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.