FTIR and Raman spectra of an anti-bacterial compound, N-phenylthiobenzamide (NptbH), were recorded in the solid phase and in solution. The harmonic vibration wavenumbers, for both the tautomeric forms of NptbH, as well as for its dimeric complex, have been calculated, using density functional B3LYP method involving several basis sets and the results were compared with the experimental values. Conformational studies have been also carried out regarding its tautomeric monomer forms as well as the dimer form. According to all calculations the thione form of NptbH (II) is about 41.2- 75.3 kJ/mol more stable than the thiol form (I) and the dimeric conformation (III) predicted to be more stable than the monomeric conformations (I) and (II). Vibrational assignments have been made, and it is found that the calculated normal mode frequencies of dimeric (III) and monomeric (II) molecules are required for the analysis of IR and Raman spectra of the NptbH in the solid and solution states. The intermolecular sulfur – nitrogen distance in N-H···S hydrogen bond system was found to be in the 3.421-3.448 Å range from these calculations. The calculated hydrogen bond strength was estimated to be in the 27.7-31.5 kJ/mol range (13.9-15.8 kJ/mol per bond).

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