Effect of an external periodic field on the rate of chemical reactions

The rates of direct and concert passages of nuclei through a potential barrier in an external periodic field were determined within a generalized Slater model of thermal dissociation of multiatomic molecules. The external field accelerated direct and — to a much greater degree — concert passages. The curves of the efficiency of the concert mechanism of the chemical reaction as a function of the field amplitude were estimated.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 29, No. 2, pp. 127–132, March–April, 1993.     

Statistical mechanical theory for steady state systems. VI. Variational principles

Several variational principles that have been proposed for nonequilibrium systems are analyzed. These include the principle of minimum rate of entropy production due to Prigogine [Introduction to Thermodynamics of Irreversible Processes (Interscience, New York, 1967)], the principle of maximum rate of entropy production, which is common on the internet and in the natural sciences, two principles of minimum dissipation due to Onsager [Phys. Rev. 37, 405 (1931)] and to Onsager and Machlup [Phys. Rev. 91, 1505 (1953)], and the principle of maximum second entropy due to Attard [J. Chem.. Phys. 122, 154101 (2005); Phys. Chem. Chem. Phys. 8, 3585 (2006)]. The approaches of Onsager and Attard are argued to be the only viable theories. These two are related, although their physical interpretation and mathematical approximations differ. A numerical comparison with computer simulation results indicates that Attard's expression is the only accurate theory. The implications for the Langevin and other stochastic differential equations are discussed.     

DFT-based chemical reactivity indices in the Hartree-Fock method. II. Fukui function, chemical potential, and hardness

A derivation of the density-functional-theory- (DFT) based reactivity indices in the ensemble unrestricted Hartree-Fock (eUHF) method is presented. The comparison between the properties of the reactivity indices evaluated in one and two sets of spin-orbital approach of the eUHF and hyper-unrestricted Hartree-Fock (UHF) methods are shown. All approaches give similar Fukui function irrespective of methodology used, but significantly differ for the global indices, containing important chemical information, and so their interpretation in terms of DFT- based indices can be questionable. The calculation scheme for the indices using the first- and second-order coupled perturbed eHF equations is proposed. A method for the identification of the spinorbitals involved in the change of the total number of electrons is included. The illustrative examples (water and hydrogen cyanide) show that the ground-state (GS) properties of the (Z +/- 1)-electron systems can be predicted from the GS properties of the Z-electron systems with an accuracy comparable with the UHF calculations. The relaxation effect, important for the HCN system in which a change in the symmetry of the highest-occupied spin-orbital occurs, is effectively predicted.     

A heuristic potential function for intramolecular and intrasupermolecular chemical reactions

A heuristic fitting procedure to obtain an analytical potential function for describing a reactive potential energy surface in the neighborhood of the intrinsic reaction coordinate (IRC ) has been developed. For discussion, the pairwise potential function form, ∑a_nr^?n, is assumed in order to fit ab initio quantum mechanical calculations of intramolecular (or intrasupermolecular) interaction energies and its use is found advantageous because all the calculation can be carried out by the linear least squares method. Normal modes perpendicular to IRC are utilized to prepare an initial data base for the potential fitting in the neighborhood of IRC . Some trial molecular dynamics (MD ) simulations are performed in order to check the fitted potential function and, unless they lead to reasonable energies within the tolerance assumed, their results are utilized to construct an improved data base (the dynamic sampling). The present systematic optimization procedure has been applied to the proton transfer reaction of the formamidine–water (FW ) system. The normal mode analysis in both the transition state (TS ) and the stable state (SS ) regions suggests that the present fitted potential function can reproduce satisfactorily the Born–Oppenheimer (BO ) adiabatic surface obtained by ab initio molecular orbital (MO ) calculations. We conclude that our procedure works well for the chemical reaction molecular dynamics (CRMD ) simulation.     

Convolution potential sweep voltammetry: Part IV. Homogeneous follow-up chemical reactions

The application of Convolution Potential Sweep Voltammetry to mechanism analysis and rate determination in electrochemical processes involving homogeneous chemical reaction is discussed. The formal analysis of the transition between pure diffusion control and pure kinetic condition is treated in the case of a first order follow-up reaction. The practical applicability of the method is then tested on the reductive pinacolization of acetophenone in acetonitrile using as operational parameters the sweep rate, the initial concentration and the water content of the medium.     

