Density functional theory calculations of the internal rotations and vibrational spectra of 2-, 3- and 4-formyl pyridine

The torsional potentials, molecular conformations and vibrational spectra, of 2-, 3- and 4-formyl pyridine have been investigated using density functional theory (DFT) method with 6-31 + G* basis set. From the calculations, 2-formyl pyridine and 3-formyl pyridine were predicted to exist predominantly in cis conformation with the cis–trans rotational barrier of 9.38 kcal/mol and 8.55 kcal/mol, respectively. The two equivalent planar structures of 4-formyl pyridine are separated by an energy barrier of 7.18 kcal/mol. The vibrational wavenumbers and the corresponding vibrational assignments of molecules in Cs symmetry were examined theoretically and the calculated Infrared of the molecules in the cis conformation was plotted. Observed wavenumbers for normal modes were compared with those calculated from normal mode coordinate analysis carried out on the basis of DFT force fields using the standard 6-31 + G* basis set of the theoretical optimized geometry.