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 calle d alpha-NH3) are compared with NH3 on a 91-atom Pt cluster; we find th at the latter closely mimics the extended surface. The calculations pr edict atop site occupancy for alpha-NH3 with N-down. The H-bond betwee n alpha-NH3 and an additional N-down molecule (beta-NH3) approaching f rom the gas-phase is then compared with that of two molecules in the g as 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, sl ab calculations with 1/4 ML each of alpha- and beta-NH3 support this g eometry over symmetrically coordinated beta-NH3 and predict an adsorpt ion energy in good agreement with experiment.