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Article 13

Directly vs. Indirectly Enhanced C in Dynamic Nuclear Polarization Magic Angle Spinning NMR Experiments of Nonionic Surfactant Systems Markus M. Hoffmann, Sarah Bothe, Torsten Gutmann, FrankFrederik Hartmann, Michael Reggelin, and Gerd Buntkowsky J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b13087 • Publication Date (Web): 09 Jan 2017 Downloaded from http://pubs.acs.org on January 10, 2017

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The Journal of Physical Chemistry

Directly vs. Indirectly Enhanced 13C in Dynamic Nuclear Polarization Magic Angle Spinning NMR Experiments of Nonionic Surfactant Systems Markus M. Hoffmann*1, Sarah Bothe2, Torsten Gutmann*2, Frank-Frederik Hartmann,3 Michael Reggelin,3 Gerd Buntkowsky*2 1

Department of Chemistry and Biochemistry, The College at Brockport, State University of New York, Brockport, NY, 14420, USA 2

Institute of Physical Chemistry, Technical University Darmstadt, Alarich-Weiss-Straße 8, D64287 Darmstadt, Germany

3

Institute of Organic Chemistry, Technical University Darmstadt, Alarich-Weiss-Straße 4, D64287 Darmstadt, Germany

ABSTRACT A study of Dynamic Nuclear Polarization (DNP) in polyethylene glycol and related nonionic surfactants is presented. In these experiments we found the surprising result that DNP enhanced 13 C Magic Angle Spinning (MAS) spectra display two sets of resonances, one with broad and one with sharp spectral features that are 180° opposite in phase. These two sets indicate the presence of a direct polarization transfer channel as expected for 13C MAS experiments, and a second unexpected indirect polarization transfer channel. Plots of DNP enhancements as a function of applied magnetic field for the two resonances show a superposition of two DNP enhancement profiles for AMUpol in the nonionic surfactant C10E6. The indirect polarization channel can be suppressed by application of a string of 1H 180° pulses during 13C DNP build-up. The presence of direct and indirect polarization channels is observed in a total of four different nonionic surfactants and with three different radicals, showing that these concurring polarization mechanisms are of general nature. Therefore, the presented findings, including the demonstration of how the indirect polarization channel can be suppressed, is of high importance for all future applications of direct 13C MAS DNP.

INTRODUCTION Since the discovery of nuclear magnetic resonance (NMR) spectroscopy, one major challenge has been its low intrinsic sensitivity. Many efforts to overcome this issue have been made starting in the 1950s with the description of cross-relaxation,1 a phenomenon that opens a pathway for polarization transfer from stronger polarized systems (i.e. electrons or protons) to weaker polarized nuclei such as 13C. Based on these observations, cross-polarization experiments 1 ACS Paragon Plus Environment


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such as the Hartmann-Hahn experiment2 or the Pines-Waugh-Gibby experiment3 have been developed, which in principle allow signal enhancements of heteronuclei by transfer from higher polarized nuclei such as protons. Hyperpolarization techniques were also introduced to obtain stronger signal enhancements. As a prominent example, Overhauser described in 1953 an effect named dynamic nuclear polarization (DNP).4 This method involves the transfer of the three orders of magnitude higher spin polarization of paramagnetic electrons to the desired nuclei by microwave (µW) irradiation at or near the electron Larmor frequency,5-8 and enables theoretical enhancement factors that depend on ǀγe/γnǀ, where γe is the gyromagnetic ratio of the electron and γn the gyromagnetic ratio of the nucleus. With the development of gyrotron systems as high power µW sources solid state DNP NMR became applicable at high magnetic fields. This offered the possibility to provide detailed information about the molecular structure, molecular interaction and chemical environment of molecules,9-12 even from large molecules such as proteins or polymers or from surfaces.11,

13-16

Although the theory of polarization transfer

mechanisms leading to dynamic polarization is relatively well established, new experimental phenomena referring to DNP are still discovered. For example, recent experiments by Griffin and co-workers found by chance an Overhauser effect in solid-state DNP.8, 17 In this vein, the present work reports the story of discovering a surprising phenomenon in experimental DNP studies at high magnetic field employing 13C MAS direct polarization experiments.

Specifically, we were initially interested in studying nonionic surfactants including polyethylene glycol (PEG), which has been employed as an efficient “green solvent” for synthetic chemistry for many years.18-22 The specific structures of the investigated nonionic surfactants of the type CmEn, and CmEnPq as well as PEG are shown in Scheme 1.

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b)

a)

Scheme 1 a) Surfactants: 1) PEG, 2) CmEn, 3) CmEnPq, and 4) Triton X-100; b) Radicals: I) AMUpol, II), TOTAPOL, and III) 4-amino TEMPO. For the surfactants, m indicates the number of carbon atoms in the alkyl chain (C), while n and q are the repetition units of ethylene oxide (E) and propylene glycol (P), respectively.

