Anharmonic analysis of CH and OH stretching vibrations of the formic acid dimer

The structure and IR absorption spectrum of the formic acid dimer (FAD) were calculated using the B3LYP DFT functional and cc-pVTZ basis set. Matrices of force and anharmonic constants were calculated in order to find the fundamental frequencies more accurately. The 1D and 2D potential energy surfaces (PES) were built using symmetry coordinates and stretching coordinates of СН and OH bonds. For OH stretching coordinates, the 2D PES were built with and without optimization of the remaining geometric parameters. It was shown that in the second case besides the global minimum, an additional local minimum on the unrelaxed PES is formed with the energy more than 4000 cm−1 higher than the energy of the ground state. A numerical solution of Schrödinger equations gives the values of OH and CH stretching vibrations. Comparison of the obtained theoretical results with the experimental data presented in the literature allowed us to make new assignments for IR and Raman spectral bands related to hydroxyl groups stretching vibrations in FAD.