Decoherence of Hyperfine Coupled ¹⁹F and ¹H Nuclei in Gadolinium(III) Model Complexes

Time-domain electron-nuclear double resonance (ENDOR) was employed to determine the nuclear decoherence rate (1/T_2n) of fluorine nuclei in four model Gd(III) chelates of varying structures, over the temperature range 3.7-10 K. These complexes are derivatives of Gd(III) spin labels used in ¹⁹F-ENDOR distance measurements, which emerged as an efficient method for measuring short-range distances in biomolecules. It was found that the ¹⁹F decoherence originates primarily from electron spin flips, following the relationship T_2n ≈ 2T_1e, where T_1e is the Gd(III) spin-lattice relaxation time. The obtained ¹⁹F decoherence rates (1-8 kHz) indicate that the intrinsic ¹⁹F-ENDOR line widths are small relative to other broadening contributions, and therefore do not significantly limit distance resolution of ¹⁹F ENDOR. Proton decoherence measurements were carried out for comparison, revealing two ¹H populations with different relaxation rates; the faster component was sensitive to the matrix protonation and attributed to dipolar flip-flops among weakly coupled protons, a mechanism absent for ¹⁹F. These results elucidate key factors affecting ENDOR resolution and provide new insights into the nuclear spin relaxation mechanism in paramagnetic systems.