These nonionic surfactants are manufactured industrially in large quantities as polydisperse mixtures and are thus inexpensively available. Taking into account that the cost of corresponding monodisperse compounds are several orders of magnitude larger, any largescale application of nonionic surfactants will use the polydisperse compounds. Consequently, there is a need to study these polydisperse surfactants in detail. We initially intended to use DNP to investigate intermolecular interactions between the different surfactant molecules and the stable free radical that is used as the polarization source in DNP experiments. Here, we recognized that the majority of stable radicals that have been developed and investigated as a polarization source are nitroxyl containing monoradicals, such as 4-amino TEMPO, and biradicals such as AMUpol and TOTAPOL.9,

14, 23-24

As one can see from the molecular structures shown in Scheme 1, these

three radicals contain polar functionalities and indeed are well soluble in water. Therefore, these radicals can serve as model compounds for polar solutes in a solvent matrix. While the original 3 ACS Paragon Plus Environment


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motivation of this study was to inspect the structural-chemical information of nonionic surfactants with the help of dynamic nuclear polarization enhanced (DNP) solid state NMR techniques, we noticed unusual spectral features in the 13C MAS spectrum under µW irradiation in our initial experimentation. We thus initiated a thorough study to characterize the phenomenon and present our findings in this report. Hereto, we investigate effects of a number of experimental variables including the type of surfactant, type of radical, the concentration of radical, and the magnetic field strength (magnetic field sweep). The combined observations and their implications are then discussed in terms of the observed DNP phenomenon. Finally, a strategy is presented to separate the unusual DNP phenomenon from the typical observed direct polarized 13C CP MAS signals. Overall, the presented findings are generally important for future DNP studies with 13C MAS direct polarization.

EXPERIMENTAL SEXTION Chemicals The nonionic surfactants were generously received from Rochester Midland Corporation who purchased these in large quantities from industrial providers: PEG 200 (Dow Chemical Company), C10E6 (Air Products), C11E6P1 (Huntsman) and Triton X-100 (Dow Chemical Company). General information about the nonionic surfactants is provided in Table S1 in the Supporting Information. The surfactants were used as received unless otherwise stated in the text. Since polydispersity and average molecular weight may vary from batch to batch, the exact compositions were analyzed as described in detail in the ESI, and the analysis results are shown in Tables S2-S5 and Figures S1-S3.

The radicals were used without further purification as received from the following sources: AMUpol (SATT, Universités d’Aix-Marseille-CNRS) and TOTAPOL (DyNuPol DNP NMR Chemicals), and 4-amino TEMPO from synthesis as described in the Supporting Information. In general, radical stock solutions of around 40 mmol·kg-1 were prepared by weighing the appropriate radical at room temperature and dissolved in the surfactant solvent using an EMMI4 ultrasonic bath (EMAG, Germany) for ca. 3 min. Based on a recent radical decomposition study done for micromolar radical concentrations, which is about a factor 1000 more dilute as the concentrations in this study, the effective initial concentration may have been lower, due to 4 ACS Paragon Plus Environment

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decomposition of the radicals during sonification.25 Concentrations of nominal 20 mmol·kg-1, 10 mmol·kg-1, 5 mmol·kg-1 and 2.5 mmol·kg-1 were obtained from the next higher stock solution by 1:1 dilution. For simplicity, these nominal solution concentrations are stated in this report, while the actual masses are listed in Table S6 in the Supporting Information. Unless specifically stated, dissolved oxygen was not removed from the samples.

DNP-Enhanced Solid-State NMR Experiments All solid-state DNP experiments were carried out on a Bruker Avance III 400 MHz NMR spectrometer equipped with an AscendTM 400 sweep-able DNP magnet and a 3.2 mm triple resonance 1H/X/Y low-temperature MAS probe. A µW frequency of 263 GHz was employed, which was generated by a 9.7 T Bruker gyrotron system. The 1H MAS, 13

Polarization Magic Angle Spinning), and

13

C CP MAS (Cross

C MAS NMR spectra with and without µW

irradiation were acquired with a MAS frequency of 8 kHz (unless stated otherwise). The sample temperature was nominally 107 K (without µW) and 117 K (with µW, which causes higher sample temperature for fixed maximum cooling power), and was stabilized by a Bruker BioSpin low temperature MAS cooling system. All 1H spectra were recorded at frequencies of 400.02 MHz, and all

13

C spectra at 100.59 MHz.

13

C CP MAS NMR spectra were recorded with a

contact time of 2000 µs, a repetition delay of 2 s and 256 scans resulting in a measurement time of 8 min 35 s. Dipolar interactions with protons were decoupled employing SPINAL-64.26 13

C MAS experiments were performed employing an initial standard saturation recovery

experiments (Figure S4) in which thermal

13

C magnetization was quenched at the beginning of

the experiment by a saturation pulse train containing twenty 2.2 µs pulses with a pulse spacing of 5 ms.

