Applying Intermolecular Hydrogen Bonding Interactions of Phenol Derivatives to the Harmonic Oscillator Using Infrared Spectroscopy

A physical chemistry laboratory experiment was designed to connect the theory of infrared spectroscopy to prior knowledge learned in organic chemistry. The infrared spectra of phenol, p-cresol, 4-isopropylphenol, and 4-fluorophenol were studied in 2.0 M solutions of dichloromethane to analyze their ability to form hydrogen bonds. Geometry optimizations and vibrational frequency calculations of the monomer, dimer, and trimer of these systems were completed using density functional theory [(B3LYP/6-311++G(d,p)]. Computational software was used to calculate frequencies, assign the peaks in the IR spectra, and visualize their motions. Both experimental and computational evidence was used to confirm that the electron donating and withdrawing nature of these substituents resulted in varying degrees of hydrogen bonding. The highest degree of hydrogen bonding was observed for 4-isopropylphenol followed by p-cresol, phenol, and 4-fluorophenol. It was concluded that the electron donating nature of the isopropyl and methyl substituent encouraged hydrogen bonding. By the end of the lab, students were able to explain the role of hydrogen bonding and substituent effects using the harmonic oscillator model.