Hydrogen-bond structure and anharmonicity in croconic acid.

First-principles molecular dynamics simulations and neutron-scattering experiments have been employed to investigate the structure and underlying vibrational motions in croconic acid as a function of temperature over the range 4-400 K. Calculated hydroxyl-bond distances were within 4% of the experimentally determined bond lengths. Temperature-dependent structures have been explored using large-scale molecular dynamics simulations. From the calculated radial distribution functions, it is found that medium-range order associated with O···H and O···O correlations are affected by an increase in temperature, yet the characteristic long-range layered structure of this material remains unaltered. Hydrogen-bond anharmonicity has been assessed from the molecular dynamics simulations, showing a red shift of ca. 50 cm(-1) of the O-H stretch frequency relative to quasi-harmonic results. This shift shows the importance of anharmonic corrections on hydrogen bonds in solid croconic acid.