Vibrational spectra and molecular structure of sulfanilic acid: IR and low temperature Raman studies and DFT investigation of monomeric and dimeric forms

Room temperature IR and Raman spectra, low temperature Raman spectra and UV–vis spectrum of sulfanilic acid (SA) were measured. Molecular structures of monomeric SA in the OH and zwitter-ion (ZI) forms and dimeric SA were optimized at the b3lyp/6-311++g** level using the Gaussian 09 software. Computations of APT charges, IR, Raman and UV–vis spectra and related quantities were also carried out. MEP, ESP plots and pictorial representations of the HOMO-LUMO were drawn using the optimized structures of the different forms of the SA molecule. Barrier heights were calculated for different tops using one dimensional potential energy scan versus dihedral angle at the same level of theory. The IR, Raman and UV–vis spectra were interpreted in light of the computed quantities. Thermodynamic functions were computed in the temperature range 100–600 K. Bioactivity scores of the SA molecule were also determined. The present investigation suggests that the SA molecule exists in the ZI form as a monomer and in the OH form as a dimer. Appearance of the intense and broad IR band in the range 2200 to 3300 cm−1 could be explained only in terms of the OH stretching mode of the dimeric form of SA.