Vibrational spectroscopic and DFT study of trimethoprim

Structural investigations by different vibrational spectroscopic methods: FTIR, FT-Raman and surface-enhanced Raman scattering (SERS) spectroscopy, as well as density functional theory (DFT) calculations were performed on trimethoprim (5-(3,4,5-trimethoxybenzyl)pyrimidine-2,4-diamine). A reliable assignment of vibrational IR, Raman and SERS bands was possible by a proper choice of model used in quantum chemical calculations. Based on SERS spectrum analysis it is shown that the molecule is adsorbed on the silver surface through the pyrimidine ring, in a perpendicular orientation. Two theoretical models were used in order to simulate the silver surface and the interaction with trimethoprim molecule, the accuracy of the models being evaluated by comparing the predicted bands position of the two complexes with the SERS result.

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► The vibrational spectra of trimethoprim were assigned based on DFT calculations.
► We studied the orientation of trimethoprim to the silver surface.
► Two theoretical models were used to simulate the silver surface.
► A correlation between SERS and Raman calculated spectra of the two models was made.