Abstract:
We present a cluster model study on the two most likely urea adsorption complexes at Pt(100) electrodes. A goal of the investigation is to determine whether urea or ureylene, a HNCONH biradical species. forms the most stable adsorption complex. Geometry optimisations of both urea species adsorbed at various surface sites have been carried out with the parallel PARAGAUSS program package using gradient-corrected density functionals. Scalar-relativistic vibelectron calculations as well as pseudopotential calculations have been performed. For each optimised structure, vibrational frequencies have been calculated. From energy considerations, ureylene adsorption is found in both types of calculations to be preferred over urea adsorption, consistent with the interpretation of electrochemical measurements, even though effective core potential results tend to underestimate the binding energies. This assignment is supported by the calculated CO vibrational stretching frequency of chemisorbed ureylene which is closer to the value obtained from "in situ" Fourier transform infrared spectroscopy in an electrochemical environment than the CO frequency calculated for adsorbed urea.