Study of the protonation of 1-methylimidazo (4,5-B)pyrazines by1H and13C NMR methods

Conclusions A study of the¹H and¹³C NMR spectra of the neutral molecules, monocations, and dications of 1-methyl- and 1,5,6-trlmethylimidazo(4,5-b)pyrazLnes has shown that the first protonation of these compounds occurs at the 3-N atom of the imidazole fragment and the second at the 4-N atom of the pyrazine ring.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1060–1068, May, 1977.     

A study of the structure of the cations of n-oxides of monosubstituted pyrazines and quinoxalines by the pmr method

The dependence of the chemical shifts of the protons on the concentration of D₂SO₄ in D₂O in a number of N-oxides of monosubstituted pyrazines and quinoxalines has been investigated, and the parameters of the PMR spectra of the neutral and the mono- and diprotonated forms of the compounds investigated have been determined. All the pyrazine and quinoxaline N-oxides considered protonate first at the unoxidized nitrogen atom (N₄). The first protonation of 2-aminopyrazine 1,4-di-N-oxide takes place at the oxygen atom of the N O group in position 1, and that of 2-methoxypyrazine at the oxygen atom of the N O group in position 4. The effect of the delocalization of the positive charge in the monocations of the compounds investigated has been considered.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1115–1123, August, 1973.     

Effect of intramolecular hydrogen bond in unsaturated dicarboxylic acid molecules on the formation of cobalt(II) and nickel(II) carboxylates

The energies of formation of intramolecular hydrogen bonds (IHB) in unsaturated dicarboxylic acid molecules in aqueous solutions were calculated by the DFT method (B3LYP hybrid functional, 6-31G** basis set) using the GAMESS software, Version 1, May 2013 (R1). The medium effect was taken into account in terms of the Polarized Continuum Model (PCM). In the case of allylmalonic, itaconic, and maleic acids, fairly weak IHB with an energy of 5.8–13.9 kJ/mol were found to form. The enthalpies of formation of acid and normal cobalt and nickel salts of carboxylic acids in aqueous solutions were calculated by the semiempirical PM3 method using the HyperChem 8.0.8 software (Hypercube Inc.). The results indicate that in aqueous solutions, the formation of cobalt and nickel acid salts of allylmalonic, itaconic, and cis,cis-muconic acids is energetically more favorable than the formation of normal carboxylates, which is consistent with experimental data.     

Redox transformations of 2-formylquinoxaline hydrate and diethylacetal N,N′-dioxides

Depending on the pH of the medium, when 2-formylquinoxaline hydrate N,N-dioxide is subjected to attack by the OH^– anion, it undergoes redox transformations to give 2-carboxyquinoxaline 4-oxide or quinoline N,N-dioxide and the formate ion. In the case of attack by the OH^– anion 2-formylquinoxaline diethylacetal N,N-dioxide undergoes only the first reaction, regardless of the pH.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1106–1108, August, 1980.     

Electronic spectra and structures of protonated pyrazine and quinoxaline N-oxides

The electronic spectra of neutral molecules and monocations of N-oxides of 2- and 3-substituted pyrazines and quinoxalines were measured, and their ionization constants in water were determined. The position of the protonation center in the investigated compounds was established.     

Calculation of energetic characteristics for the complexation of unsaturated dicarboxylic acids with cobalt(II)

Energetic characteristics for the complexation (preorganization energy and complementarity) of unsaturated dicarboxylic acids (UDAs) toward cobalt(II) were calculated with the use of HostDesigner software developed by B. Hay and T. Firman in 2002. Maleic acid (MA), itaconic acid (IA), cis,cis-muconic acid (cis,cis-MA), and allylmalonic acid (AA) were considered as UDAs. The calculated complementarity and preorganization energy, which are characterized by RMSD parameter (root-mean-square deviation) and conformation energy, respectively, are combined in a generalized desirability function D that enables one to judge the relative values of complexation energy. In terms of D and RMSD values, UDAs are arranged in the following sequence: IA > AA > MA > cis,cis-MA. A conclusion was drawn on the determining effect of complementarity of the considered UDAs on the energy of cobalt complexation with these ligands.