Journal of Magnetic Resonance 250 (2015) 25–28
Contents lists available at ScienceDirect
Journal of Magnetic Resonance journal homepage: www.elsevier.com/locate/jmr
Communication
Enhancement of quantum rotor NMR signals by frequency-selective pulses Soumya Singha Roy a, Jean-Nicolas Dumez a,b, Gabriele Stevanato a, Benno Meier a, Joseph T. Hill-Cousins a, Richard C.D. Brown a, Giuseppe Pileio a, Malcolm H. Levitt a,⇑ a b
School of Chemistry, University of Southampton, SO17 1BJ Southampton, United Kingdom Institut de Chimie des Substances Naturelles, CNRS UPR2301, Avenue de la Terrasse, 91190 Gif-sur-Yvette, France
a r t i c l e
i n f o
Article history: Received 7 October 2014 Revised 8 November 2014 Available online 21 November 2014 Keywords: Hyperpolarisation Quantum-rotor-induced polarisation QRIP Long-lived states Tunneling Methyl groups
a b s t r a c t Quantum-rotor-induced polarisation (QRIP) enhancement is exhibited by substances which contain freely rotating methyl groups in the solid state, provided that the methyl groups contain a 13C nucleus. Strong signal enhancements are observed in solution NMR when the material is first equilibrated at cryogenic temperatures, then rapidly dissolved with a warm solvent and transferred into an NMR magnet. QRIP leads to strongly-enhanced 13C NMR signals, but relatively weak enhancements of the 1H signals. We show that the 1H signals suffer from a partial cancellation of degenerate contributions, which may be corrected by applying a frequency-selective p pulse to the inner peaks of the 13C multiplet prior to 1 H observation. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Icker and Berger recently demonstrated a remarkable hyperpolarisation effect in solution NMR [1,2]. When substances containing freely rotating methyl groups are equilibrated at liquid helium temperature, dissolved rapidly with a hot solvent, and transferred into the NMR magnet, the methyl 13C solution NMR signals induced by a radiofrequency pulse are enhanced by up to 3 orders of magnitude relative to ordinary thermal NMR signals. The hyperpolarised 13C NMR signals appear as a curious antiphase spectral pattern. The 1H NMR signals are also enhanced, although by a much lower factor, and only in the case that the CH3 group contains a 13C nucleus. The enhancement persists for a time considerably longer than the relaxation time constant for longitudinal magnetisation T 1 [3]. The effect was observed on a few substances known to have large tunnelling splittings in the solid state, including 4-methylpyridine (also known as c-picoline), some acetate salts, and a few other compounds [4]. The effect is now understood to be a quantum-rotor-induced polarisation (QRIP) effect associated with the large solid-state tunnelling splitting [3]. If the tunnelling splitting is large enough, equilibration at cryogenic temperatures creates a substantial ⇑ Corresponding author at: School of Chemistry, University of Southampton, University Road, SO17 1BJ Southampton, UK. Fax: +44 23 8059 3781. E-mail address: [email protected] (M.H. Levitt). http://dx.doi.org/10.1016/j.jmr.2014.11.004 1090-7807/Ó 2014 Elsevier Inc. All rights reserved.
population difference between the lowest energy levels of the quantum rotor, which have symmetry labels A (for the lowest level) and Ea ; Eb (for the doubly degenerate first excited state), where the symmetry labels refer to the irreducible representations of the group C 3 [5]. Since the 1H nuclei of the CH3 group are identical fermions, and the elements of the group C 3 are all even permutations, the spatial symmetries are associated with symmetries of the nuclear spin states such that the total wavefunction has symmetry A. The A representation is associated with 1H nuclear spin states with total spin I ¼ 3=2 and symmetry A, while the Ea and Eb representations are each associated with 1H nuclear spin states with total spin I ¼ 1=2 and symmetry Eb and Ea respectively. Tunneling polarisation at low temperature therefore corresponds to overpopulating the state with A symmetry and spin I ¼ 3=2 states with respect to the state with E symmetry and I ¼ 1=2 spin states. When the cold sample is rapidly warmed and brought into solution the large A=E population imbalance persists; it forms a non-magnetic long-lived nuclear spin state (a slowly decaying component of the spin density operator) [3]. In the case of rapidly rotating methyl groups in solution, the relaxation time constant T LLS of this long-lived state may greatly exceed T 1 [3]. The 13C nucleus introduces weak cross-relaxation processes which convert the non-magnetic long-lived state into observable antiphase 13C and 1H magnetisation, which may be observed by applying a resonant radiofrequency pulse and detecting the induced NMR signals. Simulations of the phenomenon based on these
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Communication / Journal of Magnetic Resonance 250 (2015) 25–28
principles lead to a qualitative match with the observed experimental results [3]. These simulations provide an explanation for the relatively weak signal enhancements observed for the 1H signals of the 13 CH3 group in the QRIP experiment. As shown in Fig. 1, each component of the 13C-coupled 1H doublet is a superposition of signals from five degenerate transitions: three A transitions, and two Ea or Eb transitions. The QRIP mechanism taking into account the strong cross-correlations between heteronuclear and homonuclear dipole–dipole couplings, leads to the typical population pattern shown in Fig. 1b. The three A transitions belonging to each component of the 13C doublet are polarised in one sense, while the two E transitions belonging to the same doublet component are polarised in the opposite sense. Since the A and E transitions are degenerate in isotropic phase, these contributions partially cancel, leading to an attenuation of the QRIP-enhanced 1H NMR signals. This explanation immediately leads to a simple scheme for amplifying the QRIP-enhanced 1H NMR signals: a pair of selective p pulses applied to the two inner peaks of the 13C quartet, which are spectrally resolved, exchanges populations between pairs of states in such a way that the cancellation effect on the 1H spectrum is reduced (see Fig. 1c). Transition-selective population inversion of the two inner 13C quartet transitions is therefore expected to boost the QRIP-enhanced 1H signal strength. In this work, we employ an I-BURP (Inversion-band-selective, uniform response, pure-phase) pulse [6,7] of duration 30 ms applied on the 13C channel, which provides a sharp ‘‘top-hat’’ inversion profile, with bandwidth 168 Hz. The same results could also be achieved by selective inversion of the two outer 13C quartet transitions.
