On the anomalous vibration spectra and OH⋯N bond dictated structure of 3-fluoroisonicotinic acid

The vibrational structure of solid 3-fluoroisonicotinic acid, as measured by IR absorption and Raman spectra in the 4000-50 cm−1 region and aided by B3LYP/6-311++G(d,p) and NBO calculations has been analyzed. While anomalous spectral behavior similar to that in pyridine-carboxylic acids has been observed as two broad medium absorptions 2436 and 1870 cm−1, the inverted bands observed in isonicotinic acid, are present as 'normal' bands near 1488, 1308, 1132, 1062, and 823 cm−1. It has been shown by dimer modeling that the absorptions at 2436 and 1308 cm−1 correspond to OH stretching and in-plane vibrations originating from inter-molecular OH⋯N bonding. Formation of a network of dimer chains has been shown to be a result of CH⋯O bond links, giving rise to a local center of inversion between monomer species in the network. This has been validated by observing one broad low intensity Raman band at 1708 cm−1 and one strong broad IR band at 1727 cm−1 assigned as symmetric and anti-symmetric carbonyl modes respectively. In the proposed dimer model, the OH⋯N bond length is calculated to be 2.742 Å, shorter than the van der Waals approach, 2.750 Å, placing it in the category of a strong hydrogen bond. The CH⋯O bond length for the dimer is calculated to be ∼3.282 Å. It has been found that the dimer model is only suggestive of the complex nature of OH⋯N bonding but the results generally are in agreement with experiment.