Phototautomeric reaction, tautomerism, and infrared spectra of 6-thiopurine. Experimental matrix isolation and quantum-mechanical (conventional ab initio and density-functional theory) studies

The infrared spectra of 6-thiopurine (6-mercaptopurine) isolated in Ar and N-2 low-temperature matrices show that the molecule exists as a mixture of thione N(1)H,N(7)H and thiol N(9)H tautomers. The photoreaction converting the thione N(I)H,N(7)H form of the compound into the thiol N(7)H form is ob served for the matrix-isolated species. The assignments of the substrate to the thione N(1)H,N(7)H form and the photoproduct to the thiol N(7)H form a re confirmed by good agreement between the experimental infrared spectra an d the spectra theoretically predicted at the OFT(B3LYP)/6-31G(d,p) level. T he thiol N(9)H tautomer is photochemically stable under the conditions of U V irradiation applied in the experiment (lambda > 320 nm). The experimental ly estimated ratio of tautomers in the matrix (non-UV-irradiated) [thiol N( 9)H]/[thione N(1)H,N(7)H] = 0.24 correlates well with the theoretical estim ate of the relative energies of the tautomeric forms of the compound predic ted at conventional ab initio calculations [total energies including electr onic energies calculated at the Hartree-Fock (HF)/6-31G(d.p) level correcte d for electron-correlation contributions at the MP4(SDQ)/6-31G(d,p) level a nd zero-point energies at the HF/6-31G(d.p) level].