2. Results Fig. 2 shows the 13C spectrum of the c-picoline methyl group in CDCl3 solution, showing the quartet structure due to J-coupling of the methyl 13C to the three attached protons. A fine triplet structure due to long-range coupling to the nearest pair of aromatic protons is also visible. The spectrum in Fig. 2b was obtained by applying a 13C I-BURP pulse immediately before generation of the 13C NMR signal by a hard p=2 13C pulse. The spectrum shows that the optimised I-BURP pulse provides a clean inversion of the population differences across the inner transitions of the 13C quartet. The 1H T 1 of the methyl group in 13C-labelled c-picoline was measured by inversion recovery to be 4.3 ± 0.3 s under the experimental conditions. Fig. 3 shows the timing sequence for the QRIP experiment incorporating the 13C I-BURP pulses for enhancement of the 1H signals. After equilibration at low temperature, dissolution of the sample with a hot solvent, and transport into the high-field NMR magnet, a series of 1H pulses with flip angle of 5° was applied to induce QRIP-enhanced 1H NMR signals. After the first pulse was given, a 13 C I-BURP pulse was applied before every subsequent fourth 1H pulse, in order to reduce the partial cancellation of the degenerate A and E-manifold signals. Fig. 4a shows the sequence of 1H doublet amplitudes observed during this experiment on a sample of 13C-labelled c-picoline. The horizontal time axis takes into account 15 s delay between dissolution and acquisition of the first 1H NMR signal. The vertical scale represents the signal enhancement relative to thermal proton signal at 500 MHz, estimated by taking the ratios of corresponding peak amplitudes. The enhancements for the two peaks have the same magnitude within experimental error. The initial 1H signal enhancement due to the QRIP is observed to be 26 relative to the thermal signal. A further 1.86 times enhancement is realised
Fig. 1. Energy level diagram for a 13CH3 group at various stages of the experiment. The population of each energy levels are shown as blue circles and red arrows showing the possible transitions. For simplicity, the initial E manifolds are shown as completely depleted. Population distribution (a) immediately after dissolution and internal equilibration of the A manifold, (b) generated by cross-relaxation, and (c) after the 13C double-selective pulse on the inner quartet transitions. The green curved arrows in (b) indicate the population exchanges induced by the selective 13C pulse. Population differences within the A and E manifolds are converted into observable 1H antiphase magnetisation when rf pulses are applied. The observed NMR signal is given by the superposition of transitions occurring in both A and E manifolds. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
by the application of the selective 13C pulse. Hence an overall enhancement of 48 over thermal signal is achieved in this experiment. The large step in signal amplitude between the first and second point in this series is due to the 13C I-BURP pulse. The enhancement effect is short-lived as the populations return to the distribution shown in Fig. 1b. The effect can be repeated, however, by applying the frequency-selective 13C pulse again. The decay curve has a time constant of 25.5 ± 3.5 s, consistent with the participation of a long-lived state.
Communication / Journal of Magnetic Resonance 250 (2015) 25–28
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Fig. 2. Partial 13C spectra of a 582 mM solution (30 ll solute in 0.5 ml solvent) of 13 C-labelled c-picoline in CDCl3 at 11.7 T, displaying the quartet corresponding to the methyl group. In (a) the sample was equilibrated in high magnetic field, and a p=2 pulse used to excite the 13C NMR signals. In (b) a I-BURP pulse of duration 30 ms was applied immediately before the p=2 excitation pulse.
