Analysis of the Reaction Rate Coefficients for Slow Bimolecular Chemical Reactions

Simple bimolecular reactions $A_1+A_2\rightleftharpoons A_3+A_4$ are analyzed within the framework of the Boltzmann equation in the initial stage of a chemical reaction with the system far from chemical equilibrium. The Chapman-Enskog methodology is applied to determine the coefficients of the expansion of the distribution functions in terms of Sonine polynomials for peculiar molecular velocities. The results are applied to the reaction $H_2+Cl\rightleftharpoons HCl+H$, and the influence of the non-Maxwellian distribution and of the activation-energy dependent reactive cross sections upon the forward and reverse reaction rate coefficients are discussed.Comment: 11 pages, 5 figures, to appear in vol.42 of the Brazilian Journal of Physic

Nonequilibrium thermodynamics and energy efficiency in weight loss diets

Carbohydrate restriction as a strategy for control of obesity is based on two effects: a behavioral effect, spontaneous reduction in caloric intake and a metabolic effect, an apparent reduction in energy efficiency, greater weight loss per calorie consumed. Variable energy efficiency is established in many contexts (hormonal imbalance, weight regain and knock-out experiments in animal models), but in the area of the effect of macronutrient composition on weight loss, controversy remains. Resistance to the idea comes from a perception that variable weight loss on isocaloric diets would somehow violate the laws of thermodynamics, that is, only caloric intake is important ("a calorie is a calorie"). Previous explanations of how the phenomenon occurs, based on equilibrium thermodynamics, emphasized the inefficiencies introduced by substrate cycling and requirements for increased gluconeogenesis. Living systems, however, are maintained far from equilibrium, and metabolism is controlled by the regulation of the rates of enzymatic reactions. The principles of nonequilibrium thermodynamics which emphasize kinetic fluxes as well as thermodynamic forces should therefore also be considered

Competition reaction-based prediction of polyamines’ stepwise protonation constants: a case study involving 1,4,7,10-tetraazadecane (2,2,2-tet)

Theoretical prediction of four stepwise protonation constants of 1,4,7,10-tetraazadecane (2,2,2- tet) in correct order and with the smallest (largest) deviation of about 0.1 (–0.8) log unit from experimental values was achieved by an explicit application of a competition reaction (CRn) methodology in discrete-continuum solvation model involving four explicit water molecules. This methodology performs best when (i) tested (L(1)) and reference (L(2)) molecules are structurally similar, (ii) lowest energy conformers (LECs, selected from all possible tautomers) are used and (iii) a CRn, which assures a balanced charge distribution between reactants and products, Hn–1L(1) + HnL(2) = HnL(1) + Hn–1L(2), is implemented. A 5-step EEBGB-protocol was developed to effectively and in shortest time possible select LECs (E, B and G stands for electronic-energy-, Boltzmann-distribution- and Gibbs-free-energy-based stepwise selection of conformers). The EEBGB-protocol (i) reduced (by 94%) the number of conformers subjected to the frequency calculations (to obtain G-values) from 420 MM-selected to 25 used to compute four protonation constants and (ii) is of general-purpose as it is applicable to any flexible and poly-charged molecules. Moreover, in search for LECs, a rapid pre-screening protocol was developed and tested; it was found efficient for the purpose of this study. Additional research protocols, aimed at even better prediction of protonation constants, are also suggested.This work is based on the research supported in part by the National Research Foundation of South Africa (Grant Numbers 87777) and the University of Pretoria.http://link.springer.com/journal/102672017-05-31hb2016Chemistr