Vibrational spectral analysis of dl-valine dl-valinium and dl-methionine dl-methioninium picrates

A comparative study of infrared and Raman spectra of dl-valine dl-valinium picrate (dl-VVP) and dl-methionine dl-methioninium picrate (dl-MMP) at the room temperature in 4000–50 cm−1 range helps to determine the effect of hydrogen bonds in these crystals. The existence of the zwitterion and the protonated form in both the crystals has been observed. In dl-MMP, the methionine and the methioninium residues form a dimer through a strong hydrogen bond between them in the crystal. The characteristic bands due to gem-methyl group are observed in the case of dl-VVP. Factor group analysis has been made and the numbers of vibrational modes have been calculated. The tentative assignments of the observed bands are given. The picrate group forms the anion in both the crystals and it is unaffected by the presence of the cations.