The spectra were acquired under SPINAL-64 decoupling26 8.3 s or 95 s after the

saturation pulse train with 256 and 16 scans, respectively. Additionally, the standard 13C saturation recovery experiment for 8 s build-up time was modified (Figure S5) to introduce a string of 180° pulses on the proton channel to suppress the build-up of 1

H-magnetization during the 13C hyperpolarization. Typically, the thermal magnetization of 13C

was quenched at the beginning of the experiment by a saturation pulse train containing twenty 2.2 µs pulses with a pulse spacing of 5 ms.

During the build-up time of the

13

C, rotor

synchronized 180° pulses of 5.15 µs were applied on the proton channel with a spacing of 2τ = 5 ACS Paragon Plus Environment


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500 ms. Then, a 90° detection pulse of 3.7 µs was applied on 13C. SPINAL-64 decoupling26 was applied during data acquisition. 256 scans were acquired. To record the proton magnetization at the above described experiment the pulse sequence shown in Figure S6 was employed. After saturation of the carbon magnetization a saturation pulse train was applied on 1H with twenty 2.58 µs pulses with a pulse spacing of 0.2 ms. Then, after a period of τ = 250 ms, rotor synchronized 180° pulses with 5.15 µs pulse lengths were applied on the 1H channel with a spacing of 2τ = 500 ms. The 180° pulses were applied during the 8s buildup time of the 13C. Finally, after a period of τ = 250 ms a 90° detection pulse was applied on 1H.

The DNP enhancement profiles were obtained by comparing the intensity of µW-on spectra recorded at different fields with the µW-off spectrum measured with the same acquisition parameters.

13

C direct polarization DNP enhancement profiles were obtained in the field range

from 9.36 to 9.43 T, corresponding to g = 2.0075 to 1.9926, respectively, which covered the full range where DNP enhancement was observed. The acquisition settings were as described above at a build-up time of 8.3 s.

In addition, Hahn-Echo experiments with high power decoupling were conducted for a sample of a 40 mmol·kg-1 AMUpol in C10E6 with µW irradiation. The delays for the Hahn-Echo were MAS rotor synchronized covering times between 0.25 – 2.00 ms. Four scans were acquired for each experiment after a DNP build-up time of 300 s.

RESULTS DNP Enhanced 13C CP MAS Spectra Figure S7 in the Supporting Information shows an exemplary DNP enhanced

13

C CP MAS

spectrum and the corresponding thermal spectrum without µW irradiation for 20 mmol·kg-1 AMUpol in C10E6. The resonances below 50 ppm are from the alkyl chain while the large peak above 50 ppm is from the ethyleneoxy repeat unit of C10E6. More detailed information on the peak assignments is provided in the Supporting Information. The ratio of the integrated peak areas obtained with and without µW, IµW /IµWoff, is used here as nominal enhancement factor, as


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it is commonly done. More careful considerations such as depolarization effects could be taken into account if accurate enhancement factors are desired, which is not the case in this study.27-30 For example, an enhancement of IµW/IµWoff ≈17 is found for the spectra in Figure S7 from 20 mmol·kg-1 AMUpol in C10E6. Table 1 includes the enhancement factors from the

13

C CP MAS

for a number of samples and additional experiments that will be presented in detail in later subsections. The largest 13C CP MAS spectral enhancements were observed for the samples with AMUpol radicals with enhancement factors of up to 72, followed by TOTAPOL and the mono radical 4-amino TEMPO with enhancement factors of about 5. The enhancement factors for direct 13C polarization are generally lower than for 13C CP MAS, while for example Griffin and coworkers found a more than two-fold larger enhancement factor for the

13

C CP MAS

experiment compared to direct 13C polarization.31

Table 1: Ratios of 13C magnetization (IµWon/IµWoff ) obtained from 13C MAS spectra measured with CP or direct polarization at 8kHz with µW on and off at a polarization field of 9.397 T. Radical Solvent Molality IµWon/IµWoff CP direct polarization after 8.3s after 95s mmol·kg-1 direct indirect direct indirect AMUpol PEG 200 20 72.5 62.9 -5.3 46.1 -1.1 Triton X-100 20 41.6 43.2 -10.0 20.0 -2.4 C11E6P1 20 19.4 12.9 -6.7 9.0 -2.7 C10E6 2.5 4.8 2.9 -2.7 2.4 -1.5 C10E6 5 14.5 4.5 -4.8 4.4 -3.4 C10E6 10 19.1 7.3 -6.3 4.4 -3.6 C10E6 20 16.9 9.0 -3.8 7.1 -1.5 C10E6 40 9.5 13.5 -1.0 9.8 -1.8 C10E6/N2-flushed 20 17.5 9.4 -3.0 8.3 -2.0 C10E6, 8kHz 25 11.9 -2.9 7.7 -1.8 C10E6, 7kHz 25 8.5 -2.6 6.7 -2.2 C10E6, 6kHz 25 6.5 -2.8 5.2 -2.3 TOTAPOL C10E6 40 4.54 4.9 -0.1 1.7 -0.5 4-Amino-TEMPO C10E6 40 5.41 4.9 -2.4 6.7 -1.1 13

