Механизм термической ионизации уротропина на поверхности интерметаллида NaAu-=SUB=-x-=/SUB=-

Thermal ionization of methenamine (C6H12N4) on the surface of the NaAux intermetallic compound has been studied. It has been established that the processes of decomposition, desorption and ionization of adsorbed compounds, thermally stimulated on the surface, proceed due to the accumulation of energy at the degrees of freedom of the adsorption complex, including the adsorbed compound and a solid, by the mechanism of monomolecular decomposition reactions. In this case, the decomposition of the adsorption complex is accompanied by the desorption of ions that are not in thermal equilibrium with the solid. The uniformity of the temperature dependences of the ion current and their distribution over two groups allowed us to conclude that ions are desorbed from the surface, which correspond to the decays of individual adsorbed molecules, as well as the decays of dimers formed on the surface. The decay of methenamine molecules during thermal ionization occurs in the same way as their decay in vacuum during electron ionization, which indicates the preservation of the bulk structure of methenamine molecules during adsorption and a significant lifetime of the excited state of compounds on NaAux.

Features and New Examples of Gas Chromatographic Separation of Thermally Unstable Analytes

The processes of thermal decomposition of analytes in gas chromatographic (GC) columns are classified and two new examples of them are considered in details. First of them is monomolecular decomposition of monoalkyl esters of benzene-1, 2-dicarboxylic (phthalic) acid (monoalkyl phthalates). This process has the analogy in chemical reactions in solutions and it may be responsible for the toxicity of phthalates. The second example is decomposition of non-substituted hydrazones of both aliphatic and aromatic carbonyl compounds. The analytes of the second sub-group present the first example of bimolecular (second order) decomposition in a GC column: two molecules of hydrazones form stable azines and hydrazine. Besides that this process presents the particular interest, because it is accompanied by secondary chemical reactions not in an injector, but within GC column, when a by-product of decomposition is involved into secondary interaction with other constituents of the samples. It was confirmed, that visual images of all these decomposition processes on the chromatograms are rather identical and coincide with the manifestations of interconversion of isomers or tautomers. The most often expressed features of chromatographic profiles in such cases are the presence of peaks of an initial analyte and a product of its decomposition or isomerization, connected with more or less expressed diffused “plateau” or “train” between them. The decomposition processes during sample preparation prior to chromatographic separation or in the heated injector of GC instrument are not accompanied by such features. Despite of the rather “exotic” character of the examples considered, the knowledge of them seems to be useful for better revealing the analogous situations in chromatographic practice. Thermal instability of analytes is the principal restriction of GC separation of reactive compounds and we cannot eliminate it for objective reasons. However, in some cases we can evaluate the temperature limits of chromatographic columns, which should not be exceeded during GC separation of instable compounds. The simplest (low boiling) homologs of thermally unstable compounds are often characterized by “normal” boiling point at atmospheric pressure (T b, °C) without decomposition, that means the possibility of their GC analysis unambiguously. Therefore, we can select such T b values as GC and/or GC–MS temperature limit (T lim) for other members of series of thermally unstable homologs. If GC separation is carried out not in isothermal, but in temperature programming conditions, so-called retention temperature (T R) of unstable analytes should not exceed the evaluated T lim value.

Formation of hexacyanomanganate(II) in cyanide medium and kinetics and mechanism of its oxidation by chromate

The rate of formation of hexacyanomanganate(II) is first-order in Mn(II) and depends on both the cyanide and hydroxide concentrations. The experimental results agree with a reaction mechanism which involves manganese(II) cyano- and hydroxo-complexes. Chromate slowly oxidizes hexacyanomanganate(II). The kinetics of this reaction is first-order on hexacyanomanganate(II), and the experimental rate constant depends on the chromate concentration according to the equation kexp = 3kK[Cr(VI)]/(1 + K[Cr(VI)]). The effect of both the CN− and the OH− ions is slighter. A tentative mechanism is proposed which involves a pre-equilibrium with the formation of a hexacyanomanganate(II)–chromate complex, followed by a monomolecular decomposition. By comparing this reaction mechanism with that of other one-electron chromate reductions, it appears that hexacyanomanganate(II) behaves like octacyanomolybdate(IV) rather than like hexacyanoferrate(II) and other substitution-inert reductants. Both the equilibrium constants for the complex formation and the rate constant for its decomposition are evaluated, and voltammetric evidence of the formation of hexacyanomanganate(III)–chromate complexes is also presented. Keywords: hexacyanomanganate(II), formation rate, oxidation with chromate, kinetics, mechanism.