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Basic principles of DNP
The aim of dynamic nuclear polarization is to transfer the large Boltzmann equilibrium electron polarization to nuclei under continuous wave (CW) microwave irradiation. Electrons have a high gyromagnetic ratio compared to any nucleus and, thus, under similar experimental conditions, electrons will have a larger Boltzmann polarization than any nucleus. When a nucleus is coupled to
electron spins (via so-called hyperfine couplings), the polarization can readily be transferred to the nuclear spins.
The idea of transferring electron polarization to nuclei was first proposed by Overhauser in 1953,28 and subsequently verified by Carver and Slichter,35 the phenomenon is nowadays called the Overhauser effect. They demonstrated it experimentally in lithium metals by enhancing intensities of 6Li NMR signals. In the 1950’s, many DNP theories were investigated and developed; from a methodological as well as from an experimental perspective. Later in 1957, DNP was also employed for non-conducting solids at low temperatures by Jeffries36. In 1958, Abragam and Proctor described the effect of DNP for these nonconducting solids, experimentally and theoretically, calling the described effect the solid effect.37 During 1960’s and 1980’s, theories of DNP in solids were explored by Provotorov,38 Borghini,39 and Abragam and Goldman40 leading to the discovery of a new DNP mechanism called thermal mixing.41 Yet another DNP mechanism, called cross effect, was observed by Hwang and Hill in 1967.42-43.
DNP Mechanisms
Several mechanisms of polarization transfer from electrons to nuclear spins can be distinguished according to the source of the electron spin polarization. In a coarse-grained manner we may distinguish: the solid effect (SE), the cross effect (CE) and thermal mixing (TM), although the CE is often described as a special form of TM.
In my work, I have used TEMPOL as a polarizing agent. Due to its broad EPR line, the most probable dominant mechanism for TEMPOL is a combination of differential solid effect and of the cross effect, as pointed out by Vega and coworkers50- 51. However other mechanisms, like thermal mixing (TM) can also contribute but are less intense.
Microwave Source
Our DNP experiments are usually performed at frequencies 187.5 < fμw < 188.5 GHz with a maximum microwave power of 350 mW at the output of the microwave source. Microwaves are generated with an ELVA source which provides a frequency span of fμw = 94 GHz ± 250 MHz. With the help of a frequency doubler (Virginia diodes) a final frequency of fμw = 188 GHz ± 500 MHz can be reached.
To enhance the polarization of the nuclear spins by DNP, a paramagnetic species is required. In our laboratory, we use 4-hydroxy-2,2,6,6-tetramethyl piperidine- 1-oxyl (TEMPOL) as a polarizing agent. To saturate a broad spectral width of the EPR line, frequency modulation can be used.73 Experimentally, we use an amplitude of Δfμw = 100 MHz with a modulation frequency of fmod ≈ 2 kHz. Frequency modulation provides a better enhancement in dissolution DNP experiments,73 especially at 1.2 K.
Magnetic tunnel
During the transfer of the hyperpolarized sample, longitudinal magnetization decays back to its equilibrium state. Several mechanisms, e. g., dipole-dipole interactions, chemical shift anisotropy (CSA), scalar coupling and coupling with the paramagnetic species are responsible for the longitudinal relaxation. These relaxation mechanisms are briefly discussed in chapter 3.
To overcome this problem several solutions have been proposed. In the group of Walter Kockenberger, the DNP setup is based on a dual magnet74 where the transfer occurs in less than a second. However, most DNP polarizers are placed at a few meters distance from the NMR or MRI instrument and the transfer time can vary from a few seconds75 to about a minute76 depending on the hyperpolarized fluid. Recently B. Meier from Levitt’s group has proposed rapid transfer DNP,77 where the transfer of the solid sample can be done in about 100 ms. In our laboratory, the transfer of the sample from the polarizer to the magnet can vary from 1-10 s. During the transfer, the hyperpolarized sample may travel through very low field regions like the Earth’s magnetic field. To overcome some of the above-mentioned relaxation effects or, in other words, to preserve the maximum polarization, a magnetic tunnel was designed by Jonas Milani at EPFL78. We use the same tunnel in our laboratory. In our setup the hyperpolarized sample is transferred through the magnetic tunnel that maintains a field of 0.9 T between the polarizer at a magnetic field of 6.7 T to the spectrometers at 9.4 T or 18.8 T.
