Far-infrared spectra of 2,2'-biphenol and 4,4'-biphenol measured by terahertz time-domain spectroscopy

The far-infrared spectra of 2,2'-biphenol(2BP) and 4,4'-biphenol(4BP) in the frequency range between 0.1 and 1.6 THz were measured by terahertz; time-domain spectroscopy at 295 K. We observed distinct differences between the two isomers. 2BP has an absorption band at 1.45 THz while 4BP shows no peaks through all experimental frequency range. To investigate the relationship between the THz spectra and the low-energy modes of the molecules, quantum chemical calculations with full geometry optimizations and frequency analysis of the optimized structures were performed using the Gaussian 03 program package. The structure analyses with AM1 show that the hydroxyl groups in 2BP can form intramolecular hydrogen bond. The origin of the observed absorption at 1.45 THz is assigned to the low-frequency torsions of the two-phenyl rings which contain the hydrogen bond.