Direct Measurements of Electric Fields in Weak OH-π Hydrogen Bonds

Hydrogen bonds and aromatic interactions are of widespread importance in chemistry, biology, and materials science. Electrostatics play a fundamental role in these interactions, but the magnitude of the electric fields that support them has not been quantified experimentally. Phenol forms a weak hydrogen bond complex with the pi-cloud of benzene, and we used this as a model system to study the role of electric fields in weak OH center dot center dot center dot pi hydrogen bonds. The effects of complex formation on the vibrational frequency of the phenol OH or OD stretches were measured in a series of benzene-based aromatic solvents. Large shifts are observed and these can be converted into electric fields via the measured vibrational Stark effect. A comparison of the measured fields with quantum chemical calculations demonstrates that calculations performed in the gas phase are surprisingly effective at capturing the electrostatics observed in solution. The results provide quantitative measurements of the magnitude of electric fields and electrostatic binding energies in these interactions and suggest that electrostatics dominate them. The combination of vibrational Stark effect (VSE) measurements of electric fields and high-level quantum chemistry calculations is a general strategy for quantifying and characterizing the origins of intermolecular interactions.