Crystal structure of 3-amino-1,2,4-triazin-5(2H)-one.

because of various interesting biological activities.1 The tautomeric proton is highly effective at making strong intermolecular hydrogen binding with other heteroatoms of molecules involved with biological activity. The title compound 3-amino-1,2,4-triazin-5(2H)-one (6-azaisocytosine, Fig. 1), an isosteric isomer of isocytosine, can exist in several tautomeric forms, which have been discussed in earlier reports.2–4 Ueda and Furukawa2 concluded that the imino-oxo form is predominant, as shown by infrared spectra. Sasaki and Minamoto3 used ultraviolet and infrared spectra to show that amino-oxo form (4H-tautomer) to be predominant. Pitha et al.4 compared ultraviolet spectra and ionization constants to reveal the relative abundances as 100:1, in favor of the amino-oxo form (2H-tautomer). There is a need to obtain more precise information about the most contributed prototropic tautomerism of the title molecule and to confirm the assigned structure. So we have undertaken a critical use of X-ray crystallographic analysis. The title compound was prepared by the method of Sasaki and Minamoto.3 The physical properties of 3-amino-1,2,4-triazin-5one had been reported: mp,4–6 IR,2,3,5 UV,3–5 NMR,5,6 and dissociation exponent.4 A colorless crystal of dimensions 0.35 × 0.50 × 0.55 mm3 suitable for single-crystal X-ray diffraction measurements was obtained by recrystallization from H2O solution. The results of the X-ray structure determination are given in Tables 1 – 3. The ORTEP diagram for the title compound is shown in Fig.2. Data from the X-ray structure reveal that the oxidation site is at C-5 position and that the predominant tautomeric structure is amino-oxo form 2H-tautomer (3-amino-1,2,4-triazin-5(2H)one). This analysis reveals that the 1,2,4-triazine ring structure of 3-amino-1,2,4-triazin-5(2H)-one is slightly distorted due to the asymmetry of the electronegativity of nitrogen. Obviously, the tautomeric proton 2-H is located at N-2 (N2) with 0.885(20)Å bond distance shorter than the bond length of H–N(3) 1.009 Å,7 which means the H2–N2 single bond is strongly attracted by the greater π-deficiency triazine ring. The same reason also explains the result that the bond distance 0.966(19)Å of C3–H1 is shorter than the bond length 1.083 Å of Car–H.7 On the other hand, because of the π-electron resonance effect in the triazine ring, the C1–O bond length 1.2413(18)Å is nearly the same as the bond length 1.240 Å of Csp2 = O(1) in δlactams, and the N2–N3 bond length 1.3565(18)Å is longer than the 1.304 Å of N=N (aromatic) in pyridazine.7 The short bonds 1.3328(18)Å (C2–N1) and 1.2829(20)Å (C3–N3) in the ring have an appreciable double-bond character, and the latter may be the pathway of 2-H to resonate with 5-O. It is interesting to 723 ANALYTICAL SCIENCES JUNE 2002, VOL. 18 2002 © The Japan Society for Analytical Chemistry