Unimolecular reaction thermal studies

It has been generally accepted that the thermal decomposition of paraffinic hydrocarbons proceeds via a free radical chain mechanism [2], In order to explain the different product distributions obtained in terms of experimental conditions (temperature, pressure), two mechanisms were proposed. The first one was by Kossiakoff and Rice [3], This R-K model comes from the studies of low molecular weight alkanes at high temperature (> 600 °C) and atmospheric pressure. In these conditions, the unimolecular reactions are favoured. The alkyl radicals undergo successive decomposition by [3-scission, the main primary products are methane, ethane and 1-alkenes [4], The second one was proposed by Fabuss, Smith and Satterfield [5]. It is adapted to low temperature (< 450 °C) but high pressure (> 100 bar). In this case, the bimolecular reactions are favoured (radical addition, hydrogen abstraction). Thus, an equimolar distribution ofn-alkanes and 1-alkenes is obtained. 

The slow, thermal decomposition of hydrazoic acid in a static system has been studied by Meyer and Schumacher58. It turned out to be completely governed by heterogeneous catalysis. There are no studies on the kinetics of the homogeneous decomposition of this substance save for the investigation of its decomposition flame59. From the variation of flame properties with pressure it can be deduced that second-order reactions control the over-all rate. The unimolecular reaction... 

The thermal decomposition of F202 was studied by Schumacher and Frisch401 who found it to be a homogeneous, unimolecular reaction with the first-order rate coefficient given by... 

Our interest in thermally activated unimolecular reactions is in the change of kuni with pressure from the high to the zero pressure limit, and in the pressure dependence of the isotope effect over that range. One particularly interesting study carried out by Rabinovitch and Schneider (reading list) focused on the isomerization of methyl isocyanide, CH3NC, to methyl cyanide, CH3CN... 

The thermal isomerization of cyclopropane to propylene is perhaps the most important single example of a unimolecular reaction. This system has been studied by numerous workers. Following the work of Trautz and Winkler (1922), who showed that the reaction was first order and had an energy of activation of about 63,900 cal mole measured in the temperature range 550-650° C, Chambers and Kistiakowsky (1934) studied the reaction in greater detail and with higher precision from 469-519° C. They confirmed that it was first order and, for the reaction at its high-pressure limit, obtained the Arrhenius equation... 

Current studies of unimolecular reactions can be broadly divided into three categories, based on different methods of activation of the decomposing species. The first, most classical, method is that of thermal activation of the type first envisioned by Lindemann to explain unimolecular dissociation phenomena brought about by heat energy. The second method involves chemical activation, ... 

For some time the thermal decomposition of phosphine at high temperatures was believed to be a homogeneous unimolecular reaction. It was studied by Trautz and Bhandarkar, who concluded that under the conditions of their experiments, namely in a 3-litre porcelain bulb, the reaction on the walls of the vessel was negligible above 945° abs., in comparison with the homogeneous reaction. 

From a structural point of view, mechanism in a single crystal can be much closer to a set of identical atomic trajectories than to the kind of fuzzy statistical average with which one must be content in solution. It is not surprising that with this kind of structural uniformity the site problems that plague kinetic studies in rigid glasses disappear. Adherence to first-order rate laws can be as close in single crystals as it is in fluids, and equally valid activation parameters can be obtained for thermal unimolecular reactions of reaction intermediates [12]. 

The initial decomposition chemistry involves unimolecular reactions. This was the conclusion of the first gas-phase kinetics study [84] and has been repeatedly confirmed by subsequent bulb and shock-tube experiments [85, 86]. That first study used shock heating to induce thermal decomposition [84], The data were interpreted in terms of simple C-N bond fission to give CH2 and N02. A more extensive and definitive shock-tube study was reported by Zhang and Bauer in 1997 [85]. Zhang and Bauer presented a detailed kinetics model based on 99 chemical reactions that reproduced their own data and that of other shock-tube experiments [84, 86]. An interesting conclusion is that about 40% of the nitromethane is lost in secondary reactions. 

In Sect. 7, we raised the question of what were the chemical stimuli to which the reactivity indices defined in Sect. 6, the softness kernels, were presumed to be the responses, our seventh issue. Now there are various broad categories of reactions to be considered, unimolecular, bimolecular, and multimolecular. The former occur via thermal activation over a barrier, tunneling through the barrier, or some combination of both. There is no stimulus, and the softness kