Comparative Study of the Magnetic Field Dependent Signal

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Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

Comparative Study of the Magnetic Field Dependent Signal Enhancement in Solid-State Dynamic Nuclear Polarization Experiments Sarah Bothe,† Markus M. Hoffmann,*,‡ Torsten Gutmann,*,† and Gerd Buntkowsky*,† †

Institute of Physical Chemistry, Technical University Darmstadt, Alarich-Weiss-Strasse 8, D-64287 Darmstadt, Germany Department of Chemistry and Biochemistry, The College at Brockport, State University of New York, Brockport, New York 14420, United States

‡

S Supporting Information *

ABSTRACT: A detailed study of the magnetic field dependent signal enhancement in solid-state dynamic nuclear polarization (DNP) experiments is presented for a specific sample consisting of AMUPol dissolved in the nonionic surfactant C10E6. C10E6 displays a superposition of “direct” and “indirect channel” resonances in 13C MAS DNP NMR spectra. The shapes of the DNP enhancement profiles are essentially identical for the 1H MAS, 1H → 13C CP MAS, and 13C MAS indirect channel signals, which confirms that the same polarization transfer process from electron to proton is responsible for the obtained enhancements of these experiments. The shape of the DNP enhancement profiles of 1H and of 13C direct channel resonances reveals that the cross effect is the dominant polarization transfer mechanism for the studied sample. The magnitudes of the 13C MAS DNP enhancement profiles for 1H → 13C CP MAS, direct and indirect channel signals were found to be not uniform. For 1 H → 13C CP MAS and the indirect channel signals, this observation is related to relaxation effects of the methyl group carbon. For the 13C MAS direct channel resonances, differences in magnitudes are discussed in terms of preferential structural orientation of the polar ethylene oxide headgroup of C10E6 toward the AMUPol radical.

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during a rotor cycle.7,8 Compared to SE and CE, TM and OE are much less commonly observed in solid-state DNP NMR spectroscopy. TM requires interactions between many electron spins that as a whole transfer polarization by processes analogous to thermodynamic cooling.9 TM is only active at very low temperatures, typically smaller than 10 K.2 The OE effect in solid-state DNP NMR has only been recently discovered.10 In contrast to SE, the microwave irradiation excites in the OE electron spin transitions, and the polarization transfers to the nuclear spin through relaxation, which requires that the nuclear spin is exposed to fluctuating scalar or dipolar hyperfine interactions and strong isotropic hyperfine coupling is in effect.11 The DNP signal enhancement is generally reported as the ratio of spectral intensities obtained with and without microwave, IμWon/IμWoff, although some authors noted that quantitative evaluation should also consider side effects such as depolarization.12−14 Stable organic radical compounds serve most commonly as electron-spin sources in DNP. They are introduced into the samples for DNP measurements typically using a glass-forming solvent matrix often called “DNP juice”.15,16 Commonly, the electron spin polarization transfers

INTRODUCTION High-field dynamic nuclear polarization (DNP) is one of the hyperpolarization methods that has displayed great potential for increasing NMR sensitivity over the past 10 years.1−4 The DNP-induced enhancement is achieved by the transfer of polarization from electron spins of paramagnetic polarizing agents to the desired nuclei by microwave (μW) irradiation.5 As reviewed several times,2,5,6 there are presently four mechanisms known for the transfer of electron spin polarization to nuclear spin polarization: the solid effect (SE), the cross effect (CE), the thermal mixing (TM), and the Overhauser effect (OE). Briefly, in the SE the electron spin polarization transfers to nuclear spin polarization via the excitation of formally forbidden single or double quantum e−n transitions. CE is a more complicated process involving two coupled electron spins and a nuclear spin. Originally, the CE has been explained by a mixing of degenerate electron and nuclear spin states that can occur when the difference in electron resonance frequency matches that of the Larmor frequency of the nucleus. Only recently, it has been realized that this frequency-matching condition suffices only for understanding CE in static samples. Typically, however, solid-state samples are explored with DNP NMR spectroscopy under magic angle spinning (MAS). Under these MAS conditions, it has been shown that polarization is transferred by level anticrossing (LAC) events that occur © XXXX American Chemical Society

Received: August 10, 2017 Revised: November 14, 2017 Published: November 17, 2017 A

DOI: 10.1021/acs.jpcc.7b07967 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Scheme 1. (a) Surfactant CmEn, Where m Indicates the Number of Carbon Atoms in the Alkyl Chain (C) While n Is the Repetition Unit of Ethylene Oxide (E); (b) Biradical AMUPol

to the protons in the NMR sample and spreads through 1H−1H spin diffusion.8,15,17 Experimentally, the enhanced proton polarization is then actively transferred to other nuclei such as 13C by cross-polarization. Alternatively, in many samples 13C signal enhancements can also be realized by direct polarization transfer from electron spins to 13C nuclei.2 Recently, a surprising effect has been observed18−20 where the electron spin polarization was transferred to 13C concurrently by two different paths: directly from electron spin to 13C (direct channel) and indirectly via the proton spin reservoir through inherent nuclear Overhauser enhancement (NOE) type cross-relaxation processes (indirect channel).21−25 This effect was observed for polyethylene glycol (PEG) and related surfactants,18,19 as well as for a number of amino acids20 in combination with different kinds of polarizing agents. Several open questions remaining from these first studies on concurrent polarization transfer mechanisms concern their magnetic field dependencies. First, if the electron spin polarization indeed transfers to 13C via the proton spin reservoir for the indirect mechanism, then the DNP magnetic field sweep enhancement profiles (or short, DNP enhancement profiles) of proton and indirect channel 13C signals should be of the same shape. This validity test is still needed. Second, separate DNP enhancement profiles were shown for the direct and indirect channel 13C signals,18 which were obtained from a rather challenging deconvolution process of the 13C spectra that concurrently showed both sets of signals. For an accurate comparison of the magnetic field dependence, however, a complete separation of the different enhancement profiles is required, which may be realized by application of modified 13C MAS NMR experiments that are selective of each mechanism.18−20 Third, additional studies on the DNP enhancement profiles are needed to resolve which of the known DNP mechanisms for solid-state samples dominates. To address these questions, the present study examines the DNP enhancement profiles for 1H MAS, 1H → 13C CP MAS, and the direct and indirect channel 13C MAS. This model study is conducted for the radical AMUPol dissolved in a polydisperse ethoxylated alcohol (C10E6) shown in Scheme 1 where the presence of the concurrent direct and indirect channel polarization transfer has been previously observed.18,19 Furthermore, C10E6 offers several distinct 13C signals. Therefore, we also inspect in this report if DNP enhancement profiles are dependent on the chemical nature of the carbon atom. To the best of our knowledge, this is the first study to inspect differences in DNP enhancement profiles for different 13C resonances in direct and indirect channel 13C MAS DNP NMR experiments. The presented findings not only deepen an understanding of the mechanistic details of DNP but also show

how structural information may be derived from studying chemical samples with DNP NMR.

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EXPERIMENTAL SECTION Chemicals. The nonionic surfactant C10E6 was used as obtained from Rochester Midland Corporation who acquired this surfactant in large quantities from the industrial provider Air Products under the trade name TOMADOL91-6. Since it is a polydisperse material, polydispersity and average molecular weight may vary for each batch. The exact composition of the polydisperse C10E6 can be found in Tables S1 and S2 in the Supporting Information. The radical AMUPol was used as received from SATT (Universités d’Aix-Marseille-CNRS) without further purification. The radical sample solution of 18.2 mmol·kg−1 was prepared by weighing 1.47 mg of AMUPol at room temperature, which was then dissolved in 111.27 mg of the surfactant solvent C10E6 by shaking. DNP-Enhanced Solid-State NMR Experiments. The solid-state DNP NMR experiments were performed on a Bruker Avance III 400 MHz NMR spectrometer equipped with an Ascend 400 sweepable DNP magnet and a 3.2 mm tripleresonance 1H/X/Y low-temperature MAS probe. A 9.7 T Bruker gyrotron was used to generate a microwave frequency of 263 GHz. All experiments were carried out with a spinning speed of 8 kHz. The temperature was maintained by a Bruker BioSpin low-temperature MAS cooling system. Because μW irradiation at fixed maximum cooling power causes higher sample temperature, the nominal sample temperatures were 107 K without μW irradiation and 117 K with μW irradiation. The sample was inserted into a 3.2 mm sapphire rotor and sealed with a Teflon insert. A ZrO2 drive cap was applied on top of the rotor. To prevent the liquid sample from leaking, the sample in the rotor was first frozen in liquid nitrogen before it was quickly inserted into the probe. All 1H MAS (magic angle spinning) spectra were recorded at frequencies of 400.02 MHz, and all 13C CP MAS (cross-polarization magic angle spinning), and 13C MAS NMR spectra, were recorded at 100.59 MHz. 1 H MAS experiments were performed employing a background suppression pulse sequence in which 90° excitation pulses of 2.58 μs were applied followed by two 180° pulses with 0.1 μs spacing between the pulses. Sixteen scans were performed with 4 s repetition delay. 13 C CP MAS NMR spectra were acquired with 4 scans with a repetition delay of 4 s, using a contact time of 2 ms. The decoupling of the dipolar interactions with protons was achieved with SPINAL-64.26 The standard 13C MAS saturation recovery experiment was modified by introducing a sequence of rotor-synchronized 180° B

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Figure 1. Microwave-on (a) and microwave-off (b) 13C CP MAS NMR spectra of the studied 18 mmol·kg−1 AMUPol in C10E6 at 9.395 T measured at 8 kHz spinning with peak assignment: ethylene oxide carbons [−CH2−CH2−O−]n (1), carbon of terminal −CH2−OH (2), alkyl chain carbons −CH2−CH2−CH2− (3), alkyl chain carbon next to the methyl group −CH2−CH3 (4), and carbon of the terminal methyl group −CH3 (5). Signals marked with asterisks are spinning sidebands.

Figure 2. DNP enhancement profiles of 18 mmol·kg−1 AMUPol in C10E6 for (a) 1H MAS, (b) 1H → 13C CP MAS, (c) 13C MAS indirect channel, and (d) 13C MAS direct channel resonances. ○ represents the profile for the ethylene oxide carbons (1), □ represents the profile for the alkyl chain carbons (3), and ▽ represents the profile of the carbon of the terminal methyl group (5). Note: The profile of the direct channel resonance for the carbon of the terminal methyl group (5) is not shown since at each magnetic field no significant signal enhancement was obtained.

extended time periods. The effectiveness of this sequence was checked by detecting the proton magnetization, which was found to be suppressed by at least 90%. In a typical experiment, at the beginning the thermal magnetization of 13C was saturated by a pulse train of twenty 2.2 μs pulses with a spacing of 5 ms between the pulses. Then, during an 8 s

pulses on the proton channel to be able to suppress the buildup of 1H-magnetization during the 13C hyperpolarization as described in our previous work.18 The reason for the use of a sequence of rotor-synchronized phase-cycled 180° pulses instead of a decoupling sequence is that the probe head would be damaged if a decoupling pulse was irradiated for C

