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-hydroxyphenaleno ne (IHP) and its deuterated analog (IDP) were measured at temperatures betw een 5 K and ambient temperature. A strong absorption at 68 cm(-1) in IHP, f or which the counterpart is missing in IDP, was assigned to the proton tunn eling. A heat capacity anomaly due to a phase transition occurred at 36.0 K in IDP with Delta H-trs = 167 J mol(-1) and Delta S-trs = 5.45 J K-1 mol(- 1). No phase transition occurred in IHP, proving mutual exclusion of the tu nneling and order-disorder transition The results provide firm experimental evidence for discrete tunnel states in hydrogen-bonded crystals, the key c oncept in the theory of deuterium isotope effects in ferroelectricity. (C) 2001 Elsevier Science B.V. All rights reserved.