Fig. 4. (a) Experimental signal enhancement of the two 1H peaks as a function of time for the pulse sequence in Fig. 3, relative to the thermal proton NMR signal. Small flip-angle 5° pulses were used to induce the 1H signal. The dashed curves show exponential fits to the data yielding a time constant of 25.5 ± 3.5 s. (b) Simulated trajectories of 1H signal enhancement with respect to the thermal 1H signal at 11.7 T for the pulse sequence shown in Fig. 3.
Fig. 3. Timing sequence for the QRIP experiment, including a frequency-selective 13 C I-BURP pulse before every fifth 1H observation pulse. The timing axis is not uniform. The magnetic field during the cryogenic cooling interval is not essential and may be omitted. In practice the 1H detection pulses had a flip angle of 5° and were repeated every 5 s.
Fig. 4b shows a numerical simulation of the 1H peak amplitudes during the observation segment of the I-BURP enhanced QRIP procedure, obtained using the SpinDynamica software platform [8]. The simulation assumes an initial state corresponding to a population difference of 0.6 between the A and E levels, as estimated from the Boltzmann distribution for a system with a tunnelling splitting of 126 GHz at a temperature of 4.2 K [3]. The simulated evaluation includes all relevant relaxation mechanisms, including heteronuclear dipole–dipole coupling, homonuclear dipole–dipole coupling, chemical shift anisotropy relaxation, spin-rotation and all crosscorrelations terms. There is qualitative agreement with experiment, although the enhancement factor predicted by simulation is roughly 3 times higher than the experimental values. This discrepancy is attributed to losses during dissolution and incomplete equilibration at low temperature. Fig. 5 shows the QRIP-enhanced 1H spectra in more detail. The 1 H spectrum obtained by the Fourier transform of the transient
Fig. 5. 1H QRIP spectra of the methyl group of 13C-labelled c-picoline in CHCl3. (a) Fourier transform of the transient induced by the first 5° 1H pulse after dissolution and transport to high field; (b) Fourier transform of the transient induced by the second 5° 1H pulse, which is preceded by the frequency-selective 13C pulse; and (c) Fourier transform of the sum of 25 1H transients, observed on the same sample upon thermal equilibrium, with a flip angle of 5°, and keeping all other parameters unchanged.
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Communication / Journal of Magnetic Resonance 250 (2015) 25–28
induced by the first 5° 1H pulse is shown in Fig. 5a. Fig. 5b shows the 1H spectrum obtained from the second 1H pulse, which is preceded by the 13C I-BURP pulse. The signal enhancement from (a) to (b) is obvious. Fig. 5c shows the 1H spectrum of the same sample after thermal equilibration in the high-field magnet, summing together 25 transients. In this case the overall enhancement of the 1H signal, estimated by comparing peaks integrals, is 48 ± 3 and is identical for the two 1H peaks within experimental error. The enhanced 1H spectrum in Fig. 5b displays strikingly different line widths for the two peaks. The measured line widths (full widths at half height) are 4.3 ± 0.1 Hz, 2.7 ± 0.1 Hz for the peaks in Fig. 5a and 5.1 ± 0.1 Hz, 1.9 ± 0.1 Hz for Fig. 5b. In contrast, the thermal equilibrated spectra shown in Fig. 5c displays equal line widths of 2.8 ± 0.1 Hz for both peaks. The line widths were observed to vary between one experiment and the next, particularly in the initial part of the decay. However, the integrals did not vary significantly. We measured the temperature dependence of the chemical shifts and J-coupling parameters in independent experiments, but found this to be insufficient to explain the line width effects. At this point the line width variations are unexplained. 3. Methods The experiments were carried out on a home-built dissolutionDNP apparatus with a 3.35 T cold-bore magnet, based on the design from Ref. [9]. In the current experiments, the dissolution apparatus was only used for cooling and dissolving the sample; The DNP system was not used and the magnetic field is not expected to have an effect [1]. A cup filled with 30 ll of 13Clabelled c-picoline was inserted into the cryostat, which was already filled with liquid helium. The sample temperature was lowered to 3 K by reducing the He pressure using a pump. The sample was left at 3 K for about 2 h. The sample cylinder at the top of the dissolution stick was filled with 4 ml of degassed chloroform which was heated to 120 ° C. When the pressure inside the hot solvent chamber reached 5 bar, the dissolution stick was inserted manually into the cryostat, making contact with the sample cup and flushing the sample out through a plastic tube, chased by warm He gas. The dissolved sample was collected in a syringe and injected into a 5 mm NMR tube inside a 11.7 T high-resolution NMR magnet. It took around 15 s from dissolution to the injection of the sample into the NMR tube. All high-field NMR experiments were performed on a 500 MHz Bruker Avance III spectrometer equipped with a 5 mm broadband NMR probe. The shape of the amplitude-modulated I-BURP pulse is specified in Ref. [7]. The pulse had a total duration of 30 ms, with a maximum rf amplitude corresponding to a nutation frequency of 200 Hz. These parameters provide an inversion bandwidth of 168 Hz. The simulations parameters were taken from Ref. [3]. The correlation time was sc ¼ 5 ps for the overall molecular tumbling and sR ¼ 0:6 ps for the methyl group rotation. 