A Surface-Enhanced Raman and ab Initio Study of Spectra of Lumazine Molecules

Abstract

Surface-enhanced Raman spectroscopy (SERS) at a silver electrode was used to obtain the Raman spectrum of lumazine free of interference from fluorescence. The study was carried out varying the experimental conditions of voltage and pH. SERS spectra were compared with Fourier transform infrared (FTIR), coherent anti-Stokes Raman spectroscopy (CARS), normal Raman spectroscopy (NRS), and ultraviolet resonance Raman spectroscopy (UV RRS) spectra. Ab initio calculations at the Hartree−Fock/6-31G(d) level were made of the vibrational spectra of lumazine [2,4(3H,8H)-pterdinedione, LUM] and dimethyllumazine [6,7-dimethyl-,2,4(3H,8H)-pterdinedione, DML], and these were compared with the experimental results. Band assignments were made for LUM and DML. In addition, to determine the structure of the molecules on the metal surface as well as to examine the effects of the surface on the observed spectrum, we compared the results with the FTIR spectrum of a known metal complex of LUM. We also analyzed band shifts and the intensity ratio of in-plane and out-of-plane modes as a function of potential. This analysis indicates that the LUM molecules are tilted at an angle of ca. 45° (0.0 to −0.3 V) to the surface interacting as dimers with silver (Ag) sites through the lone pairs on the 1N or 5N atoms of the LUM ring. The tilt angle moves to ca. 50° as the potential is made more negative (−0.6 to −0.7 V).