Spectroscopic and calorimetric studies of proton tunneling and deuteration-induced phase transition in crystalline iodo-hydroxyphenalenone and iodo-deuteroxyphenalenone

Far-infrared spectra and molar heat capacities of 5-iodo-9-hydroxyphenalenone (IHP) and its deuterated analog (IDP) were measured at temperatures between 5 K and ambient temperature. A strong absorption at 68 cm−1 in IHP, for which the counterpart is missing in IDP, was assigned to the proton tunneling. A heat capacity anomaly due to a phase transition occurred at 36.0 K in IDP with ΔtrsH=167 J mol−1 and ΔtrsS=5.45 J K−1 mol−1. No phase transition occurred in IHP, proving mutual exclusion of the tunneling and order–disorder transition. The results provide firm experimental evidence for discrete tunnel states in hydrogen-bonded crystals, the key concept in the theory of deuterium isotope effects in ferroelectricity.