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

The mechanism and kinetics of aspartic acid (Asp, HOCCH(NH)CHCOH) 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-C (HOCCH(NH)CHCOH), l-Asp- d (DOCCD(ND)CDCOD), l-Asp-2,3,3- d (HOCCD(NH)CDCOH), and l-Asp-N-2,3,3- d (HOCCD(NH)CDCOH). The monolayer of Asp adsorbed on the Cu(100) surface is in a doubly deprotonated bi-aspartate form (-OCCH(NH)CHCO-). During heating, Asp decomposes on Cu(100) with kinetics consistent with a vacancy-mediated explosion mechanism. The mechanistic steps yield CO by sequential cleavage of the C3-C4 and C1-C2 bonds, and N≡CCH and H via decomposition of the remaining CH(NH)CH intermediate. Deuterium labeling has been used to demonstrate that scrambling of H(D) occurs during the decomposition to acetonitrile of the CD(NH)CD intermediate formed by decarboxylation of l-Asp-2,3,3- d and l-Asp-N-2,3,3- d.