Charge Transfer between Triphenyl Phosphine and Colloidal Silver: A SERS Study Combined with DFT Calculations

The adsorption of triphenyl phosphine (PPh 3 ) on colloidal silver has been investigated by means of surface-enhanced Raman spectroscopy (SERS). On the basis of surface selection rule, it is deduced from SERS results that PPh 3 adsorbs on silver surface via its P atom with three phenyl rings tilted with respect to the silver surface. The electron-donor effect of PPh 3 can be sensitively probed by the coadsorbed SCN - . The Raman frequency of ν CN of the adsorbed SCN - shifts to lower frequencies when PPh 3 is coadsorbed with SCN -, and the red shift of C⋮N stretching frequency is found to increase with increasing the surface coverage of PPh 3 . This could be explained in terms of the electron-donor effect of PPh 3: PPh 3 adsorbed on silver surface donates its lone pair of electrons to silver surface, and then the negative charge of silver surface transfers to the π* orbital of C⋮N bond via S−Ag bond. Consequently, the C⋮N bond is weakened, and the frequency of ν CN shifts to lower frequencies. Density functional theory (DFT) calculations further confirm the experimental results that the charge transfer is from PPh 3 to silver surface rather than reversely. The information obtained from the adsorption of PPh 3 on silver by SERS may be helpful to understand the mechanism of heterogeneous catalysis involving phosphine ligands coordinated on transition-metal surfaces.