Unusual Local Molecular Motions in the Solid State Detected by

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Unusual Local Molecular Motions in the Solid State Detected by Dynamic Nuclear Polarization Enhanced NMR Spectroscopy Markus M. Hoffmann, Sarah Bothe, Torsten Gutmann, and Gerd Buntkowsky J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07965 • Publication Date (Web): 26 Sep 2017 Downloaded from http://pubs.acs.org on September 30, 2017

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The Journal of Physical Chemistry

Unusual Local Molecular Motions in the Solid State Detected by Dynamic Nuclear Polarization Enhanced NMR Spectroscopy Markus M. Hoffmann*1, Sarah Bothe2, Torsten Gutmann*2, 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

ABSTRACT Polyethylene glycol (PEG) and three related surfactants were studied by Dynamic Nuclear Polarization (DNP) enhanced solid state NMR spectroscopy and differential scanning calorimetry (DSC). DNP enhanced solid state NMR surprisingly reveals the presence of local molecular motions that are normally understood to be inactive at temperatures ~ 100K. This surprising phenomenon could be explained by the experimentally necessary rapid freezing of the studied samples. Specifically, DSC shows that PEG 200 forms a glass upon freezing and that the three PEG related surfactants are at least partially in a glass state or some other thermodynamic non-equilibrium state when rapidly frozen to the temperatures of the DNP enhanced solid state NMR experiments. This effect of preserving local molar motions by rapid freezing also holds true for solutions of organic solutes in the PEG 200 solvent matrix. INTRODUCTION Dynamic Nuclear Polarization (DNP) has recently received increasing attention as a hyperpolarizing method to overcome the limited sensitivity of Nuclear Magnetic Resonance (NMR) spectroscopy as shown in some recent reviews.1-4 It has been applied to a variety of spectroscopic challenges including, for example, the detection of species on surfaces,5-9 the study of biological samples,10-14 polymeric samples such as cellulose15-16 and generally the detection of weak NMR active nuclei at natural abundance.17-22 In DNP, the sample of interest must include species that contain one or more unpaired electrons. Exposure to microwave (µW) irradiation in 1 ACS Paragon Plus Environment


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the presence of an external magnetic field causes an increase in the samples nuclear spin polarization of nearby nuclei on the expense of the electronic spin polarization. In the case of protons, the high spin polarization of these nuclei is spread via spin diffusion to other protons.23 From there, it can be transferred to other nuclei such as e.g.

13

C or

15

N via conventional cross

polarization techniques. For some samples, the electron polarization may also directly transfer to these nuclei. The principle quantum mechanics of the polarization transfer from electron to nuclei are fairly well understood, and reviews on this subject are available.3-4 Briefly, the two most commonly occurring mechanisms of polarization transfer are known as the solid effect (SE) and the cross effect (CE). The SE involves simultaneous quantum mechanically forbidden zero or double quantum transitions of electron and nuclear spin. In contrast, the CE requires two coupled electrons having a separation in Lamor frequencies that matches the nuclear Lamor frequency so that polarization can transfer through efficient mixing of energy states. Besides the SE and CE two additional mechanisms are known. Thermal mixing (TM) is similar to CE but involves many electrons and nuclei interacting strongly, which therefore requires rather low temperatures of typically < 10 K.4 Most recently, it has been shown that the Overhauser effect (OE) can also be an operative mechanism in particular in solid samples where strong hyperfine couplings are present.24-25 Significant research is ongoing to gain a deeper understanding of the polarization transfer mechanisms and to further optimize sensitivity gains.26-36 Recently, a new DNP phenomenon was discovered, where two sets of oppositely phased 13

C spectral resonances were observed for amino acid solutions and for polyethylene

glycol (PEG) and related nonionic surfactants shown and defined in Scheme 1.37-38 For simplicity, we will collectively refer to the compounds shown in Scheme 1 as “surfactants”. The two sets of oppositely phased

13

C spectral resonances were explained

by the superposition of a direct polarization transfer from electron spin to

13

C (“direct

channel”) and an indirect polarization transfer via NOE type processes between the

13

C

and the proton spin reservoir (“indirect channel”). While 1H-13C NOEs in solids are known for a long time,39-43 they are generally regarded as very ineffective in most solids compared to standard CP-MAS experiments. However, their intensities can be drastically increased with DNP. A prerequisite for NOE processes is the presence of motional fluctuations with correlation times that are smaller than the inverse resonance frequency42 for modulating the heteronuclear dipolar interactions on the time-scale of the 1H-13C zero2 ACS Paragon Plus Environment

