Aqueous cytosine, which exists mainly as the 1 (//)-aminooxo form (la), is shown by temperature-jump spectroscopy to tautomerize slightly to the 3(//)-aminooxo form (Ila); the equilibrium constant K = (IIa)/(Ia) is estimated to be (2.5 ± 0.5) X 10-3 at 25 °C and the tautomerization enthalpy is 3.1 ± 0.1 kcal M_1. At 10 °C, The interconversion process is catalyzed by water [kH2o = (1.8 ± 0.3) X 103 s~1 ], by H+ [kH+ = (1.6 ± 0.3) X 1010M-' s-'],by OH" [fc0H-= (1.2 ±0.05) X 1010 M_1 s_1], and by the cytosinium cation [kcat;on = (4.4 ± 0.2) X 108 M_l s-1]. On the other hand, 3-methylcytosine is shown by uv and ir spectroscopy to exist as a mixture of two tautomeric forms, the aminooxo form lib and the iminooxo form Illb; while Illb is the major form in nonpolar solvents, the amino form lib largely predominates in water, the equilibrium constant K' = (Illb)/(lib) being estimated as ca. 3 X 10-2 at 25 °C. The Ilb^Illb interconversion mechanism was investigated in D20 solution at 10 °C by temperature-jump spectroscopy. The interconversion is catalyzed by D20 [ko2o = (2.9 ± 0.3) X 104 s~'], by OD~[ £od-= (4.3 ± 0.2) X 109 M_1 s-1], and by cationic 3-methylcytosine [kCation = (2.2 ± 0.1) X 108 M_l

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