Puzzle of the Intramolecular Hydrogen Bond of Dibenzoylmethane Resolved by Molecular Dynamics Simulations

The enol form of dibenzoylmethane has been the subject of many experimental and theoretical studies, yet the symmetry and the spectral response of the OHO intramolecular hydrogen bond remains mysterious due to conflicting assignments. In order to qualitatively understand the complex proton dynamics, it is necessary to probe the neighborhood of stationary points on the potential energy landscape. Here, we employ density functional theory-based molecular dynamics (DFT-MD) simulations to sample the coupling between the intermolecular proton transfer and all other molecular modes. To account for the quantum nature of the proton motion, we employ the path integral formalism within the DFT-MD simulations. Our results reveal that the hydrogen-bonded proton is delocalized between two oxygen atoms with sightly higher probability to be observed in the asymmetric than the symmetric position. The simulated infrared spectrum is found to be in a reasonably good agreement with the experimental spectrum. The computed ν_OH band is remarkably broad and centered around 2640 cm^-1. The origin of the discrepancy between the simulated and experimental intensities of the ν_OH band is discussed.