Conformation of bis-nitroxide polarizing agents by multi-frequency EPR spectroscopy

Abstract

Multi-frequency EPR is successfully implemented to determine the conformation of bis-nitroxide radicals in a frozen solvent matrix. This structural information is highly relevant for the future design and synthesis of polarizing agents for cross-effect DNP/MAS NMR.

De novo prediction of cross-effect efficiency for magic angle spinning dynamic nuclear polarization

“…It clearly confirms the presence of an exchange interaction of ~15 MHz and a dipolar coupling of ~35 MHz for AMUPol, as initially reported, 23,30 although it was a matter of debate 78 until recently. 68 For TEKPol, we estimate the dipolar interaction to 30 MHz while the J exchange interaction is negligible in the bTbK family.…”

“…These values as well as the relative orientations for AMUPol obtained here are in very good agreement with a recent multi-frequency EPR analysis. 68 The experimental data were fitted using a step-by-step simplex approach, fitting first the gvalues, then the exchange interaction and finally the relative orientations.…”

“…This allows finding a satisfying solution using only one EPR frequency, whereas multi-frequency EPR fitting is usually required. [68][69][70] We also check the robustness of the fit by varying each parameter around its optimum value. This was used to estimate the associated errors for each of the values shown in Table 1.…”

ELDOR-detected NMR beyond hyperfine couplings: a case study with Cu(ii)-porphyrin dimers

“…The emergence of anti-holes in exchange-coupled systems with rather large dipolar couplings also implies that ELDOR-detected NMR or related sequences with additional pulses might be useful to study biradicals with moderately strong dipole-dipole and exchange couplings that are commonly employed in dynamic nuclear polarization (DNP) experiments and are difficult to characterize only via their CW EPR spectra. 54…”

Pulse Shaped Dynamic Nuclear Polarization Under Magic Angle Spinning

“…It has been reported in the literature that e cient CE-DNP is obtained using a tethered radical which has strong e e coupling, in particular, exchange (J) coupling because of its isotropic component. [42][43][44][45][46] However, owing to di↵erent radical conformers (e.g. rotamers), a wide variation in the relative g-tensors and the J-coupling between the two e spins is observed in the widely used biradical PAs.…”