DFT Study On Surface-Enhanced Raman Spectroscopy (SERS) Of Adenine

Surface-enhanced Raman spectroscopy(SERS) is one of the powerful techniques to detect good-quality Raman signals of surface species due to its high sensitivity.But at present,the mechanism of SERS is still not clear.It is widely accepted so far that there are two enhancement mechanisms:the electromagnetic enhancement and the chemical enhancement.In the thesis,we choose the adenine molecule as a probing molecule,to study the chemical enhancement effect by analysis of the stable configuration and its Raman spectra of adsorbed adenine molecules.These studied systems include isolated adenine molecule,protonated adenine molecules,and adsorbed adenine molecule on silver surfaces.Then the assignments of the vibrational frequencies of the molecules are presented.The theoretical results of the work are listed as below:(1) The stable structures and their Raman spectra of the adenine molecule itself were obtained.We further re-assigned several vibrational modes on the basis of potential energy distribution from normal mode analysis;and we presented the simulated pre-resonance Raman spectra,which depend on the excitation wavelength.(2) The DFT calculations(at the B3LYP/6-311+G^** level) and the scaled Quantum Mechanical Force field(SQMF) method are used to obtain the stable structures and to assign the fundamentals of the protonated adenine.Furthermore,we analyzed the Raman spectrum of the protonated adenine molecule in an acidic aqueous solution with pH=1.(3) To study the SERS of adenine adsorbed on silver surfaces,we calculated the adsorption configurations of adenine on different surface sites on the basis of the metallic cluster model.Then we analyzed the influence of different bonding interactions on the structures and the Raman spectra of adenine molecules.The results show that the interactions through N1,N3 and N7 positions binding to Ag clusters makes only a minor influence on the Raman spectra of adsorbed adenine.But the significant change can be observed when the N10 position binds to an Ag cluster. Comparison of the experimental and theoretical results suggests that the adenine molecule binds to the silver surface through the N7 position.We also assigned the SERS spectra of adenine adsorbed on the silver surface by combining B3LYP and SQMF methods.We further propose the existence of an adenine adsorption state with the N10 position binding to the silver to explain some special peaks observed in the TERS experiment.