4. Conclusion In summary, we have demonstrated an enhancement of the hyperpolarised 1H NMR signals generated in a QRIP experiment, induced by a selective manipulation of the 13C nuclear spin transitions. The observed enhancement supports our theory of the QRIP effect, in which a methyl long-lived state is hyperpolarised by
conversion of the low-temperature tunnelling polarisation to a population imbalance between the A and E symmetry species of the methyl group in solution. The nearby 13C nucleus introduces cross-relaxation which converts the hyperpolarised long-lived state into enhanced population differences across observable transitions. In the case of the proton NMR signals, there is destructive interference between the signal contributions from the A and E manifolds which may be partially corrected by frequency-selective manipulation of the 13C transitions. QRIP leads to density operator components which are products of 1H and 13C spin operators, so that there is no obvious gain in attempting to use INEPT-type pulse sequences [10] to transfer polarisation form 13C to 1H. This has been verified by simulations. In its current form, this experiment is restricted to the small number of chemical compounds which contain highly unhindered methyl rotors at low temperatures, exhibiting large enough tunnelling splittings to be polarised thermally at readily accessible temperatures. However, there is every prospect that dynamic nuclear polarisation (DNP) may be used to generate very low spin temperatures in the mK range [11–13]. This would permit differential polarisation of the A and E spin manifolds in a much wider range of compounds, including those which lack a tunnelling splitting. This research is ongoing in our laboratory. Acknowledgments We thank Walter Köckenberger and Josef Granwehr for the loan of the dissolution-DNP device, and Ole G. Johannessen for help with instrumentation. This research was supported by the Engineering and Physical Sciences Research Council (UK), by the European Research Council, the Royal Society (UK), the Wolfson Foundation (UK), and by the COST Action TD-1103. References [1] M. Icker, S. Berger, Unexpected multiplet patterns induced by the Haupt-effect, J. Magn. Reson. 219 (2012) 1–3. [2] M. Icker, P. Fricke, S. Berger, Transfer of the Haupt-hyperpolarization to neighbor spins, J. Magn. Reson. 223 (2012) 148–150. [3] B. Meier, J.-N. Dumez, G. Stevanato, J.T. Hill-Cousins, S.S. Roy, P. Håkansson, S. Mamone, R.C.D. Brown, G. Pileio, M.H. Levitt, Long-lived nuclear spin states in methyl groups and quantum-rotor-induced-polarization, J. Am. Chem. Soc. 135 (2013) 18746–18749. [4] M. Icker, P. Fricke, T. Grell, J. Hollenbach, H. Auer, S. Berger, Experimental boundaries of the quantum rotor induced polarization (QRIP) in liquid state NMR, Magn. Reson. Chem. 51 (2013) 815–820. [5] P.R. Bunker, P. Jensen, Molecular Symmetry and Spectroscopy, second ed., NRC Research Press, Ottawa, 2006. [6] R. Freeman, Shaped radiofrequency pulses in high resolution NMR, Prog. Nucl. Magn. Reson. 32 (1998) 59–106. [7] H. Geen, R. Freeman, Band-selective radiofrequency pulses, J. Magn. Reson. 93 (1991) 93–141. [8] SpinDynamica, programmed by Malcolm H. Levitt with contributions from Jyrki Rantaharju and Andreas Brinkmann . [9] J. Granwehr, J. Leggett, W. Köckenberger, A low-cost implementation of EPR detection in a dissolution DNP setup, J. Magn. Reson. 187 (2007) 266–276. [10] G.A. Morris, R. Freeman, Enhancement of nuclear magnetic resonance signals by polarization transfer, J. Am. Chem. Soc. 101 (1979) 760–762. [11] T. Maly, G.T. Debelouchina, V.S. Bajaj, K.-N. Hu, C.-G. Joo, M.L. Mak-Jurkauskas, J.R. Sirigiri, P.C.A. van der Wel, J. Herzfeld, R.J. Temkin, R.G. Griffin, Dynamic nuclear polarization at high magnetic fields, J. Chem. Phys. 128 (2008) 052211. [12] S. Jannin, A. Bornet, R. Melzi, G. Bodenhausen, High field dynamic nuclear polarization at 6.7 T: carbon-13 polarization above 70% within 20 min, Chem. Phys. Lett. 549 (2012) 99–102. [13] A. Bornet, R. Melzi, S. Jannin, G. Bodenhausen, Cross polarization for dissolution dynamic nuclear polarization experiments at readily accessible temperatures 1:2 < T < 4:2 K, Appl. Magn. Reson. 43 (2012) 107–117.
