The integral encounter theory of multistage reactions containing association-dissociation reaction stages. III. Taking account of quantum states of reactants

The formalism developed in Part I [K. L. Ivanov, N. N. Lukzen, A. A. Kipriyanov, and A. B. Doktorov, Phys. Chem. Chem. Phys. 6, 1706 (2004)] of the present contribution is extended to treat the reacting particles with internal quantum states. Initial spatial correlations of reactants are considered in the framework of this formalism as well.     

CPMG echo amplitudes with arbitrary refocusing angle: explicit expressions, asymptotic behavior, approximations

Exact explicit analytical expression for echoes in the Carr–Purcell–Meiboom–Gill sequence with arbitrary excitation and refocusing angles and resonance offset of RF pulses was obtained, employing the generating functions formalism developed earlier by authors. Asymptotic form and analytical approximation for echoes were derived in an elegant way and analyzed in details. In particular, it was shown that depending on T₁, T₂ and parameters of the pulse sequence, oscillatory behavior of echoes can take place. Accuracy of asymptotic forms and approximations were tested by comparison with exactly calculated echo amplitudes. Besides, it was shown, that the generating function approach can be applied to the consideration of terminated pulse sequences, when after-pulses echoes are registered.     

Theory of reversible associative-dissociative diffusion-influenced chemical reaction. I. Geminate reaction

A many-particle, in the general case, inhomogeneous reacting system of independent pairs of reactants, where geminate reversible reaction A+B?C takes place, is considered in the framework of the kinetic theory approach. The kinetic equations in the thermodynamic limit are obtained, and important relations between kinetic coefficients characterizing the reaction course are established, as well as the relations between the kinetic dependencies under different initial conditions including the relations that extend the familiar literature results to the case of rather realistic model of the reacting system. Universal long-term kinetic laws of the reaction course are determined.     

Analysis of lipid peroxidation kinetics. 1. Role of recombination of alkyl and peroxyl radicals

The kinetics of the lipid peroxidation reaction is only partly understood. Although the set of reactions constituting the overall reaction are believed to be known, it has not been possible to predict how the reaction will respond to a change of one or more of the parameters, e.g., initial concentrations of reactants or different ways of initiating the reaction, nor has it been possible to predict the time dependence of the products. The reason for these problems is the complicated structure of the kinetic scheme, which includes a chain reaction. In this work we perform an in-depth analysis of the importance of the individual reaction steps, and we introduce a new quasi-stationary concentration method based on the assumption that one or more concentrations vary much slower than the others. We show that it is justified to use a quasi-stationary concentration approximation for the alkyl radical L (*), but not for the peroxyl radicals LO 2 (*) as assumed in previous works. The method allows us to derive manageable analytical expressions. On the basis of literature values of the rate constants, we are able to introduce specific simplifications that allow us to obtain simple analytical expressions for the time dependence of all concentrations involved in the process.     

Averaging of the reactivity anisotropy by the reagent translational motion

The diffusion-controlled reaction of chemically anisotropic molecules is considered. The averaging of stereospecificity by the translational motion of the reagents has been taken into account, this being described as the process of random jumps with an arbitrary jump length λ. A general expression has been obtained for the steric factor which depends substantially on the mechanism of reagent motion. In the limit of small λ (diffusion) the results fit those reported earlier. Physically clear interpretation of the results obtained based on consideration of recontact statistics is given.     

Isotopic effects due to energy transfer in photechemical reactions

Strong H/D isotopic effects were observed in photochemical decomposition of acetaldehyde and some other organic substances at 77 and 4.2 K. The value of the isotopic effect increased considerably over the temperature range 77 to 4.2 K and remained constant over a wide range of the isotopic molecule ratio as was shown for acetaldehyde. The results obtained were explained in terms of trapping the electron excitation by molecules with lighter isotopes which are known to have, as a rule, lower excitation energy. A theory was developed taking into account the migration of the electron excitation among similar molecules, the energy exchange between two isotopic species, and their photochemical decomposition. The temperature and concentration dependences of the isotopic effects predicted by the theory were in accord with experiment.     

New Convenient Reagents for Chemoselective N‐Alkoxycarbonylation of (S)‐Isoserine: Application in the Isepamicin Synthesis

A synthesis of a series of N‐alkoxycarbonyl mercaptobenzothiazoles (MBTs) and their application as reagents for chemoselective protection of amino group are presented herein. It was shown that all new reagents, Z‐MBT, Fmoc‐MBT, Phoc‐MBT, and Tec‐MBT, are highly effective in the selective N‐alkoxycarbonylation of (S)‐isoserine. The transformation is a simple, fast, and low‐cost protocol, which is applicable in scale‐up experiments. The starting MBT was fully recovered at the end of the process, which is an additional advantage of the method. The efficiency of the Z‐reagent was also demonstrated by the selective protection of both gentamicin B and (S)‐isoserine before their peptide‐type coupling in the synthesis of the aminoglycoside antibiotic isepamicin.