IR, Raman and SERS spectral analysis and DFT calculations on the Herbicide O,S-Dimethyl phosphoramidothioate, metamidophos.

Infrared, Raman and SERS spectra of O,S-Dimethyl phosphoramidothioate, metamidophos, MAP, have been recorded. Density Functional Theory, DFT, with the B3LYP functional was used for the optimization of the ground state geometry and simulation of the infrared and Raman spectra of this molecule. Calculated geometrical parameters fit very well with the experimental ones. Combining the recorded data, the DFT results and a Normal Coordinate Analysis based on a scaled quantum mechanical (SQM) force field approach, a complete vibrational assignment was made for the first time. The comparison of SERS spectra obtained by using colloidal silver nanoparticles, with the corresponding Raman spectrum reveals enhancement and shifts in bands as well as information about the orientation of MAP on the nm-sized metal structures and the importance of the S atom on the SERS effect. DFT modelling of the SERS effect and Molecular Electrostatic Potentials (MEP) confirms the experimental information.