Generalization of classical mechanics for nuclear motions on nonadiabatically coupled potential energy surfaces in chemical reactions

Classical trajectory study of nuclear motion on the Born-Oppenheimer potential energy surfaces is now one of the standard methods of chemical dynamics. In particular, this approach is inevitable in the studies of large molecular systems. However, as soon as more than a single potential energy surface is involved due to nonadiabatic coupling, such a naive application of classical mechanics loses its theoretical foundation. This is a classic and fundamental issue in the foundation of chemistry. To cope with this problem, we propose a generalization of classical mechanics that provides a path even in cases where multiple potential energy surfaces are involved in a single event and the Born-Oppenheimer approximation breaks down. This generalization is made by diagonalization of the matrix representation of nuclear forces in nonadiabatic dynamics, which is derived from a mixed quantum-classical representation of the electron-nucleus entangled Hamiltonian [Takatsuka, K. J. Chem. Phys. 2006, 124, 064111]. A manifestation of quantum fluctuation on a classical subsystem that directly contacts with a quantum subsystem is discussed. We also show that the Hamiltonian thus represented gives a theoretical foundation to examine the validity of the so-called semiclassical Ehrenfest theory (or mean-field theory) for electron quantum wavepacket dynamics, and indeed, it is pointed out that the electronic Hamiltonian to be used in this theory should be slightly modified.     

New vertex invariants and topological indices of chemical graphs based on information on distances

By applying information theory to the set of topological distances from one vertex to all other graph vertices, one obtains four new types of vertex invariants (u _i,v _i,x _i,Y _i) which are real numbers (as opposed to integers). They may be combined in many ways to afford new topological indices. One such type leads to indicesU, V, X andY which show no degeneracy for alkanes with up to 15 vertices.     

Solid-phase fluorescence in chemical test methods of analysis based on the principles of planar chromatography

Morin immobilized on cellulose matrices was used as a reagent for determining Al, Be, and Zr(IV) by the length of the fluorescent zone on a test strip after its contact with the test solution and by the fluorescence intensity of the reaction zone of the indicator matrix after passing the test solution through it. A test procedure was developed for determining 0.2–200 mg/L Al in potable water using paper test strips sealed in a polymer film. Test procedures were developed for determining 0.0001–1 mg/L Al and 0.00001–0.1 mg/L Be in river water by the intensity of the fluorescent zone on the indicator viscose fabric after passing 100 mL of the test solution through it, using a test facility. The time of analysis was 15–20 min. The relative standard deviation did not exceed 40% in the former case and 60% in the latter case.     

Plotting phase portraits of oscillatory chemical reactions based on experimental data

We consider plotting phase portraits for oscillatory chemical reactions based on synchronous recording of several physical parameters for the example of the reaction of oxidation of malonic acid by bromate in the presence of cerium(IV, III). The phase portraits are plotted on the basis of the change in the physical parameter of the oscillatory system after equal time intervals for systems with steadily damped and chaotic oscillations. We isolate a special class of oscillatory reactions with pulsed changes in the potential. The phase portraits for such systems can be plotted using the intervals between the pulses or their height.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 4, pp. 488–492, July–August, 1988.In conclusion, the author thanks L. N. Zakrevskii and E. V. Rybak-Akimova for providing experimental data and discussion of this paper.     

Reactivity of polyesters in glycolysis reactions: Unexpected effect of the chemical structure of the polyester glycolic unit

Kinetic studies of PET glycolysis by diethylene glycol (DEG), dipropylene glycol (DPG), glycerol (Gly) and mixtures of these glycols have shown, in a previous study, that the order of reactivity of the glycols differs according to the conditions of temperature and catalysis. Indeed, their global reactivity depends both on their chemical reactivity and physico-chemical properties.Glycolysis of model polyesters which are liquid at the reaction temperature, which allows us to overcome the problem of the polyesters' solubility, were studied to compare the chemical reactivity of these glycols. Three oligoesters were synthesized from dimethyl terephthalate and three different glycols namely triethylene glycol, ethylene glycol and hexanediol to form, respectively, PE₃T, OET and PTHD.Results showed that the order of reactivity of the glycols is the same for PET, OET and PTHD but different for PE₃T. Indeed, DPG without catalyst has a particular and unexpected behaviour: its reactivity seems to be strongly influenced by the presence of oxygen atoms in the chain.     

