DFT studies of molecular structures conformers and vibrational characteristics of sulfanilamide

Molecular structures and vibrational parameters for all the four possible conformers of sulfanilamide were studied using Gaussian 09 software and PEDs were calculated using GAR2PED software. MEP and HOMO-LUMO energies were computed and NBO analysis was carried out. Optimized geometries possess C-5 symmetry. Out of 51 normal modes, 26 modes are conformer sensitive, out of which 2 modes show frequency variation above 75 cm(-1) in going from one conformer to another. Out of the 6 internal modes of NH2, 4 modes were found at lower frequencies for S-NH2 group compared to C-NH2 group, due to presence of intra-molecular O center dot center dot center dot H bonds in the SO2(NH2) group. Strength of nucleophilic attack is stronger with the H atoms of C-NH2 group compared to the H atoms of the S-NH 2 group. Intra-molecular O center dot center dot center dot H bonds also lead to difference in corresponding geometrical parameters of S-NH2 and C-NH2 groups and difference in atomic charges at corresponding sites.