C MAS Direct Polarization DNP NMR Spectra

Since

13

C CP MAS DNP does not provide information on the direct interactions between the

radical solute and the surfactant,

13

C MAS direct polarization DNP NMR experiments were

performed. These experiments resulted in unexpected spectral features as illustrated in Figure 1 for 20 mmol·kg-1 AMUpol in C10E6. Specifically, Figures 1a and 1b show the effect of µW 7 ACS Paragon Plus Environment


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irradiation on the 13C MAS spectra with 95 s build-up time after saturation, and Figures 1c and 1d after 8.3 s build-up time (see experimental). Signal enhancements are evident in each case. Interestingly, µW irradiation also results in a superposition of two spectra of opposite phase: a spectrum with broad resonances that is in phase with the corresponding spectrum obtained without µW irradiation and a spectrum with sharp resonances that is 180° out of phase with the corresponding spectrum without µW irradiation. In the following, for reasons explained below, we will refer to these broad and sharp resonance sets as “direct channel” and “indirect channel” resonances, respectively. The direct channel resonances obtained under µW irradiation are increased in intensity for 95 s build-up time in Figure 1a compared to Figure 1c for 8.3 s, in particular the ethyleneoxy resonance near 70 ppm. The linewidths of the direct channel resonances were generally observed to be narrower at 95 s build-up time compared to 8.3 s build-up time, which compared to Figure 1 can be more clearly seen in Figure S8 for 40 mmol·kg-1 AMUpol in C10E6 where the direct channel resonances dominate. Also observable in the spectra in Figure 1 is a resonance from a silicone plug insert of the MAS rotor that conveniently serves as a chemical shift reference and was set to 0 ppm. As can be seen in Figure 1, the plug insert retains the same phase regardless of µW irradiation.

To quantify the signal enhancements observed in Figure 1 the spectra were deconvoluted as shown exemplary in Figure S8 in the Supporting Information. The integration of each spectral feature was then added for the direct and indirect channel resonances, respectively, and the ratio taken with the corresponding added integration for the spectral features from the spectrum without µW irradiation. Thus, Table 1 lists enhancement factors separately for the direct and indirect channel resonances. The enhancements are generally larger for the direct channel resonances.


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a)

95s, µW on

b)

95s, µW off

c)

8.3s, µW on

d)

8.3s, µW off

200

150

100

50

0

-50

-100

ppm

Figure 1: 13C MAS NMR spectra of 20 mmol·kg-1 AMUpol in C10E6 at varying DNP build-up times after saturation (saturation recovery): a) 95 s, µW on; b) 95 s, µW off; c) 8.3 s, µW on; d) 8.3 s, µW off. Note: The spectral feature at 0 ppm is from the silicone plug insert of the MAS rotor, which conveniently serves here as a chemical shift reference.

A series of experiments were conducted to establish the experimental parameters under which the superposition of direct and indirect channel resonances are observed: (i) In Figure 2 are shown the 13C MAS NMR spectra obtained after 8.3 s DNP build-up time to illustrate the effect of changing the surfactant solvent. A superposition of direct and indirect channel resonances is also observed for AMUpol at similar concentrations in PEG, C11E6P1, as well as Triton X-100 where, however, the aromatic carbons resonating between 100 and 150 ppm appear to consist of only direct channel resonances. (ii) Next, inspection of the spectral effects of AMUpol radical concentration in C10E6 is illustrated in Figure 3. As can be seen in Figure 3, the direct channel resonances become increasingly stronger with increasing AMUpol concentration. In particular, the direct channel ethyleneoxy resonance near 70 ppm becomes the most dominant feature in Figure 3a for the highest AMUpol concentration of 40 mmol·kg-1. In contrast, the direct channel resonances, including the ethyleneoxy resoncance near 70 ppm, are hardly observable at the lowest AMUpol concentration of 2.5 mmol·kg-1. (iii) Next, Figure 4 inspects if the superposition of direct and indirect channel resonances remains observable when different radicals are used. 9 ACS Paragon Plus Environment


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We tested TOTAPOL, another bi-radical, and 4-Amino TEMPO, a mono-radical. The corresponding

13

C MAS spectra after 8.3s build-up time obtained for radical concentration of

40 mmol·kg-1 in C10E6 are shown in Figures 4a and 4b. The 13C MAS spectrum for 40 mmol·kg-1 in C10E6 is shown again in Figure 4c for easier comparison. Clearly, the superposition of direct and indirect channel resonances remains even for solutions of different radicals. The increased noise level in Figures 4a and 4b is due to lower signal enhancements.

a)

PEG 200

b)

C11E6P1

c) 200

Triton X-100

150

100

50

0

-50

-100

ppm

Figure 2: The effect of surfactants on the 13C MAS DNP NMR spectrum obtained for 8.3 s build-up time using approximately 20 mmol·kg-1 AMUpol radical in a) PEG 200, b) C11E6P1, and c) Triton X-100.


