Combined experimental and theoretical study on electronic and vibrational states of cimetidine.

Cimetidine is a drug used in the treatment of peptic ulcer, and acts as a powerful histamine H2-receptor antagonist. The study of electronic and vibrational states of cimetidine, with a combined theoretical and experimental investigation on structural and electronic properties, is of prime importance because of/since the high flexible structure and the presence of different functional groups in the molecule. The aim of this work is to use a multidisciplinary approach, involving theoretical and computational methods, photochemistry and vibrational spectroscopy, to better explain the biological and physiological effects of this drug. Quanto-mechanical (HF, DFT and semi-empirical) methods were employed to estimate the energetic stability and the vibro-electronic properties of cimetidine. In the first part of the work, the experimental vibrational spectra of cimetidine alone and after the interaction with silver particles (obtained with traditional and surface enhanced Raman spectroscopies respectively) were compared with extensive ab initio calculations. In the second part, the study of the singlet excited states and the consequent electronic spectra calculations in vacuum and in different solvents with semi-empirical and ab initio methods, were presented with experimental UV-vis absorption and fluorescence spectra in order to understand the nature of excited states and to evaluate the effects of the solvent on the electronic properties of cimetidine.