Contents lists available at ScienceDirect
Journal of Magnetic Resonance journal homepage: www.elsevier.com/locate/jmr
Communication
Enhancement of quantum rotor NMR signals by frequency-selective pulses Soumya Singha Roy a, Jean-Nicolas Dumez a,b, Gabriele Stevanato a, Benno Meier a, Joseph T. Hill-Cousins a, Richard C.D. Brown a, Giuseppe Pileio a, Malcolm H. Levitt a,⇑ a b
School of Chemistry, University of Southampton, SO17 1BJ Southampton, United Kingdom Institut de Chimie des Substances Naturelles, CNRS UPR2301, Avenue de la Terrasse, 91190 Gif-sur-Yvette, France
a r t i c l e
i n f o
Article history: Received 7 October 2014 Revised 8 November 2014 Available online 21 November 2014 Keywords: Hyperpolarisation Quantum-rotor-induced polarisation QRIP Long-lived states Tunneling Methyl groups
a b s t r a c t Quantum-rotor-induced polarisation (QRIP) enhancement is exhibited by substances which contain freely rotating methyl groups in the solid state, provided that the methyl groups contain a 13C nucleus. Strong signal enhancements are observed in solution NMR when the material is first equilibrated at cryogenic temperatures, then rapidly dissolved with a warm solvent and transferred into an NMR magnet. QRIP leads to strongly-enhanced 13C NMR signals, but relatively weak enhancements of the 1H signals. We show that the 1H signals suffer from a partial cancellation of degenerate contributions, which may be corrected by applying a frequency-selective p pulse to the inner peaks of the 13C multiplet prior to 1 H observation. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Icker and Berger recently demonstrated a remarkable hyperpolarisation effect in solution NMR [1,2]. When substances containing freely rotating methyl groups are equilibrated at liquid helium temperature, dissolved rapidly with a hot solvent, and transferred into the NMR magnet, the methyl 13C solution NMR signals induced by a radiofrequency pulse are enhanced by up to 3 orders of magnitude relative to ordinary thermal NMR signals. The hyperpolarised 13C NMR signals appear as a curious antiphase spectral pattern. The 1H NMR signals are also enhanced, although by a much lower factor, and only in the case that the CH3 group contains a 13C nucleus. The enhancement persists for a time considerably longer than the relaxation time constant for longitudinal magnetisation T 1 [3]. The effect was observed on a few substances known to have large tunnelling splittings in the solid state, including 4-methylpyridine (also known as c-picoline), some acetate salts, and a few other compounds [4]. The effect is now understood to be a quantum-rotor-induced polarisation (QRIP) effect associated with the large solid-state tunnelling splitting [3]. If the tunnelling splitting is large enough, equilibration at cryogenic temperatures creates a substantial ⇑ Corresponding author at: School of Chemistry, University of Southampton, University Road, SO17 1BJ Southampton, UK. Fax: +44 23 8059 3781. E-mail address: [email protected] (M.H. Levitt). http://dx.doi.org/10.1016/j.jmr.2014.11.004 1090-7807/Ó 2014 Elsevier Inc. All rights reserved.
population difference between the lowest energy levels of the quantum rotor, which have symmetry labels A (for the lowest level) and Ea ; Eb (for the doubly degenerate first excited state), where the symmetry labels refer to the irreducible representations of the group C 3 [5]. Since the 1H nuclei of the CH3 group are identical fermions, and the elements of the group C 3 are all even permutations, the spatial symmetries are associated with symmetries of the nuclear spin states such that the total wavefunction has symmetry A. The A representation is associated with 1H nuclear spin states with total spin I ¼ 3=2 and symmetry A, while the Ea and Eb representations are each associated with 1H nuclear spin states with total spin I ¼ 1=2 and symmetry Eb and Ea respectively. Tunneling polarisation at low temperature therefore corresponds to overpopulating the state with A symmetry and spin I ¼ 3=2 states with respect to the state with E symmetry and I ¼ 1=2 spin states. When the cold sample is rapidly warmed and brought into solution the large A=E population imbalance persists; it forms a non-magnetic long-lived nuclear spin state (a slowly decaying component of the spin density operator) [3]. In the case of rapidly rotating methyl groups in solution, the relaxation time constant T LLS of this long-lived state may greatly exceed T 1 [3]. The 13C nucleus introduces weak cross-relaxation processes which convert the non-magnetic long-lived state into observable antiphase 13C and 1H magnetisation, which may be observed by applying a resonant radiofrequency pulse and detecting the induced NMR signals. Simulations of the phenomenon based on these
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principles lead to a qualitative match with the observed experimental results [3]. These simulations provide an explanation for the relatively weak signal enhancements observed for the 1H signals of the 13 CH3 group in the QRIP experiment. As shown in Fig. 1, each component of the 13C-coupled 1H doublet is a superposition of signals from five degenerate transitions: three A transitions, and two Ea or Eb transitions. The QRIP mechanism taking into account the strong cross-correlations between heteronuclear and homonuclear dipole–dipole couplings, leads to the typical population pattern shown in Fig. 1b. The three A transitions belonging to each component of the 13C doublet are polarised in one sense, while the two E transitions belonging to the same doublet component are polarised in the opposite sense. Since the A and E transitions are degenerate in isotropic phase, these contributions partially cancel, leading to an attenuation of the QRIP-enhanced 1H NMR signals. This explanation immediately leads to a simple scheme for amplifying the QRIP-enhanced 1H NMR signals: a pair of selective p pulses applied to the two inner peaks of the 13C quartet, which are spectrally resolved, exchanges populations between pairs of states in such a way that the cancellation effect on the 1H spectrum is reduced (see Fig. 1c). Transition-selective population inversion of the two inner 13C quartet transitions is therefore expected to boost the QRIP-enhanced 1H signal strength. In this work, we employ an I-BURP (Inversion-band-selective, uniform response, pure-phase) pulse [6,7] of duration 30 ms applied on the 13C channel, which provides a sharp ‘‘top-hat’’ inversion profile, with bandwidth 168 Hz. The same results could also be achieved by selective inversion of the two outer 13C quartet transitions.