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a)

40 mmolkg-1

b)

20 mmolkg-1

c)

10 mmolkg-1

d)

5 mmolkg-1

e)

2.5 molkg-1

200

150

100

50

0

-50

-100

ppm

Figure 3: The effect of AMUpol concentration in C10E6 on the 13C MAS DNP NMR spectrum obtained for 8.3 s build-up time: a) 40 mmol·kg-1, b) 20 mmol·kg-1, c) 10 mmol·kg-1, d) 5 mmol·kg-1, and e) 2.5 mmol·kg-1.

4-AminoTEMPO a) x6

TOTAPOL

b) x4

AMUpol

c)

200

150

100

50

0

-50

-100

ppm

Figure 4: The effect of different radicals at approximately same concentration of 40 mmol·kg-1 in C10E6 on the 13C MAS DNP NMR spectrum obtained from 8.3 s saturation recovery: a) 4-Amino TEMPO, b) TOTAPOL, and c) AMUpol.



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The spectra in Figures 1-4 were all obtained from solutions without removal of dissolved oxygen. Removal of oxygen by bubbling nitrogen through the sample before filling the MAS rotor also resulted in spectra showing the superposition of direct and indirect channel resonances (not shown). A slight increase of IµWon/IµWoff was observed upon removal of oxygen (Table 1), which is consistent with works by others that showed increased DNP enhancements upon removal of dissolved oxygen.32 We tested if there are any 13C signals that experience chemical shifts wide outside the typical 13C chemical range but could not observe any signals up to 5000 ppm below and above the regular spectral offset. A blank C10E6 sample was also tested and resulted in no DNP build-up (not shown), confirming that the unusual spectral features in Figures 1-4 are indeed not from some inadvertent free radical impurity. Furthermore, we tested a 1 mmol·kg-1 solution of AMUpol in D2O and could not detect any 13C MAS signal from the radical (not shown). The effect of MAS frequency on the 13C MAS DNP NMR spectrum was also tested for a 20 mmol·kg-1 solution of AMUpol in C10E6. From Table 1, it can be seen that IµWon/IµWoff for the intensities from the direct channel resonances increases when MAS frequency is increased from 6 kHz to 8 kHz, while IµWon/IµWoff for the indirect channel resonances remains between 2 and 3. The relative increase of IµWon/IµWoff for the direct channel resonances with increasing MAS frequency results in an overall increase in positive intensity for

13

C NMR

spectra as illustrated in Figure S9 in the Supporting Information. Rotor Synchronized 13C Hahn-Echo Detection To inspect if the linewidths of the direct channel resonances arise from homogenous or inhomogeneous broadening, rotor synchronized 13C Hahn-Echo experiments were conducted for 40 mmol·kg-1 AMUpol in C10E6 where the direct channel resonances are dominant. The obtained spectra after varying times between initial 90o pulse and the Hahn-Echo are shown in Figure 5. As is evident, the linewidth of the

13

C spin echo decreases with increasing length of spin echo

time. Compared to the 13C MAS DNP spectrum obtained with single pulse excitation (Figure 5a), the line width at half height of the ethyleneoxy peak near 70 ppm reduces from ~25 ppm to ~6 ppm for the spin echo obtained after only 0.25 ms (Figure 5b), indicating that the linewidth of the direct channel resonances is due to homogeneous broadening.


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a)

no spin echo

b)

0.25 ms

c)

0.5 ms

d)

1.0 ms

e)

2.0 ms

200

150

100

50

0

-50

-100

ppm 13

Figure 5: C MAS DNP spectra of 40 mmol·kg-1 AMUpol in C10E6 recorded with single pulse excitation a); Hahn-Echo detection acquired after b) 0.25 ms, c) 0.5 ms, d) 1 ms and e) 2 ms of initial 90o pulse. Note: Spectra were recoded with a build-up time of 300 s at a B0 field of 9.397 T.

DNP Enhancement Profiles To gain further insight on the nature of the observed direct and indirect channel spectral components in Figures 1-4, 13C MAS NMR spectra for build-up times of 8.3 s were recorded for a 20 mmol·kg-1 solution of AMUpol in C10E6 at varying magnetic field strengths to obtain DNP enhancement profiles. The

13

C spectra were deconvoluted to separate the direct and indirect

channel resonances. Figure 6a shows the enhancement profiles for 13C for the direct and indirect channel resonances of the ethyleneoxy signal at 8.3 s build-up time. The DNP enhancement profiles from the spectral lines of the aliphatic

13

C atoms are provided in the Supporting

Information (Figure S10) and appear to be of very similar shape as in Figure 6a. In addition, corresponding enhancement profiles from

13

C MAS DNP spectra after 50 s build-up time are

also shown in the Supporting Information in Figure S11. A comparison of Figure S10 with Figure S11 indicates no qualitative differences for different build-up times of 8.3 s versus 50 s.



