Isotopic labeling as a tool to establish intramolecular vibrational coupling: The reaction of 2-propanol on Mo(110)

Abstract

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 γ 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.