Complex motions of X-ray induced NH3+ radicals in Tutton salt (NH4)2Zn(SO4)2•6H2O single crystals observed by EPR and ESEEM spectroscopy

EPR and pulsed EPR electron spin echo experiments were performed for NH3+ in (NH4)Zn(SO4)(2)center dot 6H(2)O 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 N-14 at room temperature. Electronic structure analysis of NH3+ showed very large pyramidal distortion of the radical molecule involved in hydrogen bonds. Proton motion in NH3+ is isotropic at room temperature and it is faster (> 10(8) s(-1)) than the anisotropic motion of the nitrogen atom (approximate to 1 x 10(8) s(-1)). Temperature affects both the dynamics and structure of the radical. The proton motion reorientation rate falls to 8 x 10(7) s(-1) below 50K where the rigid lattice limit is reached. Anomalies observed around 160K 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 NH4+ and H2O, which participate in correlated reorientations, with the rate order of 10(6) s(-1) below 100 K. The ESE dephasing rate 1/T-M ( order of 1 x 10(6) s(-1)) is strongly temperature dependent with a resonance- type enhancement at about 100K due to NH4+ reorientations with activation energy E-a = 2.8 kJ/mol.