Vibrational Solvatochromism Study of the C-H···O Improper Hydrogen Bond

The improper C-H···O hydrogen bond is an important weak interaction, with broad implications for protein and nucleic acid structure, molecular recognition, enzyme catalysis, and drug interaction. Despite its wide identification in crystal structures, the general existence of C-H···O hydrogen bonds remains elusive especially for natural C-H groups in bulk aqueous solutions at room temperature. Vibrational spectroscopy is a promising methodology to tackle this challenge, as formation of C-H···O hydrogen bonds usually causes shifts of the C-H stretch frequency. Yet, prior observations are inconclusive, as they are all based on a simple blue-shift in aqueous solution and cannot distinguish if it is an effect caused by solvent reorganization or a specific hydrogen-bonding interaction. In this work, we used vibrational solvatochromism as a calibration of the solvent reorganization effect and identified a specific H-bonding interaction. We performed vibrational solvatochromism study of C-H(D) of multiple alcohol molecules including the CH mode of CD₃CH(OH)CD₃ and the CD₃ modes of CD₃OH, CD₃CH₂OH, and CD₃CH(OH)CD₃ in a series of solvents. We found an abnormal blue-shift of the Raman frequency of the C-H and C-D bonds at both the C_α and C_β positions of alcohols in water, which lies in an opposite direction to the expected trend due to vibrational solvatochromism. This experimental evidence supports that the improper C-H···O hydrogen bonds might generally exist between nonpolarized C-H and water in liquid solutions at room temperature.