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quantum and double-quantum transitions.41 Presently, the fast rotation of methyl groups has been nearly the sole intramolecular motion observed to cause significant 1H-13C NOE enhancements in solids.42, 44 For example, the methyl group rotation was shown to be the only source of cross-relaxation in several polymers.45-46 This aspect was exploited for semi-quantitative distance measurements45 and structure elucidation of microcrystalline proteins.46 Daube et. al observed fast DNP build-up for the methyl carbons in their studied

13

C enriched amino acid samples.37 Consequently, they postulated that the

polarization transfer of the indirect channel first occurs for the methyl carbon and then further spreads to the other carbon atoms via

13

C-13C spin diffusion. However, the

superposition of direct and indirect channel polarization transfer was also observed for PEG 200, which does not possess any methyl carbons.38 This is a surprising finding because, to the best of our knowledge, 1H-13C NOEs due to local motions other than methyl rotations were only observed in studies near room temperature. These studies include polymers above the glass-transition temperature,45 bicelles,42-43 and microcrystalline proteins.47 Besides methyl rotations, segmental motions within the polyethylene oxide repeat unit are potentially of relevance for the surfactants of Scheme 1. Such motions were studied in polyethylene oxide materials by electron spin resonance and solid state NMR (SSNMR) spectroscopy among other experimental techniques,48-51 but were found to be not faster than 10-7 s at 200 K,51 which is too slow for effective NOE-type cross relaxation.

Thus, the motivation for this experimental study is to shed further light on the molecular motions responsible for the indirect channel DNP build-up. Specifically, we inspect a) 13C DNP MAS spectra of the surfactants shown in Scheme 1 at varying DNP build-up times to compare the carbon specific DNP build-up behaviour for these surfactant samples, b) 13

C DNP MAS of 5-tert-butylisophthalic acid and cyclohexane as organic solutes with

and without methyl groups in PEG 200 as the solvent matrix to test if the superposition of direct channel and indirect channel can also be observed for these solutes, and c) complementary differential scanning calorimetry at varying cooling and heating rates to better understand the phase behaviour of the studied samples.

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a)

b)

O

HO

c)

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O n

H

HO

n

CmH2m+1

CH3 CmH2m+1

O HO

q

O

n

d) O HO

CH3

n

H3C H3C

CH3

CH3

Scheme 1. a) PEG where for PEG 200 the average molecular weight is 200 g·mol-1, b) CmEn, 3) CmEnPq, and d) Triton X-100. The subscript 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.

EXPERIMENTAL

Chemicals All chemicals were generally used as received without further purification. The polydisperse nonionic surfactants shown in Scheme 1 were generously donated from Rochester Midland Corporation (RMC). They were characterized in detail in prior work with respect to composition, average molecular weight and water content (see electronic supplementary materials of our prior work38). RMC purchased the surfactants in large quantities from industrial providers: PEG 200 (Dow Chemical Company), C10E6 (Air Products), C11E6P1 (Huntsman) and Triton X-100 (Dow Chemical Company). The radical AMUpol was purchased from SATT, Université d’AixMarseille-CNRS. 5-tert-butylisophthalic acid and cyclohexane were purchased from Sigma Aldrich with respective mass purity of 98% and 99.5%. Sample solutions were prepared by weight using an electronic balance with a precision of 0.01 mg. Dissolved air was not removed from the samples.

DNP-Enhanced Solid-State NMR Experiments The solid-state DNP NMR instrumental setup was manufactured by Bruker. It consists of an Avance III 400 MHz NMR spectrometer equipped with an AscendTM 400 sweepable DNP 4 ACS Paragon Plus Environment

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magnet and a 3.2 mm triple resonance 1H/X/Y low-temperature MAS probe, a 9.7 T gyrotron system generating microwaves (µWs) at a frequency of 263 GHz, and a low temperature MAS cooling system controlling the sample temperature to 107 K or 117 K depending on whether the µW irradiation is switched off or on, respectively. The MAS frequency was set to 8 kHz unless stated otherwise. The

13

C resonance frequency is at 100.59 MHz. Dipolar interactions with

protons were generally decoupled employing SPINAL-64.52 The

13

C MAS spectra were

referenced to the signal from a silicone plug insert of the MAS rotor that was set to 0 ppm as all other 13C resonances of the samples were observed at higher chemical shifts. 13

