Cross-polarisation ENDOR for spin-1 deuterium nuclei

Abstract

Efficient transfer of spin polarisation from electron to nuclear spins is emerging as a common target of several advanced spectroscopic experiments, ranging from sensitivity enhancement in nuclear magnetic resonance (NMR) and methods for the detection of single molecules based on optically detected magnetic resonance (ODMR) to hyperfine spectroscopy. Here, we examine the feasibility of electron-nuclear cross-polarisation at a modified Hartmann-Hahn condition (called eNCP) for applications in ENDOR experiments with spin-1 deuterium nuclei, which are important targets in studies of hydrogen bonds in biological systems and materials. We have investigated a two-spin model system of deuterated malonic acid radicals in a single crystal. Energy matching conditions as well as ENDOR signal intensities were determined for a spin Hamiltonian under the effect of microwave and radiofrequency irradiation. The results were compared with numerical simulations and 94-GHz ENDOR experiments. The compelling agreement between theoretical predictions and experimental results demonstrates that spin density operator formalism in conjunction with suitable approximations in regard to spin relaxation provides a satisfactory description of the polarisation transfer effect. The results establish a basis for future numerical optimizations of polarisation transfer experiments using multiple-pulse sequences or shaped pulses and for moving from model systems to real applications in disordered systems.

¹⁷O hyperfine spectroscopy to investigate water binding to organic radicals

“…ENDOR studies at this field utilized the high-field approximation for very fast ENDOR simulations, using the analytical solutions for the transition frequencies. This proved to be accurate for high-γ nuclei such as 1 H [85] and 19 F [14] but also for the low-γ nucleus 2 H [24][25][26]86] , which has a gyromagnetic ratio similar to 17 O (γ 2H /γ 17O = −1.13 [43] ). It was justified, since in all cases, the hyperfine and quadrupole interactions were small compared to the nuclear Zeeman interaction, but also importantly, the nuclear quadrupole coupling was small, compared to the hyperfine coupling.…”

“…The simulation algorithm is based on the code developed by S. Glaser and R. Zeier (TU Munich) as well as I. Bejenke and M. Bennati (MPI NAT). [86] The Mims ENDOR routine was written in collaboration with A. Kehl in our research group and is summarized in the following pseudo-code: Three different Hamiltonians are necessary: firstly, the Hamiltonian during free evolution times Ĥfree , secondly, the Hamiltonian during microwave pulses Ĥnonsel and thirdly, the Hamiltonian during the radio-frequency pulses Ĥrf . For this approach, the Hamiltonians have to be time-independent and are therefore considered in the doubly-rotating frame.…”