Thermally induced rearrangement of hydrogen-bonded helices in solid 4-isopropylphenol as studied by calorimetric, proton NMR, dielectric and near IR spectroscopic methods

Calorimetric, dielectric and Fourier transform near infrared (IR) spectrosc opic methods were used to study molecular dynamics and structural transitio n in solid 4-isopropylphenol (4IP) above room temperature. Pulse proton nuc lear magnetic resonance (NMR) measurements were performed in the 100-340 K temperature range. A phase transition was found at 331.5 K, 1.5 K below the melting point. Energetically inequivalent methyl groups reorientations wer e observed in differently prepared samples and this suggested that a high-t emperature polymorph occurs below the transition point as a metastable phas e. Dielectric relaxation measurements showed an electric conductivity simil ar in value to that in water. This was detected as a pronounced contributio n to the imaginary part of dielectric permittivity at temperatures higher t han 310 K. Near IR spectra revealed that hydrogen bonding., are stronger in the high-temperature phase than in the room-temperature-stable one. We pro pose that thermally induced molecular rearrangements enable proton transfer in hydrogen bonds (HBs) and this stimulates protonic conduction. (C) 1999 Elsevier Science B.V. All rights reserved.