A 200 GHz dynamic nuclear polarization spectrometer

Abstract

We present our experimental setup for both dynamic nuclear polarization (DNP) and electron paramagnetic resonance (EPR) detection at 7 T using a quasi-optical bridge for propagation of the 200 GHz beam and our initial results obtained at 4 K. Our quasi-optical bridge allows the polarization of the microwave beam to be changed from linear to circular. Only the handedness of circular polarization in the direction of the Larmor precession is absorbed by the electron spins, so a gain in effective microwave power of two is expected for circular vs. linear polarization. Our results show an increase in DNP signal enhancement of 28% when using circularly vs. linearly polarized radiation. We measured a maximum signal enhancement of 65 times that of thermal polarization for a (13)C labeled urea sample corresponding to 3% nuclear spin polarization. Since the time constant for nuclear spin polarization buildup during microwave irradiation is 10 times faster than the (13)C nuclear spin T(1), the actual gain in detection sensitivity with DNP is much greater.

Correlation of the EPR properties of perchlorotriphenylmethyl radicals and their efficiency as DNP polarizers

“…In the simulations the exact number of chlorine nuclei (4 for radical 1 and 3 for radical 2) of the radicals per equivalent units was considered. In the simulations both 35 Cl and 37 Cl were taken into account (natural abundance of 76% and 24% respectively) All possible orientations of the radical with respect to the external magnetic field were computed for a given position of the magnetic field.…”

“…2). An EPR spectrum of OX63 recorded at 200 GHz resolved a small axially symmetric g--anisotropy 35 . The larger anisotropy of the PTM radicals may be attributed to the spin--orbit coupling of the chlorine substituents.…”

“…Accordingly, we assign the signals observed for radicals 1 and 2 to forbidden EPR transitions associated with flips of 35,37 Cl nuclei.…”

“…The differences in width can be partially accounted for by considering the effect of EPR forbidden transitions involving 35,37 Cl flips (see below). We note that a similar calculation of the OX63 13 C DNP sweep from the OX63 EPR spectrum gave a very good agreement with the experimental DNP frequency sweep (not shown).…”

“…Next, the 13 C nuclei present in the solvent molecules of the matrix are polarized via spin diffusion from the polarized chlorine nuclei. Moreover, it was suggested that strong mixing of the nuclear states by the 35,37 Cl nuclear quadrupole interaction could increase the transition probability of the forbidden EPR transitions enhancing the efficiency of solid effect DNP. Such an assumption was supported by Density Functional Theory (DFT) calculations that predicted large quadrupole coupling constants and non--negligible hyperfine couplings for the chlorine nuclei 14 .…”