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In the following subsections we provide first a detailed discussion of the observed DNP enhancements in the profiles in Figure 2 before we discuss their line shapes. 1 H MAS and 1H → 13C CP MAS. 1H MAS DNP enhancements of up to about 20 are observed in the sweep profile in Figure 2a. This signal enhancement is in the same range as the enhancement for the ethylene oxide carbons (1) and for the alkyl chain carbons (3) obtained in the 1H → 13C CP MAS DNP sweep profiles in Figure 2b, which show a maximum enhancement of around 15, while a larger enhancement of ca. 30 is obtained for the methyl carbon (5). 1 H−1H spin diffusion should spread the polarization evenly throughout the proton spin reservoir before it is actively transferred by the NMR pulse sequence to the 13C in the 1H → 13 C CP MAS experiment. Therefore, the 1H → 13C CP MAS DNP enhancement profiles for each 13C resonance are expected to be identical in shape and magnitude within the experimental scatter. This holds true for 1H → 13C CP MAS DNP enhancement profiles for the ethylene oxide carbons (1) and for the alkyl chain carbons (3) in Figure 2b for which the carbon signal enhancements range between about −10 and +15. However, the 1H → 13C CP MAS DNP enhancement for the methyl carbon (5) varies with a range between about −20 and +30. This nonuniform sensitivity enhancement in DNP 1H → 13C CP MAS NMR was also observed by Mollica et al. for methyl resonances in polymer material.27 These authors showed that the T1 spin−lattice relaxation time was shorter for the methyl groups, which are generally known to undergo rotational motions in the solid at low temperatures,24,28,29 and presumed that this relaxation mode is also active at the spin lock field in the rotating frame.30 To test if similar relaxation effects are at play here as well, the signal strength as a function of the CP contact time for a C10E6 blank sample was inspected as shown in Figure S7. It is evident from Figure S7 that only the methyl group signal (5) is significantly enhanced when μW irradiation is present while for ethylene oxide carbons (1) and for the alkyl chain carbons (3) the μW on/off curves show only slight deviations in their intensity. Thus, the larger 1H → 13C CP MAS DNP enhancements of the methyl carbon in Figure 2b are likely due to relaxation effects as reported by Mollica et al.27 It should be further noted that the observed methyl resonance increase upon μW irradiation of the blank sample in Figure S7 is unlikely related to quantum rotor induced hyperpolarization (QRIP),31 prior referred to as the Haupt effect,32 for experimental reasons including the too narrow temperature increase33 of 10 K at temperatures >100 K, which is too high for QRIP to be active.34 Direct and Indirect 13C DNP Enhancement Profiles. The direct and indirect 13C DNP enhancement profiles were obtained from experiments with a set DNP buildup time of 8 s. This buildup time was chosen because it allows to distinguish signals for the direct channel as well as the indirect channel at an economic measurement time. However, as shown in Figure 3, the DNP buildup is clearly not complete for all obtained signals at 8 s. Thus, the DNP enhancements at 8 s buildup time are not uniform for the direct and indirect channel resonances as the DNP buildup times are generally sensitive to the type of carbon. Inspecting the DNP enhancement profiles for the indirect channel resonances in Figure 2c, it can be seen that nevertheless the DNP enhancement profiles all range approximately between −10 and +10 except for the methyl

buildup time, rotor-synchronized 180° pulses with a pulse length of 5.15 μs were applied on the proton channel every 2τ = 500 ms. The final detection pulse on 13C was set to 3.7 μs. Spinal 64 decoupling26 was applied during data acquisition. Twenty-four scans were performed. To acquire the 13C MAS spectra without rotor-synchronized 1H 180° pulses, the same modified saturation recovery experiments were performed except that the pulse power for the rotor-synchronized 180° pulses on the proton channel were set to zero. The process of obtaining separate spectra for the direct channel and the indirect channel 13C resonances has already been described in our prior work18 but for clarity is illustrated again in Figure S1. Briefly, the 13C MAS spectrum with rotorsynchronized 180° 1H pulses represents the direct channel resonances only. The 13C MAS spectrum recorded without rotor-synchronized 180° 1H pulses represents a superposition of direct channel and indirect channel resonances. The difference spectrum represents the indirect channel resonances only. With respect to the steps of obtaining DNP enhancements, the 1H MAS and 13C MAS spectra without μW irradiation were first subtracted from the corresponding spectra with μW irradiation (see also additional exemplary 1H spectra in Figure S2). Next, the obtained 13 C spectra were deconvoluted as illustratively shown in Figure S3. The DNP enhancements were calculated as the ratio of peak heights from the adjusted spectra with μW irradiation and the spectra without μW irradiation. Spectral data acquisition and processing were repeated at varying magnetic field strength, and the DNP enhancements plotted as a function of magnetic field strength to obtain the desired DNP enhancement profiles. The DNP enhancement profiles were obtained in the field range from 9.36 to 9.42 T.

