Tautomers of 6-thiopurine in low-temperature Ar matrices: FTIR spectroscopy analysis and quantum mechanical calculations

Abstract

The structures and vibrational spectra of 6-thiopurine (6TP) molecules in an isolated state were studied by the spectroscopic and computational methods. FTIR spectra of 6TP molecules isolated in low-temperature Ar matrices (at 11 K) were obtained in the infrared range 3800–200 cm–1. The optimized structures of tautomers, model clusters and the population of tautomers were estimated by the DFT, MP2 and CCSD(T) methods. The vibrational spectra were calculated by the DFT/B3LYP method with different basis sets [6–311++G(df,pd), aug-cc-pVDZ, aug-cc-pVTZ] and the MP2/aug-cc-pVDZ/anharmonic method. In the spectral range of 1700–200 cm–1 of the experimental FTIR spectra, five combination modes enhanced by the Fermi resonance were observed. Fermi resonances with the participation of librational modes of domestic molecules were found in the 600–500 cm–1 region. It was revealed that the incorporation of 6TP between the closest packing planes of Ar lattice leads to a significant increase in the frequency of two out-of-plane “butterfly” modes.