C MAS DNP build-up spectra were obtained using the standard saturation recovery

experiment. Initially, 4 or 8 scans were recorded, while spectral acquisition was repeated with 512 scans for some shorter build-up times for improved signal to noise. A saturation pulse train containing twenty 2.2 µs pulses with a pulse spacing of 5 ms was used to initially quench the thermal 13C magnetization. 1

H→

13

C CP MAS NMR spectra were recorded with a contact time of 2000 µs, a repetition

delay of 2 s and 256 scans. Differential Scanning Calorimetry DSC measurements were carried out with a DSC 204 Phoenix® cell and a TASC 414/3A controller equipped with a CC200 cooling controller from Netzsch. Each sample was contained in a sealed aluminum crucible and analyzed in the range of 20 to -170 °C with a heating and cooling rate of 10 K min−1 and 40 K min−1. Cooling was performed with liquid nitrogen.

RESULTS AND DISCUSSION

DNP of Surfactants To inspect the local mobility close to the series of DNP enhanced

13

C nuclei in the surfactants of Scheme 1, a

13

C MAS spectra (Figure S1 in the ESI) were acquired as a

function of the build-up time employing respective 10 mM AMUpol radical solutions. Interestingly, indirect channel resonances are clearly distinguishable in Figs. S1a and S1b as inverted signals for C11E6P1 and C10E6 already after 0.2 s build-up time. To better 5 ACS Paragon Plus Environment


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observe these indirect channel features during the initial DNP build-up, a subset of these spectra with build-up times ranging from 0.2 to 4.0 seconds were acquired with increased number of scans to improve signal-to-noise and are shown in Figure 1. It can be seen in Figures 1a and 1b that the indirect channel methyl resonance near 15 ppm is clearly discernible for both C11E6P1 and C10E6. By comparison, the indirect channel methyl resonance of Triton X-100 near 30 ppm is not as strongly observable in Figure 1c at 0.2 s. Instead, it becomes clearly visible at 0.5 s build-up time along with the ethylene oxide indirect channel resonance near 70 ppm. PEG 200 does not possess any alkyl chains and thus shows in Figure 1d only a resonance for the ethylene oxide repeat unit near 70 ppm and another resonance near 60 ppm for the two terminal carbon atoms to which the hydroxyl end groups are attached.


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Figure 1. 13C MAS DNP build-up spectra for 10 mM AMUpol in a) C11E6P1, b) C10E6, c) Triton X-100, d) PEG 200. The peak at 0 ppm is from the silicone plug insert of the rotor and was chosen as a convenient chemical shift reference.

As shown previously, the indirect channel resonances can be suppressed by employing rotor synchronized 180° 1H proton pulses during build-up time when acquiring DNP spectra.38 These

13

C MAS

13

C MAS DNP build-up spectra are shown in Figure 2 and

represent the direct channel signals only. Similar trends for the build-up of the methyl group signal can be observed in Figure 2 for the direct channel signals as described above for the indirect channel signals in Figure 1. The methyl group resonances for C11E6P1 and C10E6 are prominently present at a build-up time of 0.5 s in Figure 2a and 2b, 7 ACS Paragon Plus Environment


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respectively. In contrast, the methyl group resonance for Triton X-100 in Figure 2c is not dominant at 0.5 s and instead builds up approximately in parallel with the ethylene oxide resonance. It is interesting to observe these similar trends for the direct and indirect channel resonances build-up, as these suggest that the molecular motions responsible for the indirect channel DNP transfer are somehow correlated to the effectiveness of the direct channel DNP transfer. However, the direct channel DNP transfer via CE or SE should not require any local motions and such correlation, if indeed not coincidental, would be unexpected.

Figure 2. Direct channel 13C MAS DNP build-up spectra for 10 mM AMUpol in a) C11E6P1, b) C10E6, c) Triton X-100, d) PEG 200. The peak at 0 ppm is from the silicone plug insert of the rotor and was chosen as a convenient chemical shift reference.

Difference spectra obtained by subtracting the spectra in Figure 2 from the corresponding spectra in Figure 1 are shown in Figure S2. These represent the indirect channel signals only. Besides 8 ACS Paragon Plus Environment

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confirming the trends described above for Figure 1, Figure S2c for Triton X-100 shows that indirect channel resonances for the aromatic carbons become visible at longer build-up times, starting at 4 s. This observation is confirmed with additional corresponding spectral data for Triton X-100 at 8 s build-up time (included in Figure S2c). The slower DNP build-up of the aromatic ring 13C atoms could be because of a low mobility of the aromatic rings, for example due to intermolecular π-π interactions of the aromatic rings in Triton X-100.

