In the present work, structures of two monomeric forms and the dimer of Nalidixic acid in accordance to the experimental reported have been theoretically studied by using B3LYP/6-311++G** calculations in gas and methanol environments to predict its reactivities, stabilities and, to perform the assignments of all the normal modes of vibration. Thus, based in the scaled quantum mechanical force field (SQMFF) methodology, the assignments of 168 and 81 normal modes of vibration for the dimer and monomers, respectively have been performed. Very good correlations between experimental and theoretical IR, Raman, and Ultraviolet spectra are acquired evidencing in the same the presence of those three forms of acid, as was experimentally observed. NBO and AIM calculations suggest that both forms present the same stabilities in the gas phase while the form II is less stable in methanol solution. Both studies revealed the existence of intermolecular contacts, as observed in the experimental structures. The form I has a solvation energy of -89.22 kJ/mol, the form II -60.34 kJ/mol and the dimer -73.32 kJ/mol. The present study confirms some of the assignments previously made with some modifications indicated by the most accurate force field developed in the present work. In addition, the scaled force constants are also reported.

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