2. Results Fig. 2 shows the 13C spectrum of the c-picoline methyl group in CDCl3 solution, showing the quartet structure due to J-coupling of the methyl 13C to the three attached protons. A fine triplet structure due to long-range coupling to the nearest pair of aromatic protons is also visible. The spectrum in Fig. 2b was obtained by applying a 13C I-BURP pulse immediately before generation of the 13C NMR signal by a hard p=2 13C pulse. The spectrum shows that the optimised I-BURP pulse provides a clean inversion of the population differences across the inner transitions of the 13C quartet. The 1H T 1 of the methyl group in 13C-labelled c-picoline was measured by inversion recovery to be 4.3 ± 0.3 s under the experimental conditions. Fig. 3 shows the timing sequence for the QRIP experiment incorporating the 13C I-BURP pulses for enhancement of the 1H signals. After equilibration at low temperature, dissolution of the sample with a hot solvent, and transport into the high-field NMR magnet, a series of 1H pulses with flip angle of 5° was applied to induce QRIP-enhanced 1H NMR signals. After the first pulse was given, a 13 C I-BURP pulse was applied before every subsequent fourth 1H pulse, in order to reduce the partial cancellation of the degenerate A and E-manifold signals. Fig. 4a shows the sequence of 1H doublet amplitudes observed during this experiment on a sample of 13C-labelled c-picoline. The horizontal time axis takes into account 15 s delay between dissolution and acquisition of the first 1H NMR signal. The vertical scale represents the signal enhancement relative to thermal proton signal at 500 MHz, estimated by taking the ratios of corresponding peak amplitudes. The enhancements for the two peaks have the same magnitude within experimental error. The initial 1H signal enhancement due to the QRIP is observed to be 26 relative to the thermal signal. A further 1.86 times enhancement is realised
Fig. 1. Energy level diagram for a 13CH3 group at various stages of the experiment. The population of each energy levels are shown as blue circles and red arrows showing the possible transitions. For simplicity, the initial E manifolds are shown as completely depleted. Population distribution (a) immediately after dissolution and internal equilibration of the A manifold, (b) generated by cross-relaxation, and (c) after the 13C double-selective pulse on the inner quartet transitions. The green curved arrows in (b) indicate the population exchanges induced by the selective 13C pulse. Population differences within the A and E manifolds are converted into observable 1H antiphase magnetisation when rf pulses are applied. The observed NMR signal is given by the superposition of transitions occurring in both A and E manifolds. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
by the application of the selective 13C pulse. Hence an overall enhancement of 48 over thermal signal is achieved in this experiment. The large step in signal amplitude between the first and second point in this series is due to the 13C I-BURP pulse. The enhancement effect is short-lived as the populations return to the distribution shown in Fig. 1b. The effect can be repeated, however, by applying the frequency-selective 13C pulse again. The decay curve has a time constant of 25.5 ± 3.5 s, consistent with the participation of a long-lived state.
Communication / Journal of Magnetic Resonance 250 (2015) 25–28
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Fig. 2. Partial 13C spectra of a 582 mM solution (30 ll solute in 0.5 ml solvent) of 13 C-labelled c-picoline in CDCl3 at 11.7 T, displaying the quartet corresponding to the methyl group. In (a) the sample was equilibrated in high magnetic field, and a p=2 pulse used to excite the 13C NMR signals. In (b) a I-BURP pulse of duration 30 ms was applied immediately before the p=2 excitation pulse.