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Figure 6: a) Magnetic field sweep enhancement profile of 20 mmol·kg-1 AMUpol in C10E6 for the ethyleneoxy group at 72 ppm obtained from 13C MAS NMR peak intensities at 8.3 s DNP build-up time: indirect channel resonances (dots) and direct channel resonances (cross). The black lines are present to guide the eye. The blue dashed lines indicate the field strength for which the corresponding 13C MAS DNP NMR spectra are shown in b) – d), which illustrate that under certain magnetic field strengths the presence of one of the spectral components is largely suppressed.

It can be seen in Figure 6a that the

13

C DNP enhancement profile of the direct channel

resonances contains a maximum and minimum at magnetic field strengths of 9.395 T and 9.369 T, respectively. In contrast, the enhancement profile of the indirect channel spectral component displays a maximum and minimum at 9.375 T and 9.395 T, respectively. Also, it spans a range 14 ACS Paragon Plus Environment

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from 9.370 T to 9.405 T, while for the direct channel spectral component the range is from about 9.360 T to 9.410 T. Representative 13C spectra after 8.3 s DNP build-up time are shown in Figure 6b-d at magnetic field strengths near 9.377 T, 9.395 T, and 9.368 T, respectively. Corresponding spectra are also shown for 50 s DNP build-up time in Figure S12. These spectra illustrate that under certain magnetic field strengths the presence of one of the spectral components is largely suppressed in the 13C MAS NMR spectrum.

A low-temperature ESR spectrum at a similar magnetic field range was obtained as well for a 20 mmol·kg-1 solution of AMUpol in C10E6 (Figure S13) and did not differ significantly from a prior reported ESR spectrum of a 15mM AMUpol solution in D8-glycerol/D2O/H2O33 that is also similar to ESR spectra of other biradical solutions.34-35 13

C MAS DNP NMR with Proton Pulses

During the final stages of manuscript preparation, we have become aware that the group of B. Corzilius (Goethe University Frankfurt) also independently observed a similar effect, which they attributed to a Nuclear Overhauser type process.36 This prompted us to test if protons are indeed involved in the polarization transfer process. Thus, we inserted rotor synchronized 180° pulses on the proton channel during the DNP build-up time of the

13

C MAS DNP NMR experiment

according to the pulse sequence described in the experimental section and in Figure S5. Instead of high power proton decoupling with a train of 90° pulses that would be too demanding for the rf probe and the amplifiers, these rotor-synchronized 180° pulses on the proton channel periodically invert the proton spins magnetization, which results in a net zero proton magnetization. The effectiveness of achieving net zero 1H magnetization was inspected directly by monitoring the 1H MAS spectra (not shown). The results of these experiments conducted on the sample of 20 mmol·kg-1 AMUpol in C10E6 with µW on are shown in Figure 7a. Indeed, compared to the spectrum without proton pulses (Figure 7a, black line), the direct channel resonances are removed with the application of the proton 180° pulses (Figure 7a, blue line), and they are partially reintroduced when the proton pulse length is increased to 360° (Figure 7a, red line). The fact that the direct channel resonances are only partially reintroduced is likely due to partial saturation of the proton magnetization from the train of 1H 360 pulses. Furthermore, the same series of experiments were performed at this sample with µW off (Figure 7b). Again, 15 ACS Paragon Plus Environment


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differences in the signal intensities are observed for the different experiments, which are, however, much less pronounced. Similar effects are observed with 20 mmol·kg-1 AMUpol in PEG 200 as shown in Figure S14 in the Supporting Information which confirms that the observed effect is not induced by the µW irradiation. Furthermore, as a blank sample, we conducted the same series of experiments with a neat C10E6 sample without any radical present. As can be seen in Figure S15, a small but reproducible effect is noticeable where the spectral features are increased in intensity when 180° proton pulses are applied but not when 360° proton pulses are applied. Finally, we repeated the same set of experiments for the standard DNP “juice” containing 20 mmol·kg-1 TOTAPOL in glycerol-d8/D2O/H2O (60:30:10 w/w/w). The resulting spectra in Figure S16 indicate the presence of a superposition of direct and indirect channel resonances only for the silicon plug at 0 ppm. For the CH2 / CD2 and CH / CD groups of the glycerol only the direct channel resonances are observed. A similar result was found for a solution containing glycerol-h8/ H2O (60:40 w/w, not shown).

Figure 7: 13C MAS NMR spectra of 20 mmol·kg-1 AMUpol in C10E6 obtained with 8 s DNP build-up time a) with µW and b) without µW irradiation, where spectra were recorded with additional 180° pulses (red line), 360° pulses (blue line) and without additional pulses (black line) on 1H. The spacing between the pulses on 1H was 500 ms.