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RESULTS AND DISCUSSION

General Aspects on Signal Assignment and DNP Enhancement Profiles. To illustrate typical DNP enhancement for the studied sample and for clarity in peak assignment, we show in Figure 1 the 13C CP MAS spectra of 18 mmol·kg−1 AMUPol in C10E6 in the frozen state. For the μW on spectrum, it is feasible to clearly distinguish five 13C signals that are located near 70 ppm (ethylene oxide carbons [−CH2−CH2− O−]n, 1), near 60 ppm (carbon of terminal −CH2−OH, 2), near 30 ppm (alkyl chain carbons −CH2−CH2−CH2−, 3), near 25 ppm (alkyl chain carbon next to the methyl group −CH2−CH3, 4), and near 15 ppm (carbon of terminal methyl group −CH3 (5). Subsequently, the field dependency of the DNP signal enhancement for 1H MAS, 1H → 13C CP MAS, as well as for the direct and indirect channel resonances from 13C MAS, was investigated, and the data points are plotted in Supporting Information Figures S4−S6. DNP enhancement profiles with reasonably low scatter were fitted by deconvolution with Gauss and Lorentz functions of fixed widths and positions. Figure 2a shows the 1H MAS DNP enhancement profile, and Figures 2b−d show the 13C DNP enhancement curves derived for the ethylene oxide carbons (1), for the alkyl chain carbons (3) and the carbon of the terminal methyl group (5). For the carbon signal of the terminal −CH2−OH (2) and the alkyl chain carbon next to the methyl group −CH2−CH3 (4) which are obtained as shoulder peaks in the 13C CP MAS spectrum, a detailed analysis was not reasonable due to the large uncertainty of the data. D

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Additional interesting points need to be made about the DNP enhancement profiles in Figure 2c and the DNP buildup curves in Figure 4. Daube et al. pointed out that the fast relaxation of the methyl group leads to a quicker DNP buildup of the indirect channel resonance methyl group carbon resonance in their 13C-enriched amino acid samples. They hypothesized that the other 13C resonances would receive the polarization buildup from the methyl carbon via carbon spin diffusion.20 For samples with natural abundance 13C, as is the case for the C10E6 sample in this study, carbon−carbon spin diffusion is expected to be inefficient. The amplitudes of the DNP enhancement curve in Figures 2c for the indirect channel resonances provide further support that carbon−carbon spin diffusion is indeed not involved here. At a buildup time of 8 s, where enhancement buildup is incomplete, the DNP amplitudes would be expected to decrease with the structural distance of the 13C site from the methyl group if polarization would be transferred from the methyl carbon to these carbons by 13C−13C spin diffusion. This is not indicated in Figure 2c. Instead, as can be seen in Figure 4, the 13C indirect channel DNP buildup of the various carbon atoms (ethylene oxide carbons (1) and alkyl chain carbons (3)) in C10E6 other than the methyl carbon is approximately similar in behavior. To explain the apparent magnitude decrease of IμWon/IμWoff at longer buildup times for the ethylene oxide carbons (1), and the alkyl chain carbons (3) signals in Figure 4c, it is conceivable that additional, secondary polarization transfer processes occur between carbon atoms that receive direct channel DNP enhancement and carbon atoms that receive indirect channel enhancement. If protons can mediate polarization from radicals to 13C, then they should also be able to mediate polarization between the two types of polarized carbon atoms. Since the direct channel DNP builds up slower than the indirect channel DNP,18 this secondary effect would be consistent with the observed loss of negative enhancement in Figure 4c at long buildup times.

