ISOTOPIC LABELING AS A TOOL TO ESTABLISH INTRAMOLECULAR VIBRATIONAL COUPLING - THE REACTION OF 2-PROPANOL ON MO(110)

The reactions of 2-propanol on Mo(110) were investigated using temperature programmed reaction, high resolution electron energy loss, and x-ray photoelectron spectroscopies. 2-Propanol forms 2-propoxide upon adsorption at 120 K on Mo(110). The 2-propoxide intermediate deoxygenates via selective gamma C-H bond scission to eliminate propene as well as C-O bond hydrogenolysis to form trace amounts of propane. The C-O bond of 2-propoxide is estimated to be nearly perpendicular to the surface. Selective isotopic labeling was used to establish the coupling between the C-O stretch and modes associated with the hydrocarbon framework. The degree of coupling was strongly affected by bonding to the surface, primarily due to weakening of the C-O bond when 2-propoxide is bound to Mo(110). Selective isotopic labeling was, therefore, essential in making vibrational assignments and in identifying key reaction steps. Only a small kinetic isotope effect was observed during reaction of (CD3) (CH3) CHOH, consistent with a substantial component of C-O bond breaking in the transition state for propene elimination. Coupling of the C-O stretch to motion of the methyl group is also suggested to be important in the transition state for propene elimination.