Mass Spectrometric Study of Stability, Thermochemistry and Structures of the Gaseous Oxyacid Salts
Experimental data obtained by Knudsen Effusion Mass Spectrometry and results of quantum-chemical calculations on the structure and thermodynamic properties of gaseous oxyacid salts were systematized. A criterion of thermal stability of these compounds was offered and regularities in the atomization enthalpies were established.
Thermodynamic properties of gaseous BaSnO2 and Ba2 O2 studied by Knudsen effusion mass spectrometry.
RATIONALE
BaSnO3 is an interesting technical and industrial ceramic, with uses in many areas of electronic technology. Currently, there is great interest in these ceramics because of their potential as transparent conductive oxides. Due to their good chemical stability, they are also used as surface processing materials in the synthesis of electroluminophores. When heated, the stannates of alkaline earth metals can pass into the vapor phase with or without dissociation. Until the present investigation, gaseous salts where SnO plays the role of an anion-forming oxide had been unknown. The formation enthalpy of gaseous Ba2 O2 also needs to be determined.


METHODS
Knudsen effusion mass spectrometry was used to determine the partial pressures of vapor species, equilibrium constants and enthalpies of studied gas-phase reactions, as well as the formation and atomization enthalpies of gaseous BaSnO2 and Ba2 O2 : a mixture of BaO and SnO2 was evaporated from a platinum effusion cell. For the evaporation of gold (pressure standard) a molybdenum effusion cell was used. A theoretical study of gaseous BaSnO2 and Ba2 O2 was performed by several quantum chemistry methods.


RESULTS
Ba, BaO, Ba2 O2 , SnO and BaSnO2 were found to be the main species in the vapor over the BaO-SnO2 mixture in the temperature range of 1680 - 1920 K. The standard formation enthalpies of gaseous BaSnO2 and Ba2 O2 were determined on the basis of the equilibrium constants of the studied gas-phase reactions. Energetically favorable structures of these gaseous species were found and vibrational frequencies were evaluated in the harmonic approximation. The formation enthalpy of gaseous Ba2 O2 was clarified; in addition, the formation enthalpies of gaseous SrSnO3 and CaSnO3 were estimated.


CONCLUSIONS
The thermal stability of gaseous BaSnO2 was confirmed by Knudsen effusion mass spectrometry. The reaction enthalpies of gaseous BaSnO2 from gaseous barium and tin oxides were theoretically evaluated and the obtained values were found to be in reasonable agreement with the experimental ones.
New method for analysis of enzymatic reactions: Thermolysin-catalyzed synthesis of aspartame precursor
Aspartame is an artificial sweetener which is 200 times sweeter than sucrose. A precursor of aspartame, N-(benzyloxycarbonyl)-l-aspartyl-l-phenylalanine methyl ester (Z-APM) was synthesized and hydrolyzed with thermolysin in an aqueous monophasic system. Graphical plotting methods and/or nonlinear regression have been used to analyze enzymatic reactions because of the nonlinearity of enzymatic rate equations. There are, however, problems: the former require accurate measurement of the initial reaction rate and the latter gives no analytical information. The authors proposed a method to solve the nonlinear enzymatic rate equations with respect to the concentration of a product with the Taylor expansion, especially for the reversible Theorell-Chance mechanism (Murakami, Y., Hirata, M., and Hirata, A., J. Ferment. Bioeng., 82, 246–252, 1996). This solution, however, is complex for practical use. In this paper, a simpler solution to explain the thermolysin-catalyzed synthesis of Z-APM is obtained. New methods to determine the kinetic parameters, especially for the reversible Theorell-Chance mechanism, based on the approximated solution are proposed. The results calculated using the obtained kinetic parameters can satisfactorily explain the experimental results, which demonstrates the effectiveness of the proposed analysis. We must know the relation between the concentrations of substrates and a product, and accurately evaluate of the result obtained using Taylor expansion when we determine the kinetic parameters using our newly developed methods. The methods to determine the kinetic parameters of the reversible Theorell-Chance mechanism described in this paper is, however, expected to be applied to general enzymatic reactions.