Search results for "reaction rate constant"
showing 10 items of 210 documents
Atom-based non-stochastic and stochastic bilinear indices: Application to QSPR/QSAR studies of organic compounds
2008
The recently introduced bilinear indices are applied to the QSAR/QSPR studies of heteroatomic molecules. These novel atom-based molecular fingerprints are used to predict the boiling point of 28 alkyl-alcohols and partition coefficient, specific rate constant and antibacterial activity of 34 2-furylethylenes derivatives. The obtained models are statistically significant and show rather very good stability in a cross-validation experiment. The comparison with other approaches exposes a good behavior of our method in this QSPR studies. The obtained results suggest that with the present method, it is possible to obtain a good estimation of physical, chemical and physicochemical properties for …
Organic solvents vapor pressure and relative humidity effects on the phase transition rate of α and β forms of tegafur.
2011
The objective of this work was to investigate the relative humidity (RH) and solvent vapor pressure effects on the phase transition dynamics between tegafur polymorphic forms that do not form hydrates and solvates. The commercially available α and β modifications of 5-fluoro-1-(tetrahydro-2-furyl)-uracil, known as the antitumor agent tegafur, were used as model materials for this study. While investigating the phase transitions of α and β tegafur under various partial pressures of methanol, n-propanol, n-butanol, and water vapor, it was determined that the phase transition rate increased in the presence of solvent vapors, even though no solvates were formed. By increasing the relative air h…
Modelling phase transition kinetics of chenodeoxycholic acid with the Runge–Kutta method
2009
Abstract The phase transition kinetics of two chenodeoxycholic acid polymorphic modifications— form I (stable at high temperature), form III (stable at low temperature) and the amorphous phase has been examined under various conditions of temperature and relative humidity. Form III conversion to form I was examined at high temperature conditions and was found to be non-spontaneous, requiring seed crystals for initiation. The formation kinetic model of form I was created incorporating the three-dimensional seed crystal growth, the phase transition rate proportion to the surface area of form I crystals, and the influence of the amorphous phase surface area changes with an empirical stage poin…
Solvent-mediated phase transformation between two tegafur polymorphs in several solvents
2014
This paper describes a study of the solvent-mediated polymorphic transformation (SMPT) of the metastable α tegafur to the thermodynamically stable β tegafur in several solvents. Phase transformation in acetone, ethanol, i-propanol, toluene, and water at 22 °C was described using the solid-state kinetic model P2; the rate constants for this process were in the range from 0.028 min−1 to 0.0056 min−1. In all of the employed solvents, an induction time was observed. Kinetic, solubility and scanning electron microscopy data indicated that nucleation kinetics corresponded to a second-order power function and according to the kinetic model, the nuclei growth rate was constant in the examined SMPT.…
Electro- and photo-electrooxidation of 2,4,5-trichlorophenoxiacetic acid (2,4,5-T) in aqueous media with PbO2, Sb-doped SnO2, BDD and TiO2-NTs anodes…
2020
Abstract In this work, the 2,4,5-T degradation in aqueous media was studied by electro- (EC) and photoelectrochemical (PEC) oxidation methods, which are two of the most important advanced oxidation processes (AOPs) for wastewater treatment. Both EC and PEC experiments were carried out in a single compartment cell under galvanostatic conditions (30 mA cm−2) at 298 K by using 0.05 M Na2SO4 + 200 ppm 2,4,5-T at pH 3 and pH 9, as model solutions. EC oxidation was performed using Sb-doped SnO2, PbO2 and boron doped diamond (BDD) as anodic materials. Besides these anodes, TiO2 in a nanotubular structure (TiO2 NTs) with or without PbO2 dispersed nanoparticles (TiO2::PbO2) were also used for PEC ox…
Charge-Separation and Charge-Recombination Rate Constants in a Donor–Acceptor Buckybowl-Based Supramolecular Complex: Multistate and Solvent Effects
2021
The kinetics of the nonradiative photoinduced processes (charge-separation and charge-recombination) experimented in solution by a supramolecular complex formed by an electron-donating bowl-shaped truxene-tetrathiafulvalene (truxTTF) derivative and an electron-accepting fullerene fragment (hemifullerene, C30H12) has been theoretically investigated. The truxTTF·C30H12 heterodimer shows a complex decay mechanism after photoexcitation with the participation of several low-lying excited states of different nature (local and charge-transfer excitations) all close in energy. In this scenario, the absolute rate constants for all of the plausible charge-separation (CS) and charge-recombination (CR)…
Picosecond time-resolved study on the nature of high-energy-state quenching in isolated pea thylakoids different localization of zeaxanthin dependent…
1996
Abstract The influence of the transthylakoid proton gradient on the kinetics of picosecond fluorescence decay was examined using isolated pea thylakoids having high or low zeaxanthin contents. Fluorescence lifetime measurements were performed with open (Fo) and closed (Fm) PS II reaction centers. Zeaxanthin formation in membrane energized isolated thylakoids led to a marked decrease of the average fluorescence lifetime at both Fm and Fo. In contrast, when zeaxanthin synthesis was blocked by the inhibitor DTT, the fluorescence lifetime decrease was less pronounced in the Fm state and totally missing in the Fo state. Samples containing the uncoupler ammonium chloride did not exhinit any zeaxa…
First-Principles Computed Rate Constant for the O + O 2 Isotopic Exchange Reaction Now Matches Experiment
2018
We show, by performing exact time-independent quantum molecular scattering calculations, that the quality of the ground electronic state global potential energy surface appears to be of utmost importance in accurately obtaining even as strongly averaged quantities as kinetic rate constants. The oxygen isotope exchange reaction, 18O + 32O2, motivated by the understanding of a complex long-standing problem of isotopic ozone anomalies in the stratosphere and laboratory experiments, is explored in this context. The thermal rate constant for this key reaction is now in quantitative agreement with all experimental data available to date. A significant recent progress at the frontier of three rese…
The influence of the solvent's mass on the location of the dividing surface for a model Hamiltonian
2019
The Transition State dividing surface is a key concept, not only for the precise calculation of the rate constant of a reaction, but also for the proper prediction of product ratios. The correct location of this surface is defined by the requirement that reactive trajectories do not recross it. In the case of reactions in solution the solvent plays an important role in the location of the dividing surface. In this paper we show with the aid of a model Hamiltonian that the effective mass of the solvent can dramatically change the location of the dividing surface. Keywords: Dynamical systems, Dividing surface, Reactions in solution, 2019 MSC: 00-01, 99-00
Quantum mechanical study of the 16O + 18O18O → 16O18O + 18O exchange reaction: Integral cross sections and rate constants
2018
The isotopic exchange reaction, 16O + 18O18O → 16O18O + 18O, involving excited ozone, O3*, as intermediate complex, was investigated by means of a time independent quantum mechanical approach using the TKTHS potential energy surface (PES) [V. G. Tyuterev et al., J. Chem. Phys. 139, 134307 (2013)] of ozone. State-to-state integral cross sections were calculated for collision energies lower than 0.4 eV. Then specific and thermal rate constants were computed between 10 K and 350 K using these cross sections. The full quantum thermal rate constant is found to be in better agreement with the most recent experimental data than with previous studies where other O3 PESs were employed, confirming th…