Trityl biradicals and 13C dynamic nuclear polarization
Trityl biradicals and 13C dynamic nuclear polarization
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DOI：
10.1039/c002699a
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发表时间：
2010-01-01
影响因子：
3.3
通讯作者：
Ardenkjaer-Larsen, Jan Henrik
中科院分区：
文献类型：
--
作者：
Macholl, Sven;Johannesson, Haukur;Ardenkjaer-Larsen, Jan Henrik
本研究的目的是研究在低温（约1k）下，三基双自由基是否能比三基单自由基更有效地产生低伽马核自旋的动态核极化（DNP）。在3.35 T和4.64 T的超极化条件下，合成了3种不同尺寸的新型三烷基双自由基，对其进行了表征，并将其应用于[1-C-13]丙酮酸的超极化反应。稳态DNP水平和积累时间作为自由基浓度，磁场强度和微波频率为每个双基的函数进行测量。所有研究的双基都获得了相似的最大DNP，而单基可以获得两倍的高极化。当场强从3.35 T增加到4.64 T时，双自由基和单根自由基的极化都增加了大约一倍，最大极化时的双自由基浓度比最佳单根自由基浓度低几倍，但代价是积累时间长得多。在样品中加入少量Gd3+（摩尔分数通常为100ppm）对双自由基和单自由基的DNP有相同的影响。本研究发现电子纵向弛豫时间T-1e与自由基类型和场强无关。对所有自由基，T-1e对三烷基浓度的依赖性相同。双自由基的C-13纵向弛豫时间大大缩短，这与在相同三烷基浓度下，与单根自由基相比，C-13纵向弛豫时间缩短一致。这可能是三基双自由基DNP水平较低的原因。
The objective of this study is to investigate if trityl biradicals could lead to more efficient dynamic nuclear polarization (DNP) for low gamma nuclear spins at low temperature (approximate to 1 K) than a trityl monoradical. Three novel trityl biradicals of different size are synthesized, characterized and employed for hyperpolarization of [1-C-13]pyruvic acid at 3.35 T and 4.64 T. Intramolecular electron-electron distances are obtained via dipolar couplings from electron paramagnetic resonance (EPR) spectroscopy at X-band and W-band that match well with calculated molecular structures. Steady-state DNP levels and build-up times are measured as function of radical concentration, magnetic field strength and microwave frequency for each biradical. Similar maximal DNP is obtained with all studied biradicals whereas a twice as high polarization is achievable with the monoradical. Both the biradicals and the monoradical show approximately a doubling of the polarization when increasing the field strength from 3.35 T to 4.64 T. Biradical concentrations at maximum polarization are several times lower than the optimum monoradical concentration, but the penalty is a much longer build-up time. Adding a small amount of Gd3+ to the samples (molar fraction of typically 100 ppm) has the same effect on DNP with the biradicals as with the monoradical. The electron longitudinal relaxation time T-1e is found to be independent of the radical type and the field strength in this study. The same dependence of T-1e on the trityl concentration is observed for all radicals. A considerable shortening of the C-13 longitudinal relaxation time is observed for biradicals which agrees with the shortened build-up time compared to the monoradical at the same trityl concentration. This is probably the reason for lower DNP levels with trityl biradicals.