Kinetics and Mechanism of Aspartic Acid Adsorption and Its Explosive Decomposition on Cu(100)

The mechanism and kinetics of aspartic acid (Asp, HO₂CCH(NH₂)CH₂CO₂H) decomposition on Cu(100) have been studied using X-ray photoemission spectroscopy and temperature-programmed reaction spectroscopy. We investigate the Asp decomposition mechanism in detail using unlabeled d-Asp and isotopically labeled l-Asp-4-¹³C (HO₂CCH(NH₂)CH₂¹³CO₂H), l-Asp- d₇ (DO₂CCD(ND₂)CD₂CO₂D), l-Asp-2,3,3- d₃ (HO₂CCD(NH₂)CD₂CO₂H), and l-Asp-¹⁵N-2,3,3- d₃ (HO₂CCD(¹⁵NH₂)CD₂CO₂H). The monolayer of Asp adsorbed on the Cu(100) surface is in a doubly deprotonated bi-aspartate form (-O₂CCH(NH₂)CH₂CO₂-). During heating, Asp decomposes on Cu(100) with kinetics consistent with a vacancy-mediated explosion mechanism. The mechanistic steps yield CO₂ by sequential cleavage of the C3-C4 and C1-C2 bonds, and N≡CCH₃ and H₂ via decomposition of the remaining CH(NH₂)CH₂ intermediate. Deuterium labeling has been used to demonstrate that scrambling of H(D) occurs during the decomposition to acetonitrile of the CD(NH₂)CD₂ intermediate formed by decarboxylation of l-Asp-2,3,3- d₃ and l-Asp-¹⁵N-2,3,3- d₃.