Hyperfine coupling of the hydrogen atom in high temperature water

The hyperfine coupling constant of the hydrogen atom has been measured in pressurized liquid water up to 300 °C. The reduced constant Awater/Avacuum is 0.9939 at room temperature, and decreases to a minimum of 0.9918 at 240 °C. The reduced constant then increases at higher temperature. The g-factor is 2.002244(10) at room temperature and decreases to 2.00221(1) at 240 °C. The change in g-factor is proportional to the change in hyperfine coupling. The behavior below 110 °C is in excellent agreement with a previously proposed model in which the H atom is confined to a harmonic solvent cage, and vibrations within the cage mix “p-type” character into the wavefunction, resulting in Awater/Avacuum < 1. The harmonic model breaks down above 130 °C. We demonstrate that a classical binary collision model using approximate partial molar volume information can recover the observed minima near 240 °C.