L. Hwang, Jung Wang, C. Tzeng
Jun 1, 2002
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Analytical Sciences
Abstract
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