Combined matrix-isolation FT-IR and ab-initio 6-31++G** studies on tautomeric properties of nucleic acid bases and simpler model molecules

The combined matrix isolation/ab initio 6-31++G** method is used to study the tautomeric equilibria of purine, 1-CH₃-adenine, 3-CH₃-adenine and N-CH₃-adenine in isolated conditions. For the N₉H ⇆ N₇H prototropism, tautomeric equilibrium constants of 0.085, 0.090 and 0.005 are obtained for purine, 1-CH₃-adenine and N-CH₃-adenine, respectively. The other prototropic equilibria that are considered are the amino ⇆ imino tautomerism for which relative amounts of 5.5% for the rare amino form in 1-CH₃-adenine and 0.5% for the minor imino tautomer of 3-CH₃ adenine were observed, and the N₉H ⇆ N₃H prototropism where only 0.7% of the experimental spectrum could be accounted for by the presence of the less stable N₃H form in N-CH₃-adenine.