Infrared spectra of thiouracils: experimental matrix isolation and ab initio Hartree-Fock, post-Hartree-Fock and density functional theory studies

Abstract

The experimental infrared absorption spectra of 2-thiouracil, 4-thiouracil and 2,4-dithiouracil isolated in low-temperature argon matrices are compared with the spectra of these compounds computed by the density functional theory with the combined Becke3-LYP exchange-correlation energy functional (DFT(B3-LYP)) and conventional ab initio methods (Hartree-Fock (HF), MP2) using the standard 6-31G(d,p) basis set. The vibrational spectra computed at the DFT(B3-LYP)/6-31G(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 normal modes due to out-of-plane vibrations somewhat less accurately. An assignment of the bands observed in the infrared spectra of the three thiouracils was performed, based on the comparison with the spectra calculated at the DFT(B3-LYP)/6-31G(d,p) level.