ELECTRON STRUCTURE OF RADICALS OF SULFOXYLIC ACID ESTERS

The density functional theory B3LYP/6-311++G(3df,3pd) has been used to optimize the geometry of homologues of a series of sulfoxylic acid ester radicals and to obtain the electron density distributions of the first nine compounds. The hydrogen bond presented in the initial molecules of the unbranched esters of sulfoxylic acid between the hydrogen of the second carbon atom of the alkyl chain (from the ester bond) and the oxygen of the hydroxyl group of the sulfur-containing fragment (-C(Н)H-CH2-O-S-ОН) and the corresponding cycle are not observed in the radicals. The fragmentation of the structures into topological groups of CH3, CH2, and (-O-S-O)● has been proposed and their electron integral characteristics are presented including charges, unpaired electron density, energy, and volume. The steric effect of the fragment (-O-S-O)● has been established and its inductive effect has been considered based on the groups charge parameters changes triggered by an increase in the hydrocarbon chain. The scale of group electronegativities of the studied homologues has been constructed by comparing the charges of topological groups. The fact that the radical center corresponds to the fragment (-O-S-O)● has been demonstrated by the spin density delocalization results (having the largest fraction on the sulfur atom (0.57), slightly less fraction on the oxygen atom with a free valence (0.32) and an insignificant fraction on oxygen atom using the ether bond (0.10)). The increment value of the total energy contributed by the CH2 group of each subsequent homologue of the series under the study has been estimated. It is 103260 kJ/mol. The evaluation of the “standard” value of the groups’ total electron energy and the use of their relative energy (ΔE(R)) for comparing with ΔE(R) of molecules and radicals of other homologous series have been described. A decrease in the volumes of the two closest to the sulfur-containing fragment CH2 groups caused by an outflow of electron density from them toward the fragment (-O-S-O)● has been noted.

KINETICS OF THIOUREA DIOXIDE DECOMPOSITION IN WATER-ETHANOL-AMMONIA SOLUTION

A stoichiometric mechanism for thiourea dioxide decomposition in water- ethanol-ammonia solution is proposed based on dependences of concentrations of thiourea dioxide over time and literature data. The concentration of thiourea dioxide was measured with iodometry, while the intermediates were qualitatively detected using the polarography. The set of the obtained data allows to consider that at low concentration of alcohol (approximately up to 0.1 molar ratio) the thiourea dioxide decomposition proceeds on the heterolytic mechanism with formation of sulfoxylic acid, and at high concentrations of alcohol – on homolytic mechanism with the formation of anion radicals. Rate constants for individual stages of heterolytic mechanism are obtained by mathematical modeling, presented a system of differential equations. Absolute errors of rate constants, correlation coefficients, and F-factors were also calculated. The linear correlation was found between the equilibrium constant logarithm of the decay stage of thiourea dioxide and the inverse value of the dielectric constant of water-alcohol solutions. The kinetics of reduction of nickel ions with thiourea dioxide in an aqueous ammonia solution with ethanol additives was also studied. Сoncentration of nickel ions was determined using complexometric titration. Obtained kinetic data showed that at alcohol concentrations of 0.13 molar ratio and more, reaction kinetics is described by a first-order equation. At lower ethanol concentrations, a fractional order is observed on the concentration of nickel ions. A stoichiometric mechanism of this reaction is proposed due to sulfoxylic acid molecules with alcohol concentrations less than 0.1 molar ratio and due to radical ions with alcohol concentrations more than 0.1 molar ratio. Thus, the study of nickel ion reduction kinetics confirmed the conclusion about influence of alcohol concentration on the decomposition mechanism of thiourea dioxide.

Reactivity of Small Oxoacids of Sulfur

Oxidation of sulfide to sulfate is known to consist of several steps. Key intermediates in this process are the so-called small oxoacids of sulfur (SOS)—sulfenic HSOH (hydrogen thioperoxide, oxadisulfane, or sulfur hydride hydroxide) and sulfoxylic S(OH)2 acids. Sulfur monoxide can be considered as a dehydrated form of sulfoxylic acid. Although all of these species play an important role in atmospheric chemistry and in organic synthesis, and are also invoked in biochemical processes, they are quite unstable compounds so much so that their physical and chemical properties are still subject to intense studies. It is well-established that sulfoxylic acid has very strong reducing properties, while sulfenic acid is capable of both oxidizing and reducing various substrates. Here, in this review, the mechanisms of sulfide oxidation as well as data on the structure and reactivity of small sulfur-containing oxoacids, sulfur monoxide, and its precursors are discussed.