Rotational dynamics of Di-tert-butyl-nitroxide in normal and supercooled water: an electron paramagnetic resonance study

In order to obtain dynamical information on the water solvent, which is characterized by a strong anomalous behavior in its structural and transport properties especially in the supercooled region, low concentration di-tert-butyl-nitroxide (DTBN) aqueous solutions were studied by Electron Paramagnetic Resonance spectroscopy in the temperature range from 28 down to ?17°C. The accurate spectra reconstruction, achieved by a multi-parameters Monte Carlo fitting algorithm, allowed us to reliably extract some relevant spectral parameters of the spin probe, which were connected to the probe dynamics in the framework of the motional narrowing magnetic relaxation theory. The observed trend with the temperature showed however a significant deviation from what expected from the magnetic relaxation model. This anomalous behavior is discussed in terms of the influence upon the probe motion of solvent-induced local fluctuating structures which, very likely, are connected to the water hydrogen bond network dynamics.