Chemically activated cis-1,2-dimethylcyclopropane from photolysis of cis-2-butene-diazomethane mixtures in the presence of oxygen

Abstract

Experimental evidence obtained in this study verifies reciprocal relations for two nonassociating ternary systems within the limits of experimental error. In addition, it demonstrates the applicability of hydrodynamic theory to multicomponent liquid diffusion. These conclusions are based on the fact that good agreement between the diffusion and phenomenological coefficients obtained by optical methods and those obtained from friction coefficients was found for the nonassociating systems studied.Hopefully experimental techniques will be developed in the future which will enable measurement of quaternary or higher diffusion coefficients, Dij, so that those calculated from friction coefficients, ui, can be checked. Since hydrodynamic theory requires fewer independent friction coefficients than independent diffusion coeficients to describe diffusion in systems of more than three components, such experimental techniques would be extremely valuable in checking the applicability of hydrodynamic theory to such systems.A study of the geometric and structural isomerization rates of chemically activated cis-1,2-dimethylcyclopropane formed by singlet methylene radical addition to the double bond of cis-2-butene is reported. The singlet methylene radicals were produced by diazomethane photolysis a t 4358 and 3660 d in the presence of added oxygen. The structural isomerization rate was determined by an internal comparison method that eliminates uncertainties due to pentene decomposition. The rates are in excellent agreement with RRKM theory calculations at an adjusted energy for each photolysis wavelength. The energies carried into the activated molecule by the methylene radical were 113.0 and 116.8 kcal/mol at 4358 and 3660 A, respectively. These values suggest that the methylene radical carries into the addition reaction a relatively small fraction (0.30) of the total excess energy available from its formation reaction. Energies determined in this work, applied to related systems, give results that differ by at most -3 kcal/moI from earlier determinations.

Chemically activated 1,1‐dimethylcyclopropane from photolysis of isobutene–neopentane–diazomethane–oxygen mixtures

“…Since reliable thermal Arrhenius parameters are available for 1 , l-dimethylcyclopropane isomerizations [8] and since the excitation energies for the chemically activated 1,l-dimethylcyclopropane can be deduced from previous work [3,4], the results of this study may serve either as another check on the basic assumption just discussed or as a check on the reliability of the derived excitation energies.…”

“…Neopentane of unknown purity was purified by gas-liquid partition chromatography (GLPC) and vacuum distillation. Diazomethane was prepared and stored in the manner previously described [3,4]. The source and purification of oxygen was as previously discussed [3,4].…”

“…More recently, the excitation energy of chemically activated cis-1 , 2-dimethylcyclopropane in a diazomethanecis-2-butene photolysis system has been determined where full account has been taken of the effects of triplet methylene radical reactions and product pentenes decomposition [3,4]. The basic assumption in this and the previous work [2] is that RRKM theory calculations will give the correct experimental unimolecular rate constant for a particular chemically activated decomposition, provided the calculation utilizes the appropriate excitation energy E*, critical energy Eo, and a reasonable activated complex model, i.e., one that reproduces via activated complex theory [5] the experimental Arrhenius A-factor for the corresponding thermal unimolecular process.…”

“…Neopentane and oxygen were added to the reaction mixtures to serve as an internal standard and triplet or doublet radical scavenger, respectively. A discussion of the internal standard technique and the use of oxygen as a radical scavenger in methylene reaction systems has been given earlier [3,4].…”

Dinámica de reacciones unimoleculares en fase gas: Desviaciones del comportamiento estadístico

“…Los trabajos más antiguos involucraban sistemas térmicos en los cuales las moléculas se excitaban mediante calentamiento [1][2][3][4][5][6][7][8][9] . La segunda etapa comenzó con la introducción de la técnica de activación quí-mica (principios de los años 60), que permite formar una especie altamente excitada que posteriormente puede reaccionar (isomerizar o descomponerse) o bien perder energía mediante colisiones [10][11][12][13] . La tercera etapa comenzó a primeros de los años 70 con el desarrollo de métodos más generales y precisos para preparar moléculas con energías o en estados definidos, así como el desarrollo de bases de tiempo absolutas para la medida de las velocidades de reacción.…”