Observation of strongly forbidden solid effect dynamic nuclear polarization transitions via electron-electron double resonance detected NMR

Abstract

We present electron paramagnetic resonance experiments for which solid effect dynamic nuclear polarization transitions were observed indirectly via polarization loss on the electron. This use of indirect observation allows characterization of the dynamic nuclear polarization (DNP) process close to the electron. Frequency profiles of the electron-detected solid effect obtained using trityl radical showed intense saturation of the electron at the usual solid effect condition, which involves a single electron and nucleus. However, higher order solid effect transitions involving two, three, or four nuclei were also observed with surprising intensity, although these transitions did not lead to bulk nuclear polarization-suggesting that higher order transitions are important primarily in the transfer of polarization to nuclei nearby the electron. Similar results were obtained for the SA-BDPA radical where strong electron-nuclear couplings produced splittings in the spectrum of the indirectly observed solid effect conditions. Observation of high order solid effect transitions supports recent studies of the solid effect, and suggests that a multi-spin solid effect mechanism may play a major role in polarization transfer via DNP.

Gd(iii) and Mn(ii) complexes for dynamic nuclear polarization: small molecular chelate polarizing agents and applications with site-directed spin labeling of proteins

“…Possible multi-spin flips, where 1 H and 13 C are excited at the same time are theoretically and experimentally expected, but would occur at offsets of nuclear combination frequencies with respect to the SQ (EPR) frequency. 91, 92 We do not observe any splitting or shift of these features with respect to the 1 H SE peaks. Therefore, we attribute this effect to heteronuclear cross relaxation.…”

Three-spin solid effect and the spin diffusion barrier in amorphous solids

“…The polarization buildup curve (Fig. 4B) shows that the increments in the DNP enhancement for both conditions are marginal past T 1n~1 3 s. The results shown here have important implications to the observation of TSSE at higher fields (16), which was deemed difficult because of the unfavorable scaling of P TSSE~w À4 0I (Eq. 7).…”

“…Note that the TSSE transition probability P 18 (or P 45 ) has a field dependence of w À4 0I . Thus, it is not surprising that the TSSE enhancement is small or not easily observable at higher fields (15,16). Further discussion on the relevance of Fermi's golden rule, Rabi frequency w 1S , and relaxation effect on the DNP enhancement can be found in the literature for interested readers (10).…”

High frequency dynamic nuclear polarization: New directions for the 21st century

“…Narrow line radicals like trityl and BDPA mediate the solid effect (SE) and Overhauser effect (OE) with peaks in the Zeeman field profile at ω 0S ±ω 0I as shown in Figure 3. Although the theory of the SE has recently been reformulated in detail [32,33], it is not widely used in applications since the enhancements scale as (ω 1S /ω 0I ) 2 , and as we increase ©oi the enhancements are low. Furthermore, the cross effect yields larger enhancements.…”