The electron structures of 3-substituted 1,2,4-triazines and isomeric N-methyltriazinium salts were calculated by the CNDO/2 method. The results of the calculations were compared with the parameters of the '3C, *SN, and *~N NMR spectra, as well as with experimental data on the protonation, N-alkylation, and reactions of 1,2,4triazines with simple nucleophiles. The protonation centers of 1,2,4-triazines were determined by means of ~H, 'SC, and *"N N-MR spectroscopy. The covalent adducts formed by the proton salts of 3-methoxy-, 3-methylthio-, 3-morpholino-, and 3-pyrrolidino-l,2,4-triazine with water and methanol were recorded by IHNMR spectroscopy.
Although extremely limited, the literature data on the structures of N-protonated and N-alkylated 1,2,4-triazinium cations provide evidence that the protonation and quaternization of 1,2,4-triazines can, in principle, take place at any of the ring nitrogen atoms; the direction of the reaction is determined by both electronic and steric factors [1][2][3][4][5][6].
It was previously established by ~H and ~SC NMR spectroscopy that the quaternization of 3-cycloalkylamino-substituted 1,2,4-triazines by alkylating agents takes place only at the N(,) atom [i].
In the present communication we present computational indexes for the molecules of a number of 3-substituted triazines and 1,2,4-triazinium cations, which, in conjunction with experimental data on protonation and quaternization, as well as with the characteristics of the *H, *aC, *SN, and *"N NMR spectra, give an idea of the electron density distribution in triazines and triazinium salts and make it possible to evaluate the relative stabilities of isomeric triazinium cations and ascertain the most probable centers of protonation, quaternlzation, and nucleophilic addition.
The quantum-chemical calculations of triazines la-e and N-methyltriazinium cations lla, c-e with a methyl group attached to the N(,), N(a), and N(,) atoms were made by the CNDO/2 method with "averaged" geometries of the molecules [7]. The geometry of the triazine ring for all of the compounds was assumed to be invariable with the following bond lengths: C--C 13.83, C--N 13.51, N--N 13.26, and C--H 10.80 nm. As a result of the quantum-chemical study we obtained the distributions of the charges in la-e and lla, c-e and calculated the resonance energies of the bonds (Tables i and2).