DISCUSSION The immediate question that arises from the data described in the results section is about the cause of the observed superposition of broad and sharp spectral components. As both components exhibit their own distinct DNP enhancement profile (see Figure 6 and SI Figures


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S10 and S11), it is evident that they are the result of the competition of two independent polarization transfer mechanisms. The

13

C MAS DNP NMR experiments with proton 180°

pulses (Figure 7a) clearly show that protons are involved in the polarization of the sharp component, hence we call it the indirect channel. The fact, that 13C MAS NMR spectra are also affected by the addition of proton 180° pulses in the absence of µW irradiation or even any stable radical, as shown in Figure S15, strongly suggests that the indirect channel is not primarily related to DNP experiments. Instead, it is an effect inherently present in the sample’s spin system, which, however, ordinarily is very weak and thus not considered. But with the strongly increased polarization of the protons due to DNP, it becomes comparable in size to the direct polarization of the 13C and thus a major influence on the spectra. From the 180° pulse experiment it is evident that the indirect channel is related to the proton-magnetization, i.e. a heteronuclear cross-relaxation effect in the form of a nuclear Overhauser enhancement (NOE).4, 37 While NOE effects are commonly exploited in solution NMR, they are less frequently observed or applied in solid-state NMR (see e.g. refs.38-41). The latter is due to the lack of the necessary molecular motions that modulate the heteronuclear dipolar interactions on the time-scale of the 1H-13C zero-quantum and double-quantum transitions,42 i.e. with correlations times of τ c < 10 −10 s −1 . As discussed by Haw et al.39, the heteronuclear NOEs in the solid can be caused by zero-quantum and double-quantum transitions.43

They give for the enhancement factor the following

expression

η=

γS W2 − W0 , γ I 2W1 + W2 + W0

(1)

where W0 , W1 , W2 are the zero-quantum, single-quantum and double-quantum transition rates.44-45 As these rates depend on the spectral densities at the Larmor, the zero-quantum (ωI − ωS ) and double quantum (ωI + ωS ) frequencies, they are all field dependent and necessitate molecular or intramolecular motions with correlation times in this frequency range. Thus these heteronuclear NOEs are primarily observed in mobile systems like plastic crystals,39 polymers above the glasstransition temperature46, bicelles,41 or micro-crystalline proteins,47 or in systems with fast rotating methyl groups.48-49 Methyl carbons are known to provide fast relaxation through a rotational hopping mode in 13C solid state NMR.50-51 In our case, we have observed the indirect process both for surfactants containing methyl groups (C10E6) and without methyl groups (PEG 17 ACS Paragon Plus Environment


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200, see Figure S14). In the latter case it is currently not clear, which other type of motion is responsible for the modulation of the dipolar interaction. The observed negative signals in the DNP enhanced

13

C spectra show that even at 110K and 9.4 T, the indirect process is more

efficient than the direct process.

To summarize: the indirect process is a two-step process, similar to the standard CP MAS experiments, where first the proton spin reservoir is polarized by the DNP, which then serves as polarization source for the 13C-spin reservoir via cross-relaxation. In contrast to this, the broader linewidth and the strong radical concentration dependence indicate that the 13C nuclei associated with the broad spectral component are polarized directly from the electron spins via DNP. The signals of these direct channel resonances are dominated from 13C nuclei that are spatially close to the radical and experience a stronger DNP enhancement and a stronger paramagnetic broadening. This interpretation is corroborated by the Hahn-Echo experiments. With increasing spin echo delay (see Figure 5) the signal contributions of the direct channel resonances are strongly suppressed and only the more distant

13

C nuclei associated with narrower natural line

widths and longer T2 time constants are still visible. Likewise, with increasing DNP build-up time larger volumes around the radical are polarized by the direct mechanism. Spins that are further away from the radical in distance will produce narrower spectral linewidths than those that are close to the radical. This explains why the direct channel resonances show narrower linewidths at 95 s build-up time compared to 8.3 s build-up time in Figures 1 and Figure S8. It is important here to emphasize that even though the direct channel spectral components are large in intensity (area under the curve), they most probably arise from a relatively small number of highly polarized 13C nuclei in the vicinity of the radical. This is because natural abundance 13C spin-diffusion is a very inefficient process and the

13

C nuclei closer to the radical can be

expected to experience a larger polarization than those 13C nuclei further away from the radicals. As to discussing the general shape of the 13C MAS DNP enhancement profiles, four direct DNP mechanisms are generally known: the solid effect, the cross effect, the Overhauser effect, and thermal mixing. For a description of these DNP mechanisms the reader is referred to the literature.8, 52-53 The presence of the Overhauser effect can probably be discounted because one would expect a central peak of positive enhancement in the DNP build-up curve,17 which does 18 ACS Paragon Plus Environment

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not appear to be present in Figure 6. Thermal mixing is thus far not known as an efficient DNP mechanism for the temperatures and radical concentrations involved in this study,52 and thus, can probably be excluded as a relevant DNP mechanism for

