Tautomerism and infrared spectra of 2-thiopurine: an experimental matrix isolation and theoretical ab initio and density functional theory study

Infrared spectra of 2-thiopurine (2-mercaptopurine, 2-purinethiol) isolated in low-temperature Ar and N-2 matrixes are reported. These spectra indicat e that the compound adopts exclusively the thiol N9H tautomeric form. The t heoretical calculations of relative energies of 2-thiopurine tautomers have been carried out at the MP4(SDTQ)//HF level using the 6-31G(d, p) basis se t. The thiol N9H tautomer was predicted to be the most stable of all isomer s of 2-thiopurine. The infrared spectra of the tautomers of 2-thiopurine ha ve been calculated at the DFT(B3LYP)/6-31G(d, p) level. Good agreement betw een the experimental spectra and the spectra calculated for thiol N9H tauto mer supported the identification of the dominant tautomer. It has also allo wed for the reliable assignment of the bands observed in the experimental I R spectrum. (C) 2001 Elsevier Science B.V. All rights reserved.