Exploring potential energy surfaces for chemical reactions: an overview of some practical methods

Potential energy surfaces form a central concept in the application of electronic structure methods to the study of molecular structures, properties, and reactivities. Recent advances in tools for exploring potential energy surfaces are surveyed. Methods for geometry optimization of equilibrium structures, searching for transition states, following reaction paths and ab initio molecular dynamics are discussed. For geometry optimization, topics include methods for large molecules, QM/MM calculations, and simultaneous optimization of the wave function and the geometry. Path optimization methods and dynamics based techniques for transition state searching and reaction path following are outlined. Developments in the calculation of ab initio classical trajectories in the Born-Oppenheimer and Car-Parrinello approaches are described.     

Reactivity of amino acids in nitrosation reactions and its relation to the alkylating potential of their products

Nitrosation reactions of amino acids with an -NH(2) group [namely, six alpha-amino acids (glycine, alanine, alpha-aminobutyric acid, alpha-aminoisobutyric acid, valine, and norvaline); two beta-amino acids (beta-alanine and beta-aminobutyric acid), and one gamma-amino acid (gamma-aminobutyric acid)] were studied. Nitrosation was carried out in aqueous acid media, mimicking the conditions of the stomach lumen. The rate equation was r = k(3)(exp)[amino acid][nitrite](2), with a maximum k(3)(exp) value in the 2.3-2.7 pH range. The existence of an isokinetic relationship supports the argument that all the reactions share a common mechanism. A nitrosation mechanism is proposed, and the following conclusions are drawn: (i) Nitrosation reactions of amino acids with a primary amino group in acid media occur with dinitrogen trioxide as the main nitrosating agent. The finding that the nitrosation rate is proportional to the square of the nitrite concentration suggests that the yield of nitrosation products in the stomach would increase sharply with higher nitrate/nitrite intakes. (ii) Stomach hypochlorhydria could be a potential enhancer of in vivo amino acid nitrosation. (iii) The reactivity (k(3)()(exp)) [alpha-amino acids > beta-amino acids > gamma-amino acids] is the same as that found in a previous work for the alkylating potential of lactones formed from nitrosation products of the same amino acids. This implies that the nitrosation reactions of the most common natural amino acids are the most efficient precursors of the most powerful alkylating agents. (iv) The order of magnitude (10(7)-10(8) M(-1) s(-1)) of the bimolecular rate constants of nitrosation shows that such reactions occur through an encounter process.     

Reactivity and selectivity of homolytic substitution reactions on the peroxyacid group

Peracids in solution may undergo homolytic decomposition leading mostly to mixtures of acid and alcohols, these Latters being formed from either the peracid or the solvent

The peracid behaves as source of radicals and also as substrate thus presenting two radicophile centers

- with a nucleophilic radical (R' from peracid or S' from solvent) the site of attack is the 0-0 bond of the peracid yielding an alcohol (NuOH). The rate of this reaction increases with increasing nucleophilicity of the radical ; this reactivity is interpreted in terms of frontier orbital perturbation theory, taking into account both the orbital energy level difference and the orbital overlap.- when the radical is electrophilic, it abstracts rather the hydrogen of the peroxidic group Leading to decomposition of the peracid into acid.Hydrogen abstraction from the solvent may be also observed in appropriate cases : this process can be used for free radical hydroxylation of hydrocarbons.The relative importance of these three chain reactions in competition : 0-0 bound attack, hydrogen abstraction from the peracid and solvent transfer reaction were studied as a function of structure of the radicals, solvent properties and temperature.     

Unique description of chemical structures based on hierarchically ordered extended connectivities (HOC procedures). VIII. General principles for computer implementation

The computer implementation of the HOC procedures presented in Parts I, II and III of this series is based on the module principle. It contains a main program and seventeen subroutines in Fortran IV. The system is able to provide unique atom numbering and code for all kinds of chemical compounds in their tremendous variety of structures.