Structural changes caused by the conversion of 2-hydroxybenzamide (salicylamide) into the oxyanion

The structure of the salicylamide molecule and its oxyanion have been studied by IR spectra and quantum chemical calculations within the Onsager self-consistent reaction field (SCRF) model, using a density functional theory (DFT) method at the B3LYP/6-31++G** level. Structures of all conformational isomers of salicylamide and of its anion have been located, as well as their total and relative energies have been determined. According to the calculations the planar configurations with intramolecular hydrogen bonds OH… OC (molecule) and NH…O− (oxyanion) in the molecule and anion, respectively have been found to be the most stable. The molecule → oxyanion conversion leads to geometry changes in the whole species but the strongest structural variations take place near the oxyanionic centre. In agreement between theory and experiment, the conversion causes a 40 cm−1 decrease in the frequency of the carbonyl stretching band νCO, 1.8-fold increase in its integrated intensity, strong intensity increases (4.2-fold) of the aromatic skeletal bands, and other essential spectral changes. The natural bond orbital (NBO) charges changes indicate that the oxyanionic centre bears nearly the 60% of the whole net negative charge.