Combination of CP and dissolution-DNP
Radicals with narrow line-width, e.g, trityl, are efficient to polarize directly 13C even though it takes a long time to achieve high steady state polarization levels at 1.2 K.13 However, the long build up time for 13C does not allow performing DNP experiments frequently such that DNP was combined with CP to circumvent this problem. Using TEMPOL as a radical, DNP with CP methods was optimized by Bornet at a magnetic field 3.35 T and temperatures near at 1.2 K (for more details see ref. 14). TEMPOL has a rapid buildup time constant for protons and combining cross polarization with dissolution-DNP can be very efficient.9-10 In the following I will outline a work including some theoretical background that we have carried out at EPFL Lausanne and which has been published as Ref 15. It aims at enhancing the CP efficiency by employing the abovementioned microwave gating or interruption approach.
Microwave irradiation can shorten !! »
Typically, at cryogenic temperatures and high fields, the electron spin polarization in thermal equilibrium !! ! » can be close to unity (so that the electron polarization cannot be neglected in theoretical treatments), which leads to an attenuation of the transition rates within the electron spin manifold by a factor of18 ! = 1 − !! ! » ∙ !! ! ».
The advantages of microwave gating are substantial at low temperatures
Fig. 2.6 shows the !!! relaxation curves measured at different temperatures with the pulse sequence of Fig. 2.5a, with continuous or gated microwaves (fixed gating interval !! »#$ = 500 ms and !! » varied from 0 to 10 ms). With continuous microwaves, all !!! curves are alike, featuring a fast decay of the magnetization during spin locking. However, with gated microwaves, the magnetization can survive spin locking remarkably well, and especially at the lowest temperature T = 1.2 K where ! = 0.00055. On the other hand, the advantages of microwave gating diminish at higher temperatures as the thermal equilibrium electron spin polarization !! ! » becomes significantly lower that unity.
Table of contents :
Résumé
1. Introduction: NMR to Dissolution DNP
1.1 General Introduction
1.2 Theory
1.2.1 Nuclear spin polarization:
1.2.2 Basic principles of DNP
1.2.3 DNP Mechanisms
1.2.4 DNP Interactions
1.3 DNP Hardware
1.3.1 Cryostat
1.3.2 Probe
1.3.3 Microwave Source
1.3.4 Magnetic tunnel
1.3.5 Dissolution Setup
1.4 References for Chapter 1
2. Microwave Gating in DNP
2.1 Nuclear polarization transfer
2.1.1 Cross Polarization
2.1.2 Combination of CP and dissolution-DNP
2.2 Boosting CP Efficiency by Microwave Gating
2.2.1 Paramagnetic Relaxation
2.2.3 Microwave gating extends T1ρ
2.2.4 The advantages of microwave gating are substantial at low temperatures
2.2.5 Microwave gating improves the efficiency of cross polarization
2.2.6 Optimization of timing for several cross polarization steps
2.3 Conclusions
3. Dissolution DNP of Deuterated Molecules
3.1 Introduction to DNP in Deuterated Systems
3.2 General Considerations About Nuclear Relaxation
3.2.1 The Dipolar Interaction
3.2.2 Long-Lived States for Dipolar Interactions
3.2.3 CSA Relaxation
3.2.5 Quadrupolar Interaction
3.2.5 Combination of LLS with D-DNP
3.3 Dissolution DNP of Quadrupolar Nuclei
3.3.4 Theory
3.3.5 Results and Discussion
3.3.6 Conclusions
3.3.7 Experimental Details
3.3.8 Appendix
3.4 References for Chapter 3
4. Long-Lived Deuterium Spin State Imbalance in Methyl Groups
4.1 Study of Deuterated Methyl Groups by Dissolution-DNP
4.1.1 Symmetry-Adapted Basis Set
4.1.2 Results and Discussion
4.1.3 Experimental Methods
4.2 References for Chapter 4
5. Conclusions
Curriculum Vitae
