Indication of tunneling state of hydrogen atom in hydrogen-bonded material 5-bromo-9-hydroxyphenalenon studied by X-ray and neutron diffractions

The dielectric material with an isolated hydrogen-bond, 5-bromo-9-hydroxyphenalenon, was studied by means of neutron and X-ray diffractions in the temperature range between 300 and 10 K. Analyses on nuclear and electron density distributions revealed a discrepancy between the two distributions of the hydrogen atom in the hydrogen-bond region, leading to a local electronic dipole moment. Both a least-square structure analysis and the density distribution analysis by the maximum entropy method confirmed that the hydrogen atom occupies two equivalent sites near the center of the hydrogen-bond through the whole temperature range, even at the lowest temperature measured, 10 K. This feature, together with previous results on calorimetric, incoherent neutron scattering and far-infrared spectrum measurements, concludes that the hydrogen atom in the hydrogen-bond is in a tunneling state over the two sites. Consequently, 5-bromo-9-hydroxyphenalenon was experimentally identified as a quantum paraelectric material.