Theoretical Study on the Phase Transition and the H/D Isotope Effect of Squaric Acid

Abstract

We investigated the phase transition and the isotope effect in squaric acid (H2C4O4, abbreviated H2SQ), a hydrogen-bonded dielectric material. Using first-principles calculation, we found that Jahn-Teller distortion of the unit structure (C4H4O4) was the major driving force for the phase transition in the H2SQ crystal. In order to elucidate the isotope effect on the phase transition in deuterated squaric acid (D2SQ), we employed the multi-component molecular orbital (MC_MO) method, which directly takes into account the quantum effects of protons and deuterons. Using this model, we successfully predicted the difference between the phase transition temperature of H2SQ and that of D2SQ to be 192K, which is in reasonable agreement with the experimental value of 145 K. We found that the isotope effect in the H2SQ/D2SQ system was based more on shrinking distribution of the deuteron wave rather than that of the proton wave. When the MC_MO method was coupled with adequate cluster models, first-principles calculations were effective to determining the origin of the phase transition and the H/D isotope effect in hydrogen-bonded dielectric materials.