High field dynamic nuclear polarization at 6.7 T: Carbon-13 polarization above 70% within 20 min

In most applications of dissolution-DNP, the polarization of nuclei with low gyromagnetic ratios such as C is enhanced directly by irradiating the ESR transitions of radicals with narrow ESR lines such as Trityl at low temperatures T = 1.2 K in polarizing fields B0 6 5 T. In a field B0 = 6.7 T at T = 1.2 K, DNP with TEMPO leads to a rapid build-up of proton polarization P(H) = 91% with sDNP(H) = 150 s. CP at low temperatures yields a polarization P(H? C) in excess of 70% within 20 min. After rapid dissolution to room temperature, this is 122000 times larger than the Boltzmann polarization at 300 K and 6.7 T. 2012 Elsevier B.V. All rights reserved. For a variety of reasons, dynamic nuclear polarization (DNP)[1], when it is used to boost the polarization P(S) of nuclei S with low gyromagnetic ratios cS prior to rapid heating to room temperature (so-called ‘dissolution DNP’ [2]), is usually performed in fairly low magnetic fields, most frequently B0 = 3.35 T. Furthermore, DNP is usually carried out at temperatures in the vicinity of T = 1.2 K. Under such conditions, the electron spin polarization is close to unity (Pe = 95%). By irradiating the EPR transitions with microwaves, a significant fraction of this polarization can be transferred to the polarization PS of nuclear spins S such as C, which is defined as PS 1⁄4 na nb na þ nb Experimental reports on low temperature dissolution DNP at fields above 3.35 T, i.e., at B0 = 4.6 T using Trityl and at B0 = 5 T using the widely available free radical 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) [3,4] show substantial improvements in polarization levels achieved by direct DNP, i.e., P(C) = 35% at 4.6 T and P(C) = 15% at 5 T, albeit at the price of prohibitively long build-up times: sDNP(C) > 3000 s with Trityl at 4.6 T and sDNP(C) > 1000 s with TEMPO at 5 T. On the other hand, the build-up times are usually much shorter for protons than for carbon-13 when TEMPO is used as polarizing agent [5–8]. We have shown recently [9] that the combination of H DNP using TEMPO with cross-polarization (CP) to transfer the enhanced magnetization from H to C allows one to achieve dramatic improvements in both polarization levels and build-up rates.[9] At 3.35 T, polarization levels as high as