A DENSITY FUNCTIONAL THEORETICAL STUDY OF BINDING INTERACTION AND RAMAN SPECTRA OF BENZENE ADSORBED ON PLATINUM ELECTRODES

Based on density functional calculations, we found that adsorbed benzene molecules on rough platinum electrodes probably have some upright configurations through one or two Pt-C σ- bonds binding to the rough platinum surface. Surface-enhanced Raman spectroscopy (SERS) has been a powerful tool to characterize the chemisorption and chemical reaction of adsorbates on metal and electrode surfaces. There are two kinds of enhancement mechanisms that have been discussed widely (1-3). One is electromagnetic enhancement, which originates from the surface local optical electric field effect, especially when the frequencies of incident or scattered radiations approach or locate at the region of surface plasmon resonance frequency on the rough metal surfaces. The other one is chemical enhancement, which is shown by many studies to arise from the chemical adsorption or the photo-induced charge- transfer between metal and adsorbates and will directly increase the Raman scattering cross section. On the basis of surface selection rules, the surface Raman experiment can provide the information related to the chemisorbed orientation of the molecule on metal surfaces, the flat or upright configuration. Benzene, a model molecule, has been extensively studied by numerous experiments and theoretical methods (4). A flat configuration is commonly preferred on various metal surfaces. This is seemly proved by the theoretical results that the flat configuration has the largest adsorption energy for benzene adsorbed on Pt(111) (5,6). Recently, our Raman experimental study showed that adsorbed benzene molecules on rough platinum electrodes probably have some upright configurations through one or two Pt-C σ-bonds binding to the platinum electrode surface. It is necessary to analyze theoretically the binding interaction and spectral properties of benzene adsorbed on the platinum surfaces. Our calculation is on the basis of a density functional metallic cluster model. It is worthwhile to note that we consider three types of adsorption configurations, such as benzene, phenyl, and benzyne as shown in Figure 1. The symmetry of all three geometries is kept in the C2v point group in the present work. The ground state geometries were optimized using the UB3LYP method (7), which is associated with the nonlocal exchange functional B3 and the Lee-Yang-Parr nonlocal correlation functional, and adequate to describe the interaction between the transition metal and organic molecules. For the basis set, 19 electrons in the Slater-type orbitals of each platinum atom, such as 5s 5p 5d 6s 6p of Au, are treated explicitly while electrons in the inner shells are treated with the relativistic effective core potential (LanL2DZ). The corresponding basis set for atoms in pyridine is of the three-zeta quality, 6-311G(d, p), which supplements the polarization functions to C, N and H. Calculations of vibrational frequency are performed after obtaining the optimized geometries. There is no imaginary frequency appearing in the vibrational frequency analysis for all electronic ground states. The calculated result shows that the adsorbed benzene molecules have a blueshift of the vibrational frequency of the ring breath mode (vRB) with the upright configuration. While the adsorbed benzene molecules have a redshift of the corresponding vibrational frequency in the flat configuration. In this case, the present calculation (823 cm -1 ) is in agreement with the previous studies from the electron energy loss spectroscopic experiment (830 cm -1 ) (8) and the theoretical