Ab initio calculations of adsorbate hydrogen-bond strength: Ammonia on Pt(111)

Seven-layer slab results for 1/4 monolayer of NH3 on Pt(111) (so called alpha-NH3) are compared with NH3 on a 91-atom Pt cluster; we find that the latter closely mimics the extended surface. The calculations predict atop site occupancy for alpha-NH3 with N-down. The H-bond between alpha-NH3 and an additional N-down molecule (beta-NH3) approaching from the gas-phase is then compared with that of two molecules in the gas phase; we discover the H-bond on the surface is almost three times stronger and the bond length appreciably shorter. Geometry relaxation then results in a 65+/-5 degree tilt of the beta-NH3 axis. Finally, slab calculations with 1/4 ML each of alpha- and beta-NH3 support this geometry over symmetrically coordinated beta-NH3 and predict an adsorption energy in good agreement with experiment.