Rotation of Four Small Nitroxide Probes in Supercooled Bulk Water

Using a precise method of least-squares nonlinear electron paramagnetic resonance (EPR) line fitting, we have obtained experimental evidence of a decoupling of the rotational motion of four nitroxide spin probes from the viscosity of bulk water at 277 K. This decoupling is about 50 K higher than another such phenomenon observed in interstitial supercooled water of polycrystalline ice by Banerjee et al. (Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 11448−11453). Above 277 K, the activation energies of the rotation of the probes and the water viscosity are very close, while in the supercooled region, the activation energies of the probes’ rotation are greater than that of the viscosity of water. The rotational correlation times of the probes can be fit well to a power law functionality with a singular temperature. The temperature dependence of the hydrodynamic radii of the probes indicates two distinct dynamical regions that cross at 277 K.