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.