Investigating Hydrogen Bonding in Phenol Using Infrared Spectroscopy and Computational Chemistry

The hydrogen bonding of phenol can be used as an introductory model for biological systems because of its structural similarities to tyrosine, a para-substituted phenol that is an amino acid essential to the synthesis of proteins. Phenol is able to form hydrogen bonds readily in solution, which makes it a suitable model for biological interactions. This laboratory experiment studies the phenol monomer, dimer, and trimer using geometry optimization and frequency calculations. The results are validated with the infrared spectra collected over the O-H stretching region (3100-3700 cm(-1)). Solutions of varying concentrations (0.5-2.0 M) are analyzed with the expectation that the contributions from the phenol dimer and trimer will become more significant as the concentration is increased. The observed peaks in the infrared spectrum are assigned as the "free" O-H stretch at 3579 cm(-1) and "bound" O-H stretch at 3390 cm(-1). As the concentration increases, the bound O-H stretch is enhanced because of a higher population of dimers and trimers that form in solution, whereas the free O-H stretch only slightly increases in intensity. The harmonic oscillator model and the force constant equation are used, and it is revealed that the O-H bond length is inversely proportional to the observed wavenumber which is supported by the bound O-H stretch appearing at a lower frequency in the infrared spectrum.