ETHYL-3-PHENYL-2,5-DIHYDRO-1,2,4-TRIAZIN-6(1H)-ONE METHANOL SOLVATE AND ETHYL-3-PHENYL-4,5-DIHYDRO-1,2,4-TRIAZIN-6(1H)-ONE

Tautomeric detail in the two title triazinones has been accurately def ined. ethyl-3-phenyl-2,5-dihydro-1,2,4-triazin-6(1H)-one methanol solv ate, C10H11N3O.CH4O, was found to adopt a zwitterionic form with a dih edral angle between the mean planes of the dihydrotriazinone ring (r.m .s. deviation 0.12 Angstrom) and the phenyl ring of 44.5(2)degrees. In the crystal, the molecules are linked by intermolecular hydrogen bond s between the protonated ring-N atom at position 4 and the carbonyl-O atom. thyl-3-phenyl-4,5-dihydro-1,2,4-triazin-6(1H)-one, C10H11N3O, ad opts a non-ionic tautomeric form. The dihedral angle between the mean planes of the dihydrotriazinone ring (r.m.s. deviation 0.15 Angstrom) and the phenyl ring is 33.7(1)degrees. The molecules in the crystal ar e linked into dimers by hydrogen bonds between the protonated ring-N a tom at position 1 and the carbonyl-O atom.