Heterolytic activation of H-H and H-OH bonds on Lewis acid-base site pairs at metal oxide surfaces: site structures, through-space interactions, and Brønsted-Evans-Polanyi scaling relations.

Scaling relations between activation barriers and thermodynamic properties in surface catalysis require that the binding centers that stabilize products can be accessed at transition states (TS). These requirements are not met, in general, in heterolytic H-X (X = H, OH) cleavage steps mediated by concerted interactions with Lewis acid-base pairs in oxides. Density functional theory shows that residual covalency precludes stabilization of Hδ+-Hδ- fragments at the TS at the optimal binding modes available in the product state (PS). The distances required by electron sharing at the TS are replaced by more permissive distance requirements of electrostatic interactions at the dissociated PS, leading to Zr-O distances that influence TS and PS stability to different extents. TS stability depends on the extent of H-X cleavage and on concerted interactions of Hδ+-Xδ- fragments with each Zr-O pair, but these fragments shift and rotate to optimal binding modes as covalency is lost at the PS. These geometric effects cannot be captured by additional parametrization based on fragment distances, because the covalent constraints that define TS stabilization are relaxed only after cleavage, when translations and rotations of Hδ+-Xδ- fragments become accessible under dominant Coulombic interactions. The shift from covalency to Coulombic stabilization in heterolysis causes post-TS "ledges" to emerge along the H-X cleavage coordinate; these inferred product-like regions act to decouple TS stability from the energy of the fully relaxed PS. H-H and H-OH cleavage therefore depend differently on geometry. Hδ+-Hδ- interactions involve weakly directional, nearly isotropic orbital overlap and can accommodate broader ranges of fragment separations and orientations near the TS; Hδ+-OHδ- interactions require more directional overlap with O-based p orbitals, which constrains fragment orientations and delays access to Coulombically stabilized configurations until later along the reaction coordinate. Thus, scaling relations become inaccurate and impractical when substantial stabilization occurs only through the relaxation of fragments after the bond scission TS and the only readily accessible thermodynamic descriptor is the fully relaxed PS. Although a metastable product-like configuration may correlate more directly with TS stability, such a relation is not useful because this configuration is neither uniquely defined nor directly accessible from the binding properties of stable species. Accurate descriptions of heterolytic bond activation at oxide surfaces therefore require explicit ab initio treatments of concerted interactions with M-O site pairs.