Order–disorder phase transition in crystalline di-tert-butylphosphinic acid dimer: The role of the ice rules and quantum tunneling

Heat capacities of di-tert-butylphosphinic acid (abbreviated as DTPA) crystal were measured by adiabatic calorimetry. Phase transition of a second-order type was discovered at 37.8 K. The entropy of the transition was estimated to be 2.6 J K−1 mol−1, and the magnitude was interpreted as suggesting that the transition is attributed to the order–disorder process of the hydrogen-bonded protons of a DTPA dimer and that the protons rearrange with always keeping the ice rules. It was discussed that the real state of the protons should be described on the consideration of the tunneling between the two configurations which are represented classically as each proton is located close to either of the two oxygen atoms forming a hydrogen bond.