INFRARED-SPECTRA OF THIOURACILS - EXPERIMENTAL MATRIX-ISOLATION AND AB-INITIO HARTREE-FOCK, POST-HARTREE-FOCK AND DENSITY-FUNCTIONAL THEORYSTUDIES

The experimental infrared absorption spectra of 2-thiouracil, 4-thiour acil and 2,4-dithiouracil isolated in low-temperature argon matrices a re compared with the spectra of these compounds computed by the densit y functional theory with the combined Becke3-LYP exchange-correlation energy functional (DFT(B3-LYP)) and conventional ab initio methods (Ha rtree-Fock (HF), MP2) using the standard 6-31G(d,p) basis set. The vib rational spectra computed at the DFT(B3-LYP)/G-31G(d,p) and HF/6-31G(d ,p) levels of theory were found to reproduce well the experimental IR spectra. The results of MP2/6-31G(d,p) calculations predicted the norm al modes due to out-of-plane vibrations somewhat less accurately. An a ssignment of the bands observed in the infrared spectra of the three t hiouracils was performed, based on the comparison with the spectra cal culated at the DFT(B3-LYP)/6-31G(d,p) level.