Hydrogen Bonds and Molecular Orientations of Supramolecular Structure between Barbituric Acid and Melamine Derivative at the Air/Water Interface Revealed by Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy

Abstract

We studied the supramolecular structure between barbituric acid (pyrimidine-2,4,6(1<i>H</i>,3<i>H</i>,5<i>H</i>)-trione, BA) and an amphiphilic melamine derivative at the air/water interface by heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy. HD-VSFG measurements <i>in situ</i> showed a positive broad band from 2300 to 2950 cm^-1. By comparing the experimental results with <i>ab initio</i> molecular dynamics (AIMD) simulations, we assigned the broad band to the NH stretching modes of BA strongly hydrogen-bonded to the melamine derivative. In addition, we report <i>in situ</i> HD-VSFG spectra of the interfacial supramolecular structure in the CO stretching region. Two CO stretching bands were identified. On the basis of the signs of the C=O bands, we uniquely determined the orientation of BA. The strong hydrogen bonds and the molecular orientations are direct evidence for the supramolecular structure based on complementary hydrogen bonds at the air/water interface.