Fig. 4. (a) Experimental signal enhancement of the two 1H peaks as a function of time for the pulse sequence in Fig. 3, relative to the thermal proton NMR signal. Small flip-angle 5° pulses were used to induce the 1H signal. The dashed curves show exponential fits to the data yielding a time constant of 25.5 ± 3.5 s. (b) Simulated trajectories of 1H signal enhancement with respect to the thermal 1H signal at 11.7 T for the pulse sequence shown in Fig. 3.
Fig. 3. Timing sequence for the QRIP experiment, including a frequency-selective 13 C I-BURP pulse before every fifth 1H observation pulse. The timing axis is not uniform. The magnetic field during the cryogenic cooling interval is not essential and may be omitted. In practice the 1H detection pulses had a flip angle of 5° and were repeated every 5 s.
Fig. 4b shows a numerical simulation of the 1H peak amplitudes during the observation segment of the I-BURP enhanced QRIP procedure, obtained using the SpinDynamica software platform [8]. The simulation assumes an initial state corresponding to a population difference of 0.6 between the A and E levels, as estimated from the Boltzmann distribution for a system with a tunnelling splitting of 126 GHz at a temperature of 4.2 K [3]. The simulated evaluation includes all relevant relaxation mechanisms, including heteronuclear dipole–dipole coupling, homonuclear dipole–dipole coupling, chemical shift anisotropy relaxation, spin-rotation and all crosscorrelations terms. There is qualitative agreement with experiment, although the enhancement factor predicted by simulation is roughly 3 times higher than the experimental values. This discrepancy is attributed to losses during dissolution and incomplete equilibration at low temperature. Fig. 5 shows the QRIP-enhanced 1H spectra in more detail. The 1 H spectrum obtained by the Fourier transform of the transient
Fig. 5. 1H QRIP spectra of the methyl group of 13C-labelled c-picoline in CHCl3. (a) Fourier transform of the transient induced by the first 5° 1H pulse after dissolution and transport to high field; (b) Fourier transform of the transient induced by the second 5° 1H pulse, which is preceded by the frequency-selective 13C pulse; and (c) Fourier transform of the sum of 25 1H transients, observed on the same sample upon thermal equilibrium, with a flip angle of 5°, and keeping all other parameters unchanged.
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Communication / Journal of Magnetic Resonance 250 (2015) 25–28
induced by the first 5° 1H pulse is shown in Fig. 5a. Fig. 5b shows the 1H spectrum obtained from the second 1H pulse, which is preceded by the 13C I-BURP pulse. The signal enhancement from (a) to (b) is obvious. Fig. 5c shows the 1H spectrum of the same sample after thermal equilibration in the high-field magnet, summing together 25 transients. In this case the overall enhancement of the 1H signal, estimated by comparing peaks integrals, is 48 ± 3 and is identical for the two 1H peaks within experimental error. The enhanced 1H spectrum in Fig. 5b displays strikingly different line widths for the two peaks. The measured line widths (full widths at half height) are 4.3 ± 0.1 Hz, 2.7 ± 0.1 Hz for the peaks in Fig. 5a and 5.1 ± 0.1 Hz, 1.9 ± 0.1 Hz for Fig. 5b. In contrast, the thermal equilibrated spectra shown in Fig. 5c displays equal line widths of 2.8 ± 0.1 Hz for both peaks. The line widths were observed to vary between one experiment and the next, particularly in the initial part of the decay. However, the integrals did not vary significantly. We measured the temperature dependence of the chemical shifts and J-coupling parameters in independent experiments, but found this to be insufficient to explain the line width effects. At this point the line width variations are unexplained. 3. Methods The experiments were carried out on a home-built dissolutionDNP apparatus with a 3.35 T cold-bore magnet, based on the design from Ref. [9]. In the current experiments, the dissolution apparatus was only used for cooling and dissolving the sample; The DNP system was not used and the magnetic field is not expected to have an effect [1]. A cup filled with 30 ll of 13Clabelled c-picoline was inserted into the cryostat, which was already filled with liquid helium. The sample temperature was lowered to 3 K by reducing the He pressure using a pump. The sample was left at 3 K for about 2 h. The sample cylinder at the top of the dissolution stick was filled with 4 ml of degassed chloroform which was heated to 120 ° C. When the pressure inside the hot solvent chamber reached 5 bar, the dissolution stick was inserted manually into the cryostat, making contact with the sample cup and flushing the sample out through a plastic tube, chased by warm He gas. The dissolved sample was collected in a syringe and injected into a 5 mm NMR tube inside a 11.7 T high-resolution NMR magnet. It took around 15 s from dissolution to the injection of the sample into the NMR tube. All high-field NMR experiments were performed on a 500 MHz Bruker Avance III spectrometer equipped with a 5 mm broadband NMR probe. The shape of the amplitude-modulated I-BURP pulse is specified in Ref. [7]. The pulse had a total duration of 30 ms, with a maximum rf amplitude corresponding to a nutation frequency of 200 Hz. These parameters provide an inversion bandwidth of 168 Hz. The simulations parameters were taken from Ref. [3]. The correlation time was sc ¼ 5 ps for the overall molecular tumbling and sR ¼ 0:6 ps for the methyl group rotation. 