Orientational Water Bonding on Pt(111): Beyond the Frontier Orbital Principle

For decades, our understanding of water-metalbonding hasbeen dominated by the frontier orbital principle in which globallystable water-metal interactions are ruled by HOMO interactingwith metal surfaces. Using density functional theory calculations,herein, we have revealed that the frontier orbital principle cannotbe applied to metastable water bonding on Pt(111), where the decisiverole of HOMO is replaced by HOMO-1 in terms of the greatest orbitalshifts and depopulations as the two different bonding indicators.Unlike the stable water configuration in which both HOMO-1 and HOMOprefer to overlap with metal states through & sigma;-like orbital interactions,metastable configurations exhibit delicate competition or balancebetween & sigma;-like and & pi;-like orbital interactions exertedby HOMO-1 and HOMO, respectively. These findings have significantlydeepened our understanding of orbital roles in water-metalbonding interactions and bridged the gap between theoretical understandingof electrified waters at electrochemical interfaces and water scienceon metal surfaces.