Pyridine N-oxide/trichloroacetic acid complex in acetonitrile: FTIR spectra, anharmonic calculations and computations of 1–3D potential surfaces of O–H vibrations

•Two conformers of PyO⋅TCA complex in acetonitrile with short hydrogen bond were found.•For conformers IR spectra were calculated in harmonic and anharmonic approximations.•1–3D PES for the motion of the hydrogen atom in both conformers were build.•Frequencies of OH vibrations were found by numerical solution of Schrödinger equation.•We analyzed increasing of anharmonicity while solving PyO⋅TCA complex in acetonitrile.

FTIR spectra of pyridine N-oxide and trichloroacetic acid H-bonded complex in acetonitrile were studied at 20 and 50 °C. The calculations of equilibrium configurations of the complex and their IR spectra in harmonic- and anharmonic approximations were carried out at the level of B3LYP/cc-pVTZ/PCM. However both approximations turned out to be incompetent determining the frequency of the O–Н stretching vibration. In order to reveal the causes of essential discrepancies between calculated and experimental data one-, two- and three-dimensional potential energy surfaces (PES) of the O–H…O bridge proton motion in the frame of fixed other atoms in the complex were calculated. The frequencies of O–H…O stretching and bending vibrations were calculated by numerical solution of the Schrödinger equation. It is shown that only the approach of proton motion on the 3D PES allows obtaining a good agreement between the calculated and the experimental values of the frequencies of the О–Н stretching vibrations.

Graphical abstractThe contours of the O–H stretching vibration wave function corresponding to 20% of probability density from its value in the maximum as a projection on the XOZ coordinate plane for the conformer II of PyO⋅TCA in acetonitrile.Figure optionsDownload full-size imageDownload as PowerPoint slide