13

C. DNP enhancement profiles that

result from the pure solid effect are known to exhibit distinct peaks of negative and positive enhancements for the well-resolved solid effect or a dispersion shaped enhancement profile for the differential solid effect.54 Furthermore, for the solid-effect to be dominant, the inhomogeneous breadth of the ESR spectrum must be smaller than the NMR Larmor frequency. However, the ESR spectrum in Figure S13 spans about 1.2 GHz and is a factor of three larger than the NMR Larmor frequency (400 MHz). Therefore, the solid effect does not appear to be the primary DNP mechanism in the studied samples. The cross effect is known to play an important role for the DNP in the bi-radical AMUpol, and the observed MAS frequency dependence of the DNP enhancement (Table 1, 6-8 kHz) is in support for the cross effect contributing to the DNP.55 Thus, the DNP mechanism is likely to be dominated by the cross effect.56-58

Figure 8: 13C MAS NMR spectra of 20 mmol·kg-1 AMUpol in C10E6 obtained with 8 s DNP build-up time measured with µW on: Spectrum recorded without additional pulses (black line), with additional 180° pulses (red line) on 1H representing the spectrum obtained via the direct channel, and difference spectrum of both, representing the spectrum obtained via the indirect channel (violet line). The spacing between the pulses on 1H was 500 ms.

If DNP is to be applied to extract structural chemical information, as we originally intended for the case of nonionic surfactants, it is necessary to remove the contributions of the indirect 19 ACS Paragon Plus Environment


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channel by saturating the 1H magnetization, with the string of 180° proton pulses. Calculation of the difference spectrum between the original one (Figure 8, black line) and the spectrum recorded with additional 180° proton pulses (Figure 8, red line) results in the spectrum produced by the indirect polarization channel (Figure 8, violet line). Thus, this strategy allows a clear distinction between the direct and the indirect channel resonances, which will be of high value for future applications of 13C MAS direct polarization experiments. In particular, this difference spectroscopy allows in principle a clean measurement of the enhancement profile curves of the direct and indirect channel, which is beyond the scope of the current contribution.

CONCLUSION In summary, DNP enhanced

13

C MAS NMR spectra exhibit an unusual superposition of two

spectral components with different phase, which we observed in solid solutions involving a total of four different surfactants and three different radicals. To the best of our knowledge, such observation has not been reported before for DNP enhanced

13

C MAS spectroscopy. Different

polarization transfer channels are active in the studied samples. One of these directly polarizes mainly

13

C nuclei in the vicinity of the radical. The efficiency of this mechanism grows with

radical concentration indicating that two-electron processes might be involved. By choice of polarization magnetic field strength or by use of a T2 filter, this contribution can be suppressed. The second mechanism involves a cross relaxation process of the proton Zeeman reservoir to the 13

C nuclei, which is ordinarily ineffective and weak. However, because of the increased proton

polarization through DNP this mechanism is enhanced resulting in the observed sharp spectral and oppositely phased components in the 13C spectrum. These can be removed by the addition of 180° proton pulses during the DNP build-up time resulting in pure direct polarized 13C spectra. Conversely, the indirect channel resonances, which are narrower in linewidth for increased spectral resolution, can be obtained as the difference spectrum of 13C spectra acquired with and without 1H 180° pulses. These findings will need to be considered for future NMR investigations using

13

C direct polarization experiments and are thus generally of importance for the rapidly

expanding research community employing DNP enhanced spectroscopy. In future experiments, we will determine these profiles employing the proton 180°-pulse experiment and compare their profiles to the 1H-DNP profile and the CP MAS profile.


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ASSOCIATED CONTENT *Supporting Information Details on the polydisperse nonionic surfactant including chromatographic and NMR spectral analysis on the molecular weight distribution and average; 4-amino-TEMPO synthesis; sample preparation details; 13C spectral deconvolution examples; additional 13C DNP enhancement profiles, ESR spectrum of 20 mmol·kg-1 AMUpol in C10E6, pulse programs, additional 13C and 1 H spectra illustrating effect of added 1H 180˚ pulses, 13C spectral data 20 mmol·kg-1 TOTAPOL in Glycerol-d8/D2O/H2O DNP standard juice. AUTHOR INFORMATION Corresponding Authors* Markus M. Hoffmann E-mail: [email protected] Tel.: + (585) 395-5587 Fax: + (585) 395-5805 Torsten Gutmann E-mail: [email protected] Tel.: + 49 6151 16-21122 Fax: + 49 6151 16-21119 Gerd Buntkowsky E-mail: [email protected] Tel.: + 49 6151 16-21116 Fax: + 49 6151 16-21119

NOTES The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This report is based upon work supported by the Deutsche Forschungsgemeinschaft (DFG) under grants Bu 911/20-1 (DNP spectrometer) and Bu 911/24-1. The latter included a Mercator fellowship for MMH to support a sabbatical at the Technical University Darmstadt. We thank Rochester Midland Corporation for donating the industrial polydisperse surfactants and Dr. Burkhard Endeward, Goethe University Frankfurt, for the acquisition of the 263.2 GHz ESR spectrum.



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Directly vs Indirectly Enhanced 13C in ... - ACS Publications

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