X-ray, FT-IR and PM3 Studies of Hydrogen Bonds in Complexes of Some Pyridines with Trifluoroacetic Acid

The crystal structures of trifluoroacetic acid complexes with 4-NMe2-, 4-Me- and 4-CN-pyridines were determined by X-ray analysis; the N-H...O bonds are 2.724(3), 2.702(4) and 2.587(5) angstrom respectively. The H-bonds are nearly linear for 4-NMe2, 177(4)-degrees; 4-Me, 177(4)-degrees; and 4-CN, 174(6)-degrees.IR spectra (Nujol) show continuous absorption, whose intensity decreases with elongation of the H-bond length. The continuous absorption is not observed in D2O spectra. The solid-state spectra in the 1700 cm-1 region are more complex than those in D2O; the characteristic overtones of the pyridine rings borrow intensity from the continuous absorption via Fermi resonance. The overtones indicate modified structure in the 1700 cm-1 region. The results of both diffraction and FT-IR experiments are comparable.The structural parameters of the complexes were also determined by quantum-mechanical calculations with the semiempirical MNDO-PM3 method. A solvent effect was taken into account using a self-consistent reaction field theory.