Indirect detection of spectroscopically unobservable components of chemical exchange by NOESY 2D NMR

A new method for detection of intermediates in chemical exchange is suggested. It is found that the effective exchange rate constant determined from NOESY 2D NMR spectrum depends on the mixing time in the NOESY pulse sequence. The true exchange rates and relative population of the intermediates can be appreciated from this dependence. As follows from estimations, this method is applicable to detect intermediates with relative population up to 0.1% and is also useful to detect the components of exchange hidden in overlapping spectra. © 1993 Springer

Quantitative evaluation of the effect of the medium on the thermodynamic parameters of conformational transitions of substituted 1,3-dioxanes in solutions and gases with PMR data

It was shown that the reaction field model satisfactorily describes the thermodynamic parameters of conformational equilibrium of 2-acetyl-substituted 1,3-dioxanes both as a function of the properties of the solvent and the phase state of the compound studied. Quantitative agreement was obtained between the experimental and theoretical values. © 1992 Plenum Publishing Corporation

Influence of medium on thermodynamic parameters of conformational conversions of certain partially fixed seven-membered 1,3-heterocycles, as indicated by NMR data

It has been shown that the observed differences in enthalpy and entropy between equilibrium states of seven-membered 1,3-heterocycles contained contributions from the temperature dependence of permittivity of the medium. These contributions are comparable in magnitude to the differences in enthalpy and entropy characterizing the conformational equilibrium in such solvents as chloroform and methylene chloride. The changes in enthalpy difference of the conformers chair ⇄ twist and chair ⇄ boat in these compounds when the medium is changed are in good agreement with reactive field theory. © 1990 Plenum Publishing Corporation

Relationship of methane consumption with the respiration of soil and grass-moss layers in forest ecosystems of the southern taiga in Western Siberia

The consumption of methane by some soils in the southern taiga of Western Siberia was studied by the static chamber method in the summer of 2013. The median of the specific CH4 flux through the soil was −0.05 mg C/(m2 h) for the entire set of measurements (the negative flux indicates the consumption of methane by the soil). A statistically significant (R2 = 0.81) linear relationship has been found between the specific CH4 flux to the soil and the total respiration of the soil and the grass-moss layers in the studied forest ecosystems. The quantitative theoretical explanation of this relationship is based on the plant-associated and free methanotrophy

Determination of the effect of the medium on the thermodynamic parameters of the conformational transformations of 3-substituted 2,4-dithia-3,5-dihydro-1H-cycloocta[d,e]naphthalenes by double fitting of exchange-broadened PMR spectra

The thermodynamic parameters of the conformational equilibrium for 3-substituted 2,4-dithia-3,5-dihydro-IH-cycloocta[d,e] naphthalenes in various solvents were obtained by the PMR method with temperature variation. The "double fitting" procedure makes it possible to eliminate the systematic error due to the effect of temperature on the spectral parameters of each of the forms. Here the linear temperature dependence of the nonequivalence in the chemical shifts of the AB quadruplets leads to good agreement between the theoretical and experimental line shapes over the whole temperature range of the investigations. The effect of the medium on the enthalpy of the equilibrium can be explained in terms of the reactive field model. © 1991 Plenum Publishing Corporation

Study of the multipositional conformational exchange in 2,2-dimethyl-1,3-dithiepin and its 5,6-benz analog by two-dimensional NMR spectroscopy

Two-dimensional NMR spectroscopy was used to establish that the chair ⇄ chair-inversion process in 2,2-dimethyl-1,3-dithiepin and its 5,6-benz derivative is a function of transitions between intermediate boat and twist structures, while the role of direct chair ⇄ chair transitions is small. © 1990 Plenum Publishing Corporation

Electrochemical reduction of α,β-unsaturated diphosphonium salts on a mercury electrode

1. Study has been made of the electrochemical reduction of the salts of trans-1,2-diphosphonioethene on the dropping Hg electrode in protogenic and aprotic media. 2. It has been shown that stable radical anions result from the first, reversible one-electron stage of the reduction of disalts in aprotic media. A scheme is proposed to describe the reduction mechanism. © 1977 Plenum Publishing Corporation

Influence of the solvent on the activation parameters of the conformational conversions of 2-substituted 1, 3-dithia-5,6-benzocycloheptenes

The enthalpy and entropy of activation of the chair(e)-boat(e) interconversion for 2-substituted 1,3-dithia-5,6-benzocycloheptenes depend significantly on the type of the solvent, whereby the changes in the enthalpy of equilibrium and of activation are not determined by the same factors when the solvent is changed. © 1989 Plenum Publishing Corporation

Anion and cation radicals of certain unsaturated compounds whose molecules contain two phosphorus atoms

1. Cation and anion radicals of unsaturated compounds whose molecules contain two phosphorus atoms have been generated electrochemically and their properties studied by ESR methods. 2. These ion radicals are π-electron systems and in which the unpaired electron is delocalized over the P-C=C-P fragment. The stability of the ion radicals is discussed. © 1978 Plenum Publishing Corporation

Amino-nitrile cleavage in the electrochemical reduction of hydeazones of aromatic aldehydes

1. Factors which determine the possibility of amino-nitrile cleavage of hydrazones on electrochemical reduction (ECR) include the basicity of the anionic product formed in the course of the ECR and the mobility of the aldehyde hydrogen which depends on the character of the electron polarization of the hydrazone fragment and the polarity of the N-N bond. 2. The primary action in amino-nitrile cleavage under conditions of ECR is the deprotonation of the azomethine fragment in the unreduced molecule by electrochemically generated strong base (anion or dianion). © 1988 Plenum Publishing Corporation