Anharmonic effects and vibrational dynamics in H-bonded liquids by attenuated total reflectance FT-IR spectroscopy

In the present work IR measurements as a function of temperature, performed, in the (2500-3700) cm(-1) region, on ethylene glycol (EC) and its homologous systems, namely ethylene glycol monomethyl ether (EGmE) and ethylene glycol dimethyl ether (EGdE), are presented and discussed. The goal is to analyze the evolution of the O-H stretching fundamental band in a wide range of temperature for systems having identical chemical structure except for the number of hydroxyl end groups, clarifying in this way the processes characterizing the vibrational dynamics of these I-I-bonded liquids. In particular, the O-H stretching band shape, through a quantitative study performed with FT-IR in the attenuated total reflectance geometry, has been connected to the different transient local environments of hydrogen-bonded molecular groups, and the role played by inter-and intramolecular interactions discussed within the framework of current theories for associated liquids. In addition, temperature-induced isosbestic points observed in the IR spectra for liquid EG and EGmE are analyzed in terms of a simple two-state model.