Quantum-Mechanical Arrow-Pushing: Unraveling the Electron Flow in Propane ODH on V4O10

Abstract

Arrow-pushing diagrams have historically served as foundational visual tools for representing electron movement in chemical reactions. Recent methodological advances, particularly those involving intrinsic bond orbital (IBO) analysis, allow these diagrams to be constructed rigorously from quantum mechanical (QM) calculations. Despite these advances, most applications have been limited to individual steps or small portions of reaction mechanisms. Propane oxidative dehydrogenation (ODH) to propene on vanadium oxide is a catalytically significant transformation, for which an orbital-level understanding can deepen mechanistic insight. Here, we use density functional theory combined with IBO analysis to generate a step-by-step, QM-derived arrow-pushing diagram for the entire propane ODH catalytic cycle on a V₄O₁₀ cluster as a model for vanadium oxide. Our study reveals that electronic changes are confined to a small region of the catalyst, specifically a single V = O bond and one of its bridging oxygen atoms. Furthermore, the V–O bonds in this region alternate between σ and dative character to facilitate electron flow. Each propane molecule undergoes two hydrogen removals─first by hydrogen atom transfer and then by proton transfer. Additionally, we distinguish two mechanistically different isopropyl radical-trapping events: concerted carbocation-coupled electron transfer and carbon radical transfer. This work highlights the value of QM-derived arrow-pushing diagrams as powerful tools for dissecting complex catalytic processes at the orbital level.