4. Conclusion In summary, we have demonstrated an enhancement of the hyperpolarised 1H NMR signals generated in a QRIP experiment, induced by a selective manipulation of the 13C nuclear spin transitions. The observed enhancement supports our theory of the QRIP effect, in which a methyl long-lived state is hyperpolarised by
conversion of the low-temperature tunnelling polarisation to a population imbalance between the A and E symmetry species of the methyl group in solution. The nearby 13C nucleus introduces cross-relaxation which converts the hyperpolarised long-lived state into enhanced population differences across observable transitions. In the case of the proton NMR signals, there is destructive interference between the signal contributions from the A and E manifolds which may be partially corrected by frequency-selective manipulation of the 13C transitions. QRIP leads to density operator components which are products of 1H and 13C spin operators, so that there is no obvious gain in attempting to use INEPT-type pulse sequences [10] to transfer polarisation form 13C to 1H. This has been verified by simulations. In its current form, this experiment is restricted to the small number of chemical compounds which contain highly unhindered methyl rotors at low temperatures, exhibiting large enough tunnelling splittings to be polarised thermally at readily accessible temperatures. However, there is every prospect that dynamic nuclear polarisation (DNP) may be used to generate very low spin temperatures in the mK range [11–13]. This would permit differential polarisation of the A and E spin manifolds in a much wider range of compounds, including those which lack a tunnelling splitting. This research is ongoing in our laboratory. Acknowledgments We thank Walter Köckenberger and Josef Granwehr for the loan of the dissolution-DNP device, and Ole G. Johannessen for help with instrumentation. This research was supported by the Engineering and Physical Sciences Research Council (UK), by the European Research Council, the Royal Society (UK), the Wolfson Foundation (UK), and by the COST Action TD-1103. References [1] M. Icker, S. Berger, Unexpected multiplet patterns induced by the Haupt-effect, J. Magn. Reson. 219 (2012) 1–3. [2] M. Icker, P. Fricke, S. Berger, Transfer of the Haupt-hyperpolarization to neighbor spins, J. Magn. Reson. 223 (2012) 148–150. [3] B. Meier, J.-N. Dumez, G. Stevanato, J.T. Hill-Cousins, S.S. Roy, P. Håkansson, S. Mamone, R.C.D. Brown, G. Pileio, M.H. Levitt, Long-lived nuclear spin states in methyl groups and quantum-rotor-induced-polarization, J. Am. Chem. Soc. 135 (2013) 18746–18749. [4] M. Icker, P. Fricke, T. Grell, J. Hollenbach, H. Auer, S. Berger, Experimental boundaries of the quantum rotor induced polarization (QRIP) in liquid state NMR, Magn. Reson. Chem. 51 (2013) 815–820. [5] P.R. Bunker, P. Jensen, Molecular Symmetry and Spectroscopy, second ed., NRC Research Press, Ottawa, 2006. [6] R. Freeman, Shaped radiofrequency pulses in high resolution NMR, Prog. Nucl. Magn. Reson. 32 (1998) 59–106. [7] H. Geen, R. Freeman, Band-selective radiofrequency pulses, J. Magn. Reson. 93 (1991) 93–141. [8] SpinDynamica, programmed by Malcolm H. Levitt with contributions from Jyrki Rantaharju and Andreas Brinkmann . [9] J. Granwehr, J. Leggett, W. Köckenberger, A low-cost implementation of EPR detection in a dissolution DNP setup, J. Magn. Reson. 187 (2007) 266–276. [10] G.A. Morris, R. Freeman, Enhancement of nuclear magnetic resonance signals by polarization transfer, J. Am. Chem. Soc. 101 (1979) 760–762. [11] T. Maly, G.T. Debelouchina, V.S. Bajaj, K.-N. Hu, C.-G. Joo, M.L. Mak-Jurkauskas, J.R. Sirigiri, P.C.A. van der Wel, J. Herzfeld, R.J. Temkin, R.G. Griffin, Dynamic nuclear polarization at high magnetic fields, J. Chem. Phys. 128 (2008) 052211. [12] S. Jannin, A. Bornet, R. Melzi, G. Bodenhausen, High field dynamic nuclear polarization at 6.7 T: carbon-13 polarization above 70% within 20 min, Chem. Phys. Lett. 549 (2012) 99–102. [13] A. Bornet, R. Melzi, S. Jannin, G. Bodenhausen, Cross polarization for dissolution dynamic nuclear polarization experiments at readily accessible temperatures 1:2 < T < 4:2 K, Appl. Magn. Reson. 43 (2012) 107–117.
