The activation energy for water reorientation differs between IR pump-probe and NMR measurements

Molecular reorientation dynamics in liquid water are typically probed using either infrared (IR) pump-probe anisotropy experiments or the NMR spin-echo technique. While it is widely appreciated that the two yield different reorientation times based on the nature of the measurements, little attention has been paid to the implications for the corresponding activation energies. Here, the activation energies associated with reorientation of the OH bond vector in liquid water are calculated to high accuracy directly from simulations at a single temperature using a recently developed method [Z. A. Piskulich et al., J. Chem. Phys. 147, 134103 (2017)]. The results indicate that the reorientation times obtained from IR anisotropy and NMR measurements have different activation energies that, with improved accuracy, should be experimentally distinguishable. The origins of the differences in the two activation energies are examined in detail, including by a decomposition into the contributions to the activation energies due to the kinetic energy and the intermolecular interactions.