Elucidating the Nature of Π-Hydrogen Bonding in Liquid Water and Ammonia

Aromatic compounds form an unusual kind of hydrogen bond with water andammonia molecules, known as the π-hydrogen bond. In this work, we report abinitio path integral molecular dynamics simulations enhanced bymachine-learning potentials to study the structural, dynamical, andspectroscopic properties of solutions of benzene in liquid water and ammonia.Specifically, we model the spatial distribution functions of the solventsaround the benzene molecule, establish the π-hydrogen bonding interactionas a prominent structural motive, and set up existence criteria to distinguishthe π-hydrogen bonded configurations. These serve as a structural basis tocalculate binding affinities of the solvent molecules in πhydrogen bonds,identify an anticooperativity effect across the aromatic ring in water (but notammonia), and estimate π-hydrogen bond lifetimes in both solvents. Finally,we model hydration-shell-resolved vibrational spectra to clearly identify thevibrational signature of this structural motif in our simulations. Thesedecomposed spectra corroborate previous experimental findings for benzene inwater, offer additional insights, and further emphasize the contrast betweenπ-hydrogen bonds in water and in ammonia. Our simulations provide acomprehensive picture of the studied phenomenon and, at the same time, serve asa meaningful ab initio reference for an accurate description ofπ-hydrogen bonding using empirical force fields in more complex situations,such as the hydration of biological interfaces.