COMBINED MATRIX-ISOLATION FT-IR AND AB-INITIO 6-31-ASTERISK-ASTERISK STUDIES ON TAUTOMERIC PROPERTIES OF NUCLEIC-ACID BASES AND SIMPLER MODEL MOLECULES(+G)

The combined matrix-isolation/ab initio 6-31++G* method is used to st udy the tautomeric equilibria of purine, 1-CH3-adenine, 3-CH3-adenine and N-CH3-adenine in isolated conditions. For the N9H --><-- N7H proto tropism, tautomeric equilibrium constants of 0.085, 0.090 and 0.005 ar e 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 p resence of the less stable N3H form in N-CH3-adenine. (C) 1997 Elsevie r Science B.V.