 T (ω0H/2π = 600 MHz) and T = 100 K in samples spinning at ωr/2π = 8

kHz. The enhancements for o-TP were 78 and 35 for the Overhauser and solid effects,

respectively. In PS the corresponding numbers were ~18 and ~7.

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Figure 2. Microwave frequency profile illustrating the DNP enhancement of protons of the same

sample of polystyrene containing BDPA, recorded at 4.2 K and 6.7 T without spinning. The

blue line shows the profile before degassing, the red line after the removal of oxygen under

vacuum. The small shift in microwave frequencies is due to a field drift or to a difference in

calibration. Note since the μw frequency is being swept the sign of the solid effect lines is

reversed from those in Figure 1.

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Figure 3. Build-up of the proton polarization P(1H) due to the Overhauser effect at a field of 6.7 T and

temperatures of 4.2 K (red) and 1.2 K (black) (a) prior to degassing and (b) after degassing,

monitored by applying a series of pulses with 1° flip angles at 5 s intervals. All data were

recorded with P = 87.5 mW of microwave power at 187.74 GHz applied to a sample of

deuterated polystyrene doped with BDPA prepared by film casting.

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Figure 4. Build-up of the proton polarization P(1H) induced by microwave saturation at 1.2 K in a

field of 6.7 T in a glass-forming mixture of d8-toluene/d8-THF/h8-THF (v:v:v = 8:1:1)

doped with 40 mM BDPA. The build-up curves correspond to: (blue squares) microwave

irradiation frequency set on the positive lobe of the solid effect (SE); (pink squares) on the

negative lobe; (red squares) in the centre (where the OE is effective.). For comparison:

saturation recovery (SR, grey squares) by T1(1H) of the proton polarization P(1H) after

presaturation but without microwave irradiation. The black squares show the same data

amplified by a factor 8. The build-up time constants are given in Table I.

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Table 1

Build-up times in polystyrene doped with BDPA at 4.2 K and 1.2 K in a field of 6.7

Temperature (K) P(1H) (%) Tbup (s)

Before Degassing 1.2 5.15±0.002 398±0.39

4.2 0.584±0.004 50.5±0.57

After Degassing 1.2 9.47±0.04 617±6.6

4.2 0.87±0.006 102.5±1.3


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Table 2

Build-up times a glass-forming mixture of toluene-d8/THF-d8/THF (v:v:v = 8:1:1) doped with 40 mM BDPA

at 1.2 K in a field of 6.7 T.

P(1H) (%) Tbup (s)

SR 0.5±0.001 1735±10

OE 1.85±0.001 2653±4

SE+ 4.85±0.003 3104±4

SE− −4.37±0.003 3685±5


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Overhauser Effects in Non-Conducting Solids at 1.2 K