Vibrational sum frequency generation (SFG) spectroscopy has been utilized to study the spatial orientation and alignment of Re(CO)3Cl(dcbpy) (dcbpy = 4,4′-dicarboxy-2,2′-bipyridine) (or ReC0A) on the (001) and (110) surfaces of rutile single-crystalline TiO2. The SFG intensity of the CO stretching modes shows an isotropic distribution on the (001) surface and an anisotropic distribution on the (110) surfaces with respect to the in-plane rotation of the crystal relative to the surface normal (or the incident laser beam plane). By combining these results with ab initio SFG simulations and with modeling of ReC0A-TiO2 cluster binding structures at the density functional theory level, we reveal that the origin of the optical anisotropy for ReC0A on the TiO2(110) surface is associated with the binding preference of ReC0A along the [-110] axis. Along this direction, the binding structure is energetically favorable, because of the formation of proper hydrogen bonding between the carboxylate group and passivating water molecules adsorbed on the TiO2(110) surface. Simulations of dimers of ReC0A molecules binding close together with full nearest-neighbor effects give a structure that reproduces the experimental SFG polar plot. The tilt angle, defined by the bpy ring angle relative to the surface normal, of the catalyst is found to be 26° for one monomer and 18° for the other, which corresponds to an aggregate at high surface coverage.
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