Quantum Tunneling Facilitates Water Motion across the Surface of Phenanthrene

Quantum tunnelingis a fundamental phenomenon that plays a pivotalrole in the motion and interaction of atoms and molecules. In particular,its influence in the interaction between water molecules and carbonsurfaces can have significant implications for a multitude of fieldsranging from atmospheric chemistry to separation technologies. Here,we unveil at the molecular level the complex motion dynamics of asingle water molecule on the planar surface of the polycyclic aromatichydrocarbon phenanthrene, which was used as a small-scale carbon surface-likemodel. In this system, the water molecule interacts with the substratethrough weak O-H & BULL;& BULL;& BULL;& pi; hydrogen bonds, inwhich phenanthrene acts as the hydrogen-bond acceptor via the highelectron density of its aromatic cloud. The rotational spectrum, whichwas recorded using chirped-pulse Fourier transform microwave spectroscopy,exhibits characteristic line splittings as dynamical features. Thenature of the internal dynamics was elucidated in great detail withthe investigation of the isotope-substitution effect on the line splittingsin the rotational spectra of the H-2 O-18, D2O, and HDO isotopologues of the phenanthrene-H2O complex. The spectral analysis revealed a complex internaldynamic showing a concerted tunneling motion of water involving itsinternal rotation and its translation between the two equivalent peripheralrings of phenanthrene. This high-resolution spectroscopy study presentsthe observation of a tunneling motion exhibited by the water monomerwhen interacting with a planar carbon surface with an unprecedentedlevel of detail. This can serve as a small-scale analogue for watermotions on large aromatic surfaces, i.e., large polycyclic aromatichydrocarbons and graphene.