Adsorption Interaction,\nAzo Bond, and Surface-Enhanced\nRaman Spectra of Azopurine on Silver Investigated by a DFT Study

Surface-enhanced Raman scattering (SERS) spectroscopy\nhas been\nwidely employed in the study of adenine and its derivatives due to\nits ability to provide fingerprint information on interfacial adsorption\nmolecules. Adenine, as one of the most important biomolecules, dominates\nthe SERS spectra of DNA and coenzymes. In our recent work, we demonstrated\nthat adenine undergoes a photo-oxidative coupling process to yield\n(E)-1,2-di(7H-purin-6-yl) diazene (azopurine) through experiments\nand density functional theory (DFT) calculations. However, due to\nthe structural sensitivity of SERS spectroscopy and the specificity\nof azo-containing functional groups, DFT spectroscopic simulations\nare highly dependent on the functionals and parameters used. In this\nwork, we employed DFT calculations to conduct a theoretical analysis\nof the structures and spectra of azopurine and its complexes formed\nwith charged silver clusters. We investigated the effects of azopurine\nconformational isomerism, interactions with metal ions, and the choice\nof theoretical methods on the spectra and established the correlation\nbetween the isomers and Raman spectra of azopurine molecules. Additionally,\nthrough the calculations of Raman spectra for a series of molecules\ncontaining NN bonds, it has been found that the characteristic\nRaman spectral peaks of molecules with NN bonds are intimately\nrelated to the theoretical methods, bonding properties, and accuracy\nof NN bond length calculations. The solvation effect can also\nsignificantly influence the vibrational frequency shift and relative\nintensities of Raman spectral bands. Therefore, it is crucial to select\nthe appropriate theoretical approaches and comprehensively consider\nthe relevant computational conditions during theoretical calculations.