The spectra in Figure S2 were deconvoluted with Lorentzian functions to obtain individual signal heights for a quantitative comparison of the initial indirect channel DNP build-up. The resulting graphs are shown in Figure 3 where the y-axis values are the signal heights divided by the corresponding number of

13

C atoms. The quaternary

13

C

atoms in Triton X-100 were excluded in the count as their signal contribution can be assumed to be negligibly small given that no protons are attached to these carbon atoms.



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Figure 3. Indirect channel 13C MAS DNP build-up signal heights per number of 13C of various spectral features from 10 mM AMUpol in a) C11E6P1, b) C10E6, c) Triton X-100, d) PEG 200: “CH3” = terminal methyl carbon, C-CH3” = methylene carbon next to terminal alkyl group, “CH2” methylene alkyl chain, “COH” terminal ethylene oxide chain carbon, and “EO” = ethylene oxide carbon. A detailed spectral assignments can be found in the supplementary material of our prior work.38

As a general trend in Figure 3, the DNP build-up is faster for carbon atoms that belong or are near to the ends of the surfactant molecule, i.e., the terminal methyl carbon and the methylene carbons next to it as well as the terminal ethylene oxide chain carbon. This observation could be explained by the increased motional freedom of the terminal functional groups. We caution that the negative heights of the C-OH carbon in Triton X100 in Figure 3c might be inflated from the deconvolution as this signal overlaps severely with the strong ethylene oxide signal (Figure S2c). In the case of C11E6P1, the C-OH signal overlapped so severely with the ethylene oxide signal that it could not be deconvoluted with sufficient confidence. For Triton X-100, we remark that the resonance for the terminal methyl

13

C atoms overlaps with the other

13

C atoms of the alkyl chain,

where the latter, however, should only negligibly contribute to the DNP build-up compared to the five methyl 13C atoms.

As pointed out in the introduction, Daube et al. suggested that for their studied

13

C

enriched amino acid samples the indirect channel polarization transfer proceeds via the fast rotating homonuclear

13

C methyl groups and then further spreads throughout the molecule by

13

C–13C spin diffusion.37 The surfactants used in this study are not

isotopically enriched, and

13

C–13C spin diffusion should be ineffective. Nevertheless,

some transitory DNP build-up via the methyl carbons is consistent with the initial DNP build-up in Figures 3a and 3b for C11E6P1 and C10E6, where the additional methyl group of the isopropoxy group in C11E6P1 might explain why the ethylene oxide indirect channel methyl resonance builds up somewhat faster in C11E6P1 compared to C10E6. In particular, it is interesting that the magnitude of the indirect channel DNP methyl group resonance build-up decreases from 0.5 s to 1 s for C11E6P1 and C10E6 in Figures 3a and 3b. This observation is consistent with a transitory DNP build-up process for the methyl carbon that begins with a fast polarization build-up via the proton spin reservoir followed by a loss of polarization to neighbouring nuclei. The magnitude of the methyl group indirect channel resonance increases again in Figure 3 from 1 s to longer build-up times. This 10 ACS Paragon Plus Environment

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would be expected if the neighbouring nuclei have no other relaxation outlets and thus begin to transfer polarization back to the methyl carbon at longer DNP build-up times. Moreover, a careful inspection of Figure 2a and 2b shows that also the magnitude of the direct channel methyl group resonance decreases from 0.5 s to 1 s for C11E6P1 and C10E6, which again is a surprising parallel behaviour of the direct and indirect channel DNP transfer.

While the initial indirect channel build-up of C11E6P1 and C10E6 may be consistent with the hypothesis of some transitory spin diffusion via the methyl group, the initial indirect channel DNP build-up behaviour of Triton X-100 and of PEG 200 strongly suggests the presence of active local motions other than methyl group rotations. Specifically, the DNP build-up of the ethylene oxide 13C atoms of Triton X-100 in Figure 3c is approximately as fast as for the alkyl

13

C atoms despite the presence of a total of five methyl groups. PEG

200 does not possess methyl

13

C carbon atoms but all

13

C atoms experience indirect

channel DNP build-up.

