Leonid I. Trakhtenberg

Temperature dependence of cryochemical H-tunneling reactions

The temperature dependence of tunneling transition of the atomic particle in solids is studied near absolute zero. The different mechanisms of the temperature dependence are considered. They are the medium reorganization, the potential barrier parameters modulation, and the under-barrier friction The nonadiabatic effects are also considered. The rate constant K is described by formula lnu200aK=lnu200aK0+C4T4+C5T5+C6T6+C8T8 at low temperatures. It was conducted through the comparison of theory with the experimental data from the article of Kumada et al. [Chem. Phys. Lett. 261, 463 (1996)]. It turned out that good agreement takes place if one takes into account the quantum properties of the hydrogen crystal with the assumption of the dominated role of medium reorganization.

Matrix reorganization with intramolecular tunneling of H atom: Formic acid in Ar matrix

The intramolecular tunneling of hydrogen atom in formic acid HCOOH at low temperatures is considered based on literature experimental data on C-O internal rotation. The energetic and geometric parameters as well as vibrational frequencies for formic acid in cis and trans configurations surrounded by 12 Ar atoms are calculated in the frame of the MP2 approach with extended basis sets. The temperature and pressure dependence of the rate constant is analyzed taking into consideration the matrix reorganization for the Debye model of lattice motion. It has been shown that the available experimental data can be explained by the suggested matrix reorganization mechanism. Theoretical expressions for the temperature dependence of the rate constant agree well with the experimental data on the cis to trans tunneling reactions in formic acid with fitting parameters attaining reasonable values. A mechanism describing pressure dependence of the rate constant for H-atom intramolecular tunneling reactions is also proposed.

Effect of the Medium on Intramolecular H-Atom Tunneling: Cis-Trans Conversion of Formic Acid in Solid Matrixes of Noble Gases

Intramolecular tunneling of a hydrogen atom in formic acid at low temperatures has been studied theoretically on the basis of quantum-chemical modeling of HCOOH@Nb(12) clusters. Three noble matrixes (Ar, Kr, and Xe) are considered. Energetic and geometric parameters as well as vibrational frequencies for the formic acid in cis and trans configurations surrounded by 12 Nb atoms are calculated within the frame of the MP2 approach with extended basis sets. The rate constant of HCOOH cis-trans conversion is analyzed by taking into account matrix reorganization and the change of HCOOH position in the cluster. The matrix reorganization is considered within the Debye model of lattice vibrations, whereas the external motion of HCOOH in the cluster is treated using the Einstein model of solids. It has been shown that the literature experimental data on the cis to trans tunneling reaction in the formic acid can be accounted for within the proposed mechanism, which describes the matrix reorganization and the change of the HCOOH position in the noble gas matrix, with fitting parameters of the suggested theoretical model attaining reasonable values.

Complex Dielectric Permittivity of Metal-Containing Nanocomposites: Non-phenomenological Description

Addition of metal nanoparticles radically alters the complex dielectric permittivity of a matrix-insulator. A non-phenomenological theory describing these changes is developed by assuming that a large concentration of electron traps with the critical binding energy in the amorphous matrix exists. Such traps in the vicinity of the nanoparticles can be partially occupied by the electrons. The trapped electron with its neighbor nanoparticle then represents an appreciable dipole moment. Reorientations of this moment in the external electric field occurs due to electron jumps between the traps over a sphere surrounding a nanoparticle. The calculation of the interaction of the dipole moment with an external field is carried out taking into account the dielectric permittivity of the matrix. By deriving and solving the equation for the dipole relaxation function, the real and imaginary parts of the permittivity may be evaluated. The calculated dependences of the dielectric permittivity on frequency and temperature agree qualitatively with experiment.

Reagent reorganization and promotive modes in barrier preparation for H-tunneling in fluorene-acridine system

Abstract For the hydrogen-atom transfer reaction in the fluorene–acridine system the set of two-dimensional potential energy surfaces (PESs) of the double adiabatic approximation type is examined on the PM3 semi-empirical level. The rate constant expression derived for this reaction is used to find the optimal atomic configuration of reagents, for which the potential energy barrier is minimal. The energy of reagent reorganization is also evaluated and its important role for describing this process is shown. The theoretical calculations of the rate constant are in agreement with the available experimental data, when the different promotive modes (translational, librational and two intramolecular) are taken into account.

Fluctuation-induced transport of two coupled particles: Effect of the interparticle interaction

We consider a system of two coupled particles fluctuating between two states, with different interparticle interaction potentials and particle friction coefficients. An external action drives the interstate transitions that induces reciprocating motion along the internal coordinate x (the interparticle distance). The system moves unidirectionally due to rectification of the internal motion by asymmetric friction fluctuations and thus operates as a dimeric motor that converts input energy into net movement. We focus on how the law of interaction between the particles affects the dimer transport and, in particular, the role of thermal noise in the motion inducing mechanism. It is argued that if the interaction potential behaves at large distances as x(α), depending on the value of the exponent α, the thermal noise plays a constructive (α > 2), neutral (α = 2), or destructive (α < 2) role. In the case of α = 1, corresponding piecewise linear potential profiles, an exact solution is obtained and discussed in detail.

Study of the In2O3 molecule in the free state and in the crystal

ABSTRACT The nanomaterials based on the In2O3 molecule are widely used as catalysts and sensors among other applications. In the present study, we discuss the possibility of using nanoclusters of In2O3 as molecular photomotors. A comparative analysis of the electronic structure of the In2O3 molecule in the free state and in the crystal is performed. For the free In2O3 molecule the geometry of its lowest structures, V-shape and linear, was optimised at the CCSD(T) level, which is the most precise computational method applied up to date to study In2O3. Using experimental crystallographic data, we determined the geometry of In2O3 in the crystal. It has a zigzag, not symmetric structure and possesses a dipole moment with magnitude slightly smaller than that of the V-structure of the free molecule (the linear structure due to its symmetry has no dipole moment). According to the Natural Atomic population analysis, the chemical structure of the linear In2O3 can be represented as O = In−O−In = O; the V-shaped molecule has the similar double- and single-bond structure. The construction of nanoclusters from ʻbricksʼ of In2O3 with geometry extracted from crystal (or nanoclusters extracted directly from crystal) and their use as photo-driven molecular motors are discussed.

Theory of temperature dependence of hydrogen tunneling reactions

Abstract The different possible mechanisms of the temperature dependence of the tunnelling transition of an atomic particle in solids near absolute zero and the non-adiabatic effects are determined and considered. The general expression for the rate constant is obtained, taking into account the quantum properties of hydrogenous crystal. Comparison of the theoretical formula with experimental data [3] has permitted the leading mechanism, i.e. medium reorganization, of temperature dependence to be established. Good agreement has been achieved between theory and the discussed experimental data at reasonable values of the system parameters.