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. © 2022 Author(s).