Complex motions of X-ray induced radicals in Tutton salt (NH₄)₂Zn(SO₄)₂·6H₂O single crystals observed by EPR and ESEEM spectroscopy

Abstract

EPR and pulsed EPR electron spin echo experiments were performed for in (NH₄)Zn(SO₄)₂·;6H₂O single crystals over the temperature range 4.2–300 K. Well-resolved EPR spectra allowed determination of hyperfine splitting parameters, which are nearly isotropic (2.48 mT) for protons, but anisotropic (3.25; 2.0; 0.98 mT) for ¹⁴N at room temperature. Electronic structure analysis of showed very large pyramidal distortion of the radical molecule involved in hydrogen bonds. Proton motion in is isotropic at room temperature and it is faster (>10⁸ s⁻¹) than the anisotropic motion of the nitrogen atom (≈1 × 10⁸ s⁻¹). Temperature affects both the dynamics and structure of the radical. The proton motion reorientation rate falls to 8 × 10⁷ s⁻¹ below 50 K where the rigid lattice limit is reached. Anomalies observed around 160 K confirm ordering and stabilization of the hydrogen bonding network in the crystal below this temperature. Electron spin echo (ESE) decay display modulations and the FT-spectrum (ESEEM spectrum) show effects from molecular motions of the matrix molecules and H₂O, which participate in correlated reorientations, with the rate order of 10⁶ s⁻¹ below 100 K. The ESE dephasing rate 1/T _M (order of 1 × 10⁶ s⁻¹) is strongly temperature dependent with a resonance-type enhancement at about 100 K due to reorientations with activation energy E _a = 2.8 kJ/mol.