Figure 3. 13C DNP MAS buildup spectra for 18 mmol·kg−1 AMUPol in C10E6 at 9.395 T.

group carbon (5) where the enhancements range between −5 and +5. The 13C signal heights as a function of buildup time were experimentally obtained to inspect if this differentiation of the methyl carbon (5) can be explained again with the relaxation behavior of the methyl group carbon (5). The results are shown for the indirect channel signal heights in Figure 4a, and the corresponding 13C MAS signal heights under absence of μW irradiation are shown in Figure 4b. In stark contrast to the other 13C signals, it can be seen in Figure 4 that the methyl carbon (5) signal has already built up the maximum signal strength at 8 s in both Figure 4a and 4b. Spin−lattice relaxation seems to be significantly faster for the methyl carbon compared to the ethylene oxide carbons (1) and the alkyl chain carbons (3) and limits the achievable DNP enhancement of the methyl carbon, as can be seen in Figure 4c.

Figure 4. 13C MAS signal intensities as a function of buildup time of the indirect channel under microwave irradiation (a) and in absence of microwave irradiation (b) and the resulting DNP enhancements (c) at 9.395 T. ■ represents the ethylene oxide carbon (1) signals,▲ represents the alkyl chain carbon (3) signals, and ⧫ represents the terminal methyl group carbon (5) signals. E

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from the different NMR experiments. For this purpose, the DNP enhancement profiles of 1H MAS as well as the various 13 C MAS experiments in Figure 2 were normalized to unity for the maximum DNP enhancement. DNP enhancement profile data from the ethylene oxide carbon (1) resonance were representatively chosen because these display the least scatter (Figures S4−S6). Figure 5 shows a comparison of the

With respect to the molecular motions behind this interesting mechanistic relaxation behavior of C10E6, we have recently investigated this aspect in a separate study.19 We could show that the rapid freezing from room temperature to experimental conditions of 117 K the sample experiences when it is inserted into the NMR magnet may result in a glass state or some other thermodynamically unstable state. Thus, molecular motions other than methyl rotations that would normally be inactive at these low temperatures may still be operative and allow for the observed indirect channel 13C DNP buildup. Furthermore, we note that recently transient contacts of solvent and polarization agent have been recognized as a motional process with sufficiently fast correlation times to enable OE DNP.35 For the direct channel DNP enhancement profiles shown in Figure 2d, the DNP enhancements range between −20 and +10 for the ethylene oxide carbon (1) resonance, while a range between about −3 and +3 is obtained for the alkyl carbon (3) resonance. For the terminal methyl carbon (5) resonance shown in Supporting Information Figure S6e, no significant signal enhancement is obtained. This ordering in DNP enhancements is following the structural sequence of C10E6 (Scheme 1) from the polar ethylene oxide headgroup (largest enhancement) to the nonpolar alkyl chain tail (least enhancement). This trend is not only apparent for a buildup time of 8 s used to obtain the sweep profiles but also for the fully relaxed spectra measured with 900 s buildup time (see Supporting Information Figure S8). Comparing the T1 relaxation time constants for the ethylene oxide carbon (1) resonance and the alkyl carbon (3) resonance obtained from a saturation recovery experiment on a neat C10E6 sample without AMUPol (Figure S9a, c) resulted in an opposing trend: a faster time constant is obtained for the alkyl chain carbons (121 ± 23 s) compared to the ethylene oxide carbons (306 ± 12 s) (uncertainties are stated as standard errors). T1 curves for the other carbon signals (Figure S9b, d, e) were not analyzed in detail due to the large scatters of data points due to the low signal-to-noise ratio. When 20 mM AMUPol is added to C10E6, the trend of the T1 relaxation time constants stays the same (Figure S10), from faster relaxation of the alkyl chain carbons (93 ± 9 s) to slower relaxation for the ethylene oxide (177 ± 15 s). More interestingly, comparing the absolute values for the T1 relaxation time constants for the C10E6 sample with and without radical, a significant decrease of the relaxation time is only obtained for the ethylene oxide carbons (1), while for the alkyl chain carbons (3) only a tendency to faster relaxation times is observed. This suggests that for the studied 18 mmol·kg−1 radical concentration, the T1 relaxation times of the different functional groups of C10E6 are influenced by paramagnetic relaxation effects induced by AMUPol molecules in their environment, which have a r−6 distance dependency.36−40 Thus, on average AMUPol seems preferentially located near the C10E6 polar ethylene oxide headgroup since the relaxation time of this group is significantly reduced in the C10E6/AMUPol sample compared to the neat C10E6 sample. Due to the hydrophilic character of AMUPol, it will interact preferentially with the hydrophilic ethylene oxide headgroups of C10E6. Comparing DNP Enhancement Profile Shapes from Different NMR Experiments. To better understand the DNP mechanistic details, we now turn to a comparison and inspection of the shapes of the DNP enhancement profiles

