Theoretical and Spectroscopic Study of Infrared Spectra of Hydrogen-Bonded 2,4-Dithiouracil Crystal and Its Deuterated Derivative

Theoretical simulation of the bandshape and fine structure of the N-H(D) stretching bands is presented for 2,4-dithiouracil and its deuterated derivative taking into account anharmonic coupling between the high-frequency N-H(D) stretching and the low-frequency N center dot center dot center dot S stretching vibrations, resonance interaction between two equivalent hydrogen bonds in the dimer, anharmonicity of the potentials for the low-frequency vibrations in the ground and excited state of the N-H(D) stretching mode, Fermi resonance between the N-H(D) stretching and the first overtone of the N-H(D) bending vibrations, and electrical anharmonicity. The effect of deuteration has been successfully reproduced by our model calculations. Infrared, far-infrared, Raman and low-frequency Raman spectra of the polycrystalline 2,4-dithiouracil have been recorded. The geometry and experimental frequencies are compared with the results of harmonic and anharmonic DFT calculations.