Matrix-isolation FT-IR and ab initio 6-31 + + G** study of 1-CH₃-adenine tautomerism

The vibrational spectrum of 1-CH₃-adenine was studied by the combined experimental matrix-isolation and theoretical ab initio SCF/6-31 + +G ** method. The imino N₉H tautomer is largely predominant in Ar matrices in accordance with the theoretical energy predictions. Trace amounts of the imino N₇H and the amino tautomers are detected for the first time, and tautomerization constants of 0.09 and 0.05 were obtained from a combination of the experimental and the theoretical IR intensities. The results are discussed in relation to the prototropic behaviour of adenine and purine and to the mechanism of the tautomerization process.