Journal of Molecular Structure vol:410 pages:397-401
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. (C) 1997 Elsevier Science B.V.