A comparison of the initial DNP build-up between the different surfactants provides additional interesting observations. Strictly speaking, this comparison can only be quantitatively done under consideration of the number density (molecules per volume), which is not exactly known. Nevertheless, a semi-quantitative comparison is possible. Under the assumption of same mass density of the surfactants, the number density would be approximately the same for C11E6P1, C10E6 and Triton X-100 and about twice as much for PEG 200 given that its molecular weight is about half of the other surfactants. Under these assumptions, Figure 3 and, for easier comparison, Figure S3-S5, which include error bars while they are omitted in Figure 3 to avoid clutter, indicate the following: Firstly, even though it contains five methyl groups, the Triton X-100 alkyl resonance DNP buildup is slower than for the methyl

13

C atoms (Figure S3) as well as for the alkyl chain

13

C

atoms (Figure S4) in C10E6 and C11E6P1. Evidently, the local motions of the Triton X-100 branched alkyl chain appear to be more restricted compared to the linear alkyl chains of C10E6 and C11E6P1. It is known that C3 methyl group rotation with the carbon-carbon sigma bond as the axis of rotation is increasingly impeded with the size of the groups 11 ACS Paragon Plus Environment


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attached to the neighbouring carbon.53 Therefore, it is reasonable that the close proximity of the methyl groups to each other in the Triton X-100 alkyl chain restricts the motions of the methyl groups.. Secondly, even under consideration of a two-fold higher number density for PEG 200 compared to the other surfactants, the DNP build-up of its ethylene oxide

13

C atoms is apparently the fastest despite its lack of methyl groups (Figure S4).

Evidently, the 13C atoms in PEG 200 experience local motions that are less restricted than for the ethylene oxide 13C atoms in the other surfactants.

To summarize this subsection, indirect channel resonance DNP enhancements are observed for all

13

C resonances of all studied surfactants. At least for Triton X-100 and

PEG 200 other local motions must be present besides fast methyl rotations. Beyond the scope of this report, it would be interesting to study the temperature dependence of these local molecular motions. Unfortunately, this is presently not feasible by DNP enhanced techniques, owing to the technical limitations of the DNP spectrometer. Alternatively, deuterium NMR line shape studies from deuterated analogues, if available, would be feasible. Furthermore, it might be interesting to compare the DNP enhanced spectra to conventionally measured spectra of

13

C enriched surfactants. The nontrivial synthesis of

these, however, exceeds the scope of this report.

DNP of Organic Solutes in PEG 200 We wondered if indirect channel resonances could also be observed for solutes dissolved in the surfactants. To test the idea, we prepared a solution of 0.02 molar AMUpol in PEG 200 with 0.18 molar 5-tert-butylisophthalic acid, which contains three methyl groups. The obtained

13

C

DNP NMR spectra at 95 s build-up time are shown in Figure 4, with insets displaying an expanded scale to better see the resonances from the solute in the presence of the dominant PEG 200 resonances. Besides for the methyl carbons near 30 ppm, the presence of indirect channel resonances is also indicated for the aromatic carbons. Specifically, aside from the negative spinning side band peaks from the PEG 200 solvent indicated in Figure 4 by asterisks, there are small negative peaks visible in the aromatic region most notably at 125 ppm in Figure 4b. For comparison, the corresponding 1H → 13C CP DNP MAS spectrum is shown in Figure 4c.


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Figure 4: 13C MAS NMR spectra of 0.02 molar AMUpol in PEG 200 with 0.18 molar 5-tertbutylisophthalic acid under µW irradiation with 95 s build-up time after saturation and rotor frequency of a) 6 kHz, 128 scans, b) 8 kHz, 336 scans and c) 8 kHz with cross-polarization, 256 scans. Spinning side bands referring to the PEG 200 are indicated with *.

In a similar way, cyclohexane was tested as a model compound for a solute without methyl groups. As can be seen in Figure 5b, the cyclohexane indirect channel resonance near 25 ppm is stronger in relative magnitude to the PEG 200 solvent signals in comparison to the 1H → 13C CP MAS DNP spectrum (Figure 5c). For this cyclohexane sample, it was possible to obtain a

13

C

MAS spectrum with 8.3 build-up time without µW irradiation, shown in Figure 5d, with sufficient sensitivity to observe the solute. The cyclohexane signal in the spectrum of Figure 5b is enhanced by about a factor of 30 compared to the spectrum in Figure 5d. Interestingly, it appears from Figure 5 that even at 95 s only the indirect channel resonance is observable for cyclohexane, while in comparison the spectral features of the PEG 200 solvent are dominated by the direct channel resonance. This observation is confirmed by the

13

C MAS experiment 13

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employing rotor-synchronized 180° proton pulses, which suppresses the indirect channel signals. Indeed, the resulting spectrum in Figure S6 shows no evidence for a direct channel cyclohexane signal near 25 ppm.

Figure 5: 13C NMR spectra at 8 kHz MAS of 0.02 mol·kg-1 AMUpol in PEG 200 with 0.2 molar cyclohexane under µW irradiation with a 95 s (16 scans) and b) 8 s (16 scans) build-up time after saturation, and c) with 1H → 13C cross-polarization (128 scans). For comparison 8 s (16 scans) build-up time after saturation without µW irradiation in shown in d). The cyclohexane signal near 25 ppm appears inverted and strongly enhanced in a) and b).

To further investigate the cyclohexane DNP build-up, a series of 13C MAS spectra were acquired at varying DNP build-up times up to 700 s (Figure S7). Even at 700 s build-up time, cyclohexane only displays the indirect channel signal in Figure S7. The persistance of the indirect channel cyclohexane signal in Figure S7 is thus remarkable. The cyclohexane signal heights obtained from deconvolution of the spectral data are plotted in Figure S8 as a function of build-up time. 14 ACS Paragon Plus Environment

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The associated time constants obtained from a bi-exponential fit are 0.9 ± 0.1 s and 70 ± 40 s illustrating the superposition of a fast and a slow indirect channel DNP build-up process. Given that the indirect channel DNP build-up proceeds via the proton spin reservoir the magnitude of the indirect channel cyclohexane 13C signal should build up polarization in the same way as the 1

H MAS signal. To test this hypothesis, Figure 6 shows the corresponding DNP build-up curves.

Both curves in Figure 6 display the same fast initial build-up, which thus further confirms that also the indirect channel cyclohexane

13

C signal originates from NOE type cross relaxation

processes from the proton spin reservoir.

1.0

Normalized Intensity

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0.5

13

C6H12 C MAS Indirect Channel Proton MAS 0.0 0.1

1

10

100

DNP build-up time (s) Figure 6: Normalized DNP build-up curve at 117 K for the normalized magnitude of the Indirect channel 13 C MAS cyclohexane signal height (black square) and for the normalized integrated proton MAS spectrum (red circle) from a solution of 0.02 molar AMUpol in PEG 200 with 0.2 molar cyclohexane.

In summary, also organic solutes with and without methyl functional groups can display a superposition of direct and indirect channel DNP resonances. In fact, surprisingly, for 15 ACS Paragon Plus Environment


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cyclohexane only the indirect channel is effective and the direct channel DNP resonance could not be observed up to build-up times of 700 s


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DSC Studies Given that thus far only methyl rotational motions were generally known to be potentially active in solid materials at temperatures of 117 K, the question arises why are various additional local motions active in the studied samples? One possible explanation for these observed additional local motions is related to the process of freezing of the samples. This process could be sufficiently fast to prevent a completion of the liquid to solid phase transition to result in a glass state or some other form of thermodynamically non-equilibrium state. To examine this hypothesis, DSC was conducted for the four surfactants, and the results are shown in Figure 7 for a standard cooling and heating rate (red top traces) as well as of 40 K/min (black bottom traces). The latter approximates the cooling rate the NMR sample experiences in the DNP spectrometer. Here, it is assumed that it takes less than 5 min for cooling to the experimental conditions of 110 K – 117 K. 4

6

heating

heating

cooling

4

DSC (mW/mg)

DSC (mW/mg)

cooling

2

cooling 0

2 heating 0 cooling

-2

heating -2

a) C11E6P1 100

-4

150

200

250

300

b) C10E6

100

150

Temperature (K)

200

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Temperature (K)

4

4

heating

heating cooling 2

cooling

2 heating

cooling

DSC (mW/mg)

DSC (mW/mg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 cooling

heating

cooling

heating

0 cooling

heating

-2 -2 heating

c) Triton X-100 -4 100

150

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d) PEG 200 300

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Temperature (K)

200

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300

Temperature (K)

Figure 7. DSC scans of a) C11E6P1, b) C10E6, c) Triton X-100, and d) PEG 200 at cooling and heating rates of 10 K·min-1 (red top traces) and 40 K·min-1(black bottom traces). The 10 K·min-1 DSC scans are offset by +5 mW/mg for C10E6 and +3 mW/mg for the other surfactants to avoid clutter.



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A glass transition manifests itself in a DSC scan by the presence of a step-like feature instead of an enthalpic peak.54-55 Such step-like feature is only observed for PEG 200 in the 10 K/min and 40 K/min DSC scans shown in Figure 7. However, the rather complex 40 K/min DSC scans of the other surfactants in Figure 7 are distinctly different from the 10 K/min DSC scans. It is observed in the 40 K/min scans that the DSC is positive during cooling and negative during heating for a range of temperatures (mostly low temperatures). This indicates that a cooling rate of 40 K/min is generally too fast for the surfactants to respond, leading to incomplete freezing processes. These incomplete freezing processes continue to evolve during the heating cycle, once temperature is high enough, before the melting processes set in. In the case of Triton X-100 (Figure 7c), the incomplete freezing processes that continue during the heating cycle lead to a large negative peak near 240 K. Overall, the DSC results indicate that the PEG 200 sample is a glass former and at least some of the local motional dynamics of the other surfactants are preserved when they are rapidly cooled after insertion into the ~110 K cold DNP NMR probe.

DSC scans at 10 K/min and 40 K/min were also obtained for the solutions of organic solutes in PEG 200. The absence of enthalpic peaks in the 10 K/min DSC scans in Figure S8 indicates that also the PEG 200 solutions containing the organic solutes are forming glasses. The glass transition, observable as a sigmoidal step in Figures S8c and S8d, remains near 180 K compared to neat PEG 200 (Figure 7d). The 40 K/min DSC scans in Figure S9 are again more complex for the heating cycle compared to the 10 K/min scans indicating that the rapid cooling leads in this case to incomplete glass transition processes. For 5-tert-butylisophthalic acid, a C3 rotation of not only the individual methyl groups but also a C3 rotation of the 5-tert-butyl unit as a whole is conceivable within the PEG 200 glass matrix that may explain that the aromatic carbons show indirect channel resonances in Figure 4. Such motion was observed to be active at 100 K for tertbutylaldehyde.56 As for cyclohexane, chair-chair interconversion is known to freeze-out near 185 K in neat cyclohexane.57 However, it is conceivable that due to the rapid freezing this motion or related bond twisting motions are still active for the cyclohexane in the PEG 200 matrix at 117 K explaining the strong indirect channel resonance in Figure 4. Similar reductions of the freezing point are often observed in solutions of confined systems.58-66 As a final comment, it should be noted that even motions characterized by infinite long correlation times could lead to cross polarization albeit at the low-limit NOE enhancement of 18 ACS Paragon Plus Environment

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only η = 0.15 and with slowed NOE build-up.45 This however, may still be significant because of the large polarization of the proton spin reservoir from DNP.

CONCLUSIONS

Overall, it could be demonstrated that NOE type cross polarization processes from the proton spin reservoir to

13

C were observed for all

13

C signals, albeit at varying rates, in the studied

surfactants and solutions of organic solutes in PEG 200. Additional local molecular motions beyond the generally known rapid rotation of methyl groups must be active at least for PEG 200, Triton X-100, and cyclohexane at the experimental conditions of 117 K. In particular for cyclohexane, local motions were surprisingly effective resulting in a stronger enhanced indirect channel 13C resonance compared to the DNP CP MAS spectrum. These local molecular motions may be preserved in glass forming liquids and solutions, such as PEG 200, as demonstrated by the DSC results. Even for liquid or solution samples that ordinarily do not form glasses, the rapid cooling that the sample experiences upon introduction into the DNP NMR probe may at least partially preserve local motions as it was demonstrated here for C11E6P1, C10E6 and Triton X-100. This realization should be generally of high interest not only for the research communities investigating liquid and solution structures but in particular also for those employing DNP enhanced solid state NMR techniques because these require glass forming matrices for optimum DNP polarization transfer.

*Supporting Information 13

C MAS spectra and results from DNP build-up experiments. Additional DSC data for the organic solutions. This information is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Authors* Markus M. Hoffmann E-mail: [email protected] Tel.: + (585) 395-5587 19 ACS Paragon Plus Environment


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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.

Notes and References

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Unusual Local Molecular Motions in the Solid State Detected by

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