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 13 C 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 C 10 E 6 . The indirect polarization channel can be suppressed by application of a string of 1 H 180°pulses during 13 C DNP buildup. 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, are of high importance for all future applications of direct 13 C MAS DNP.

Synthesis of novel trityl-nitroxyl biradicals and their performance as polarization agents in DNP-enhanced solid-state MAS NMR spectroscopy is presented. Signal enhancements in 1H, 1H → 13C CP MAS, and 13C MAS experiments obtained with these radicals dissolved in 1,1,2,2-tetrachloroethane (TCE) solution are compared with the enhancements obtained from TCE solutions of binitroxyl radicals. The signal enhancements are correlated with the distance between the radical centers of the biradicals, as determined by theoretical structure calculations. Some of the biradical TCE solutions display direct-channel resonances in 13C MAS experiments as well as indirect channel resonances induced via the proton spin reservoir. Differential scanning calorimetry reveals that only these solutions do not form any solid crystalline phases upon rapid cooling, suggesting that molecular motions needed for polarization transfer from radicals to 13C via the proton spin reservoir remain active at the experimental low temperatures of nominal 120 K. DNP magnetic field sweep enhancement profiles for selected new biradicals are presented as well. These indicate that the DNP transfer is dominated by the cross-effect mechanism.

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 ∼100 K. 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 nonequilibrium 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.

The colligative property freezing point depression is evaluated as a means for estimating the extent of aggregation for solutions of poly(ethylene oxide) alcohol (CE) nonionic surfactant in cyclohexane. Combined with additional measurements of self-diffusion coefficients, it is shown that both unaggregated CE as well as reverse micelles are significantly present for the entire range of measured CE concentration (0.048-2.35 mol kg). A change in speciation near 0.2 mol kg is indicated by the results from both freezing point depression and self-diffusion coefficient measurements. It is shown that average reverse micelle radii and aggregation numbers obtained from the ratio of solvent and CE self-diffusion coefficients are consistent with prior reported results. However, unreasonably small radii for the reverse micelles as well as for the cyclohexane were obtained from analysis of the results by the Stokes-Einstein equation using additional measured solution viscosities. The concentration of reverse micelles and unaggregated CE was calculated from the freezing point depression results using the aggregation numbers obtained from ratio of self-diffusion coefficients. These concentrations indicate that the reverse micelles become smaller in average size and increase in number with increasing temperature without an increase in unaggregated CE.

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 C 10 E 6 . C 10 E 6 displays a superposition of "direct" and "indirect channel" resonances in 13 C MAS DNP NMR spectra. The shapes of the DNP enhancement profiles are essentially identical for the 1 H MAS, 1 H → 13 C CP MAS, and 13 C 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 1 H and of 13 C direct channel resonances reveals that the cross effect is the dominant polarization transfer mechanism for the studied sample. The magnitudes of the 13 C MAS DNP enhancement profiles for 1 H → 13 C CP MAS, direct and indirect channel signals were found to be not uniform. For 1 H → 13 C CP MAS and the indirect channel signals, this observation is related to relaxation effects of the methyl group carbon. For the 13 C MAS direct channel resonances, differences in magnitudes are discussed in terms of preferential structural orientation of the polar ethylene oxide headgroup of C 10 E 6 toward the AMUPol radical.