Molecular Dynamics Simulations of Nitroxide Probes in Supercooled Water

Watkins 2141

Under carefully controlled conditions, bulk liquid water may be cooled below its melting point (273.15 K) without forming ice. However, the microscopic details of this supercooled water remain poorly understood. Recently, electron paramagnetic resonance (EPR) has been used to directly measure the rotational diffusion correlation times of four nitroxide probes in supercooled water. Results from these experiments are consistent with the existence of a liquid-liquid transition temperature, separating low and high density water, near 228 K. In order to gain a more detailed molecular description of the interactions of water with these molecules, we used molecular dynamics to simulate these probes in liquid water within the temperature range of 253 K to 353. While the simulated molecular probes exhibited much faster rotational diffusion times compared to experiment, the liquid-liquid transition temperature was found to be approximately correct, near 210 K. Similarly, a change in the Arrhenius behavior above and below the density maximum was found, consistent with experiment. Furthermore, we found that the number of hydrogen bond interactions between the probe and the water offers an explanation for the different rotational diffusion coefficients amongst the probes.