Surface-enhanced Raman spectra of phthalimide. Interpretation of the SERS spectra of the surface complex formed on silver islands and colloids

The surface-enhanced Raman scattering (SERS) spectra for phthalimide (PIMH) vacuum evaporated, cast onto silver island films, and from colloidal silver are clearly identified with the formation of a phthalimide-silver complex (chemisorption). The phthalimide-silver complex (PIMAg) has been obtained from the PLMH potassium salt and AgNO3. The Raman scattering spectrum of the isolated complex is in agreement with the SERS spectra of the silver surface complex. The experiments were carried out using three laser lines at 514.5, 633, and 780 nm. The photochemical decomposition of the surface complex is detected with all laser lines. The spectral interpretation is aided using Hartree-Fock and local density functional theory (S-VWN) calculations, carried out to compute simulated SERS spectra using the molecular complex PIMAg and the PIMH. The calculated surface complex (SERS) Raman spectrum using S-VWN was found to be in good agreement with the observed spectrum. The observed Raman spectrum for the surface complex may contain the Raman-active modes allowed in the total irreducible representation of the complex symmetry point group. However, the number of observed vibrations can be reduced due to further constraints introduced by molecular and field orientations at the surface (surface selection rules or propensity rules).