Molecular dynamics simulations of elementary chemical processes in liquid water using combined density functional and molecular mechanics potentials. II. Charge separation processes

A new approach to carry out molecular dynamics simulations of chemical reactions in solution using combined density functional theory/molecular mechanics potentials is presented. We focus our attention on the analysis of reactive trajectories, dynamic solvent effects and transmission coefficient rather than on the evaluation of free energy which is another important topic that will be examined elsewhere. In a previous paper we have described the generalities of this hybrid molecular dynamics method and it has been employed to investigate low energy barrier proton transfer process in water. The study of processes with activation energies larger than a few kT requires the use of specific techniques adapted to “rare events” simulations. We describe here a method that consists in the simulation of short trajectories starting from an equilibrated transition state in solution, the structure of which has been approximately established. This calculation is particularly efficient when carried out with parallel computers since the study of a reactive process is decomposed in a set of short time trajectories that are completely independent. The procedure is close to that used by other authors in the context of classical molecular dynamics but present the advantage of describing the chemical system with rigorous quantum mechanical calculations. It is illustrated through the study of the first reaction step in electrophilic bromination of ethylene in water. This elementary process is representative of many charge separation reactions for which static and dynamic solvent effects play a fundamental [email protected]

An accurate coulometric method for the chlorometric determination of sulfonamides

A coulometric method for the chlorometric determination of microquantities (0.04-0.1 mg) of sulphaphenazole, sulphadimidine and sulphamerazine, is described. The titration is carried out with a constant current of 2 mA, in the presence of methyl orange as indicator. The results obtained are accurate and reproducible; the method is simple and rapid and can be applied to routine analysis of investigated sulfonamides, within the concentrations examined

Method for determination of phenylbutazone and oxyphenylbutazone by bromocoulometric titration

A method for the quantitative determination of phenylbutazone and oxyphenylbutazone by bromocoulometric titration is presented. Bromine is generated at the anode and, under the titration conditions applied, it is added quantitatively to the double bond of the enol form of dioxophenyl-pyrazolidine. The titration end-point is detected by the decolorization of methyl orange solution used as the indicator. The results obtained show that the method is reproducible and accurate for the determination of small quantities of these analgesics

Solvatation et réactivité de l'ion bromure. une étude expérimentale de la dynamique de l'équilibre [math] dans les solvants protiques

Les constantes de vitesse directe, k+ , et inverse, k-, de l'équilibre [math] sont mesurées dans l'eau, le méthanol et l'acide acétique à l'aide d’une technique de relaxation chimique par saut de température. Les constantes directes, k+, comprises entre 1 et 2,5 109 M-1s-1 ainsi que les constantes k - (107 < k < 5 107s-1) sont toutes deux sensibles au solvant. Bien que très rapide, la réaction de l'ion Br- sur le brome n’est vraisemblablement pas controlée par la diffusion. Au cours de l'interprétation de l'effet de solvant, le mécanisme classique où l'étape limitante serait la désolvatation de l’ion Br-, est critiqué eu égard à la très faible stabilité des complexes de solvatation des ions halogénure