Figure 5. Normalized DNP enhancement profiles of 18 mmol·kg−1 AMUPol in C10E6 for 1H MAS (black squares), 1H → 13C CP DNP MAS (red circles), and the 13C MAS indirect channel resonance (blue triangles).

normalized DNP enhancement profiles for 1H MAS, 1H → 13 C CP MAS, and the indirect channel 13C MAS DNP NMR experiments. The indirect channel 13C MAS DNP enhancements were inverted to allow for easier comparison. For transparency, the indirect channel 13C DNP enhancement profile without invertion is shown in Figure S11 together with the 1H MAS DNP enhancement profile. In CP experiments, the proton magnetization is transferred to the 13C nuclei under Hartmann−Hahn conditions.41 Therefore, the field sweep curve of the CP experiments is expected to be of similar shape as the 1H sweep curve. This is indeed the case in Figure 5, where the proton DNP enhancement profile generally overlaps with the 1H → 13C CP MAS DNP enhancement profile except for the region around the minimum near 9.37 T, where the negative DNP enhancement is slightly less pronounced. Figure 5 also shows that the indirect channel 13C MAS DNP enhancement profile overlaps well with the 1H → 13C CP MAS and 1H MAS DNP enhancement profiles. This clearly supports the hypothesis that the DNP enhancement profile of the indirect channel 13C MAS originates from polarization transfer via the proton spin reservoir. Furthermore, it confirms that the transfer of polarization from electron to proton in the first step of the indirect channel mechanism is identical to the electron to proton polarization transfer that is observed in proton 1H MAS DNP NMR and 1H → 13C CP MAS DNP NMR. Finally, a comparison of the indirect channel with the direct channel 13C MAS DNP enhancement profiles for the ethylene oxide carbon (1) resonance is shown in Figure 6. These DNP enhancement profiles are in qualitative agreement with but are quantitatively more accurate than the DNP enhancement profiles obtained in our prior work.18 Aside from their already mentioned overall mirror image like appearance, it is evident in F

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nuclear Larmor frequency.42 Such behavior is illustrated clearly in the case of the radicals SA-BDPA and trityl OX063.43 In contrast, for the CE the separation of the DNP enhancement profile minimum and maximum does generally not change from 13 C and 1H because the separation reflects the Larmor frequency difference between the two electron spins.42 This behavior has been observed for TOTAPOL, which displays rather similar DNP enhancement profiles as AMUPol in this study, in particular with respect to the 1H MAS DNP enhancement profile.43 The unchanging separation of DNP enhancement profile minimum and maximum in Figure 2 for the 1H MAS and direct channel 13C MAS resonances strongly supports CE as the dominant DNP mechanism. Nevertheless, it is conceivable that SE contributes to the DNP enhancement profiles observed in this study. Such combination of CE and SE has been observed in static DNP studies not only for the monoradicals TEMPOL and trityl44,45 but also for the biradical TOTAPOL.46 Inspecting closely the reported findings for TOTAPOL,46 the presence of SE was only significantly noticed at temperatures