Reorientational Water Diffusion on Tc(0001) from First-Principles: A Contrasting Model to H₂O/Pt(111)

Water diffusion on metal surfaces plays a crucial role in various electrochemical and catalytic processes. Through density functional theory (DFT) calculations, we have revealed how the occupancy of d-shell electronic structures affects the diffusion mechanisms, orbital competitions, coherent HOMO couplings, and vibronic couplings in atop-to-atop (ATA) reorientational diffusion of water on metal surfaces. Our results indicate that H-up mediated ATA mechanism on Tc(0001) is thermodynamically favorable at low temperatures, in sharp contrast to the flat-hopping mechanism favored in H₂O/Pt(111). The HVP, HUP, and HDN mechanisms in H₂O/Tc(0001) show balanced orbital competition, whereas only the HVP mechanism in H₂O/Pt(111) exhibits this type of orbital competition. The electronic rule of orientation-independent coherent HOMO coupling modes identified on Pt(111) shows a breakdown for H₂O/Tc(0001), which exhibits orientation dependence instead; the H-down-mediated mechanisms show the increased amplitude of the coherent HOMO couplings, while the H-up-mediated mechanisms show a decreased amplitude. Furthermore, the decoupled vibronic couplings between symmetric OH stretching and HOMO states observed in H₂O/Pt(111) are unexpectedly absent in H₂O/Tc(0001), whereas the Tc(0001)-supported HUF mechanism generates decoupled vibronic couplings between asymmetric OH stretching and HOMO states. Additionally, coupled vibronic couplings between water bending and HOMO states are observed in the HDW mechanism on Tc(0001). These findings offer new insights into the bonding nature of water-metal interactions during transient diffusion and lay a solid foundation for future research on electrocatalysis with electronic descriptors.