We have determined the basicity of 3,2, 4,. According to quantum chemical calculations done by the MNDO method and the ab initio method in a 6-31G+ + basis, the tetrazine ring is a nonpolar, highly aromatic system similar to benzene. The aromaticity of the tetrazine heterocycle decreases significantly upon protonation, which considerably destabilizes the protonated form.The high interest in 1,2,4,5-tetrazines is due to their high biological activity and reactivity [1, 2]. The tetrazine ring is an interesting model for physieoehemical studies as the ring most saturated with nitrogen atoms relative to stable azine [3], but information about the physicoehemical properties of tetrazines is limited or fragmentary. In particular, such an important property of a heterocycle as basicity has not been discussed previously for tetrazines at a quantitative level.In this work, we have determined the basicity constant for 3,6-diphenyl-l,2,4,5-tetrazine (I) in aqueous sulfuric acid solutions using the spectrophotometric method. This compound was chosen as a model since it is relatively stable in acid solutions and is suitable for spectrophntometric analysis. The structure and thermodynamic characteristics of the free base I and its conjugate acid II and also the corresponding forms of unsubstituted tetrazine (HI, IV) were calculated by the semiempirical MNDO method and separately for the forms (III, IV) using the ab initio method in a 6-31G+ + basis.The basicity centers of 1,2,4,5-tetrazines are the pyridine nitrogen atoms, which are equivalent for compounds with identical substituents in the 3 and 6 positions of the ring.On going from base I to conjugate acid II, we observe a substantial bathochromic shift of the maximum for the ~r-x" transition in the electronic absorption spectrum, from ~ 295 to 336 nm. Furthermore, the protonated form II, in contrast to the base I (the characteristics of the n-r" transition of base I in ethanol were: ~ 548 nm, e 2250) does not have" a pronounced maximum corresponding to the n-~r" transition. This may be explained by the substantial differences between the electronic structure of the base and the corresponding conjugate acid. Upon protonation, the color of the solutions changes: the base gives a bright raspberry color, the protonated form gives a yellow color. It is difficult to measure the electronic absorption spectrum of the unprotonated tetrazine I in aqueous sulfuric acid solutions because of the extremely low solubility of I. For the working concentrations (10 -5 moles/liter), we could measure the spectrum of the base only in 1,4--dioxane and ethanol.Because of the low solubility of base I in aqueous solutions of sulfuric acid, we calculated the basicit3, constants by the iteration method in [5]. As the PKaH+ value, we took the ratio of the y-intercept (pK'BH+) tO the slope (m) of the Eits-MacClelland [spelling unverified] linear equation [4]. The calculation results were (~ 340 nm): pKsH+ -4.8 -I-0.6,
