Quantitative analysis of high field liquid state Dynamic Nuclear Polarization.

Dynamic Nuclear Polarization (DNP) in the liquid state has become the focus of attention to improve the NMR sensitivity of mass limited samples. The Overhauser model predicts a fast reduction in DNP enhancement at high magnetic fields where the Electron Larmor frequency exceeds the typical inverse correlation time of the magnetic interaction between a radical spin and proton spins of the water molecules. Recent experiments have shown that an appreciable DNP enhancement in the liquid state is possible also at magnetic fields of 3 to 9 Tesla. At present it is not clear whether the Overhauser model needs to be adapted to explain these results. In the present paper we aim to resolve this question by a combination of in situ temperature dependent NMR relaxation measurements, EPR and DNP experiments. Enhancement factors of up to -165 are obtained with microwave powers below 500 mW. We conclude that at 3.4 Tesla (95 GHz) the various measurements are consistent with each other and in quantitative agreement with Overhauser theory. Microwave heating of the sample does play an important role to reduce the correlation times and allow a substantial Overhauser DNP. The typical enhancement factors may allow new applications in microfluidic NMR.

Pushing the limit of liquid-state dynamic nuclear polarization at high field.

Dynamic nuclear polarization in the liquid state was predicted more than 50 years ago by Overhauser. Its application for NMR sensitivity enhancement has been limited because of intrinsic and experimental problems to apply this method at high magnetic fields. Here we report on 95 GHz DNP experiments using the common TEMPO radical dissolved in water. In an efficient non-radiative microwave resonator, we observe average experimental enhancement factors up to -65. The local enhancement in the center of the resonator is calculated to reach a level of -94 at the highest microwave power. At high microwave power, the DNP enhancement shows a linear increase with no tendency to saturation. The results indicate that a substantial sensitivity enhancement is possible for liquid state NMR in nL sample volumes.

Dynamics of guest molecules in PHTP inclusion compounds as probed by solid-state NMR and fluorescence spectroscopy.

Partially deuterated 1,4-distyrylbenzene () is included into the pseudohexagonal nanochannels of perhydrotriphenylene (PHTP). The overall and intramolecular mobility of is investigated over a wide temperature range by (13)C, (2)H NMR as well as fluorescence spectroscopy. Simulations of the (2)H NMR spectral shapes reveal an overall wobble motion of in the channels with an amplitude of about 4 degrees at T = 220 K and 10 degrees at T = 410 K. Above T = 320 K the wobble motion is superimposed by localized 180 degrees flips of the terminal phenyl rings with a frequency of 10(6) Hz at T = 340 K. The activation energies of both types of motions are around 40 kJ mol(-1) which imply a strong sterical hindrance by the surrounding PHTP channels. The experimental vibrational structure of the fluorescence excitation spectra of is analyzed in terms of small amplitude ring torsional motions, which provide information about the spatial constraints on by the surrounding PHTP host matrix. Combining the results from NMR and fluorescence spectroscopy as well as of time-dependent density functional calculations yields the complete potential surfaces of the phenyl ring torsions. These results, which suggest that intramolecular mobility of is only reduced but not completely suppressed by the matrix, are corroborated by MD simulations. Unrealistically high potential barriers for phenyl ring flips are obtained from MD simulations using rigid PHTP matrices which demonstrate the importance of large amplitude motions of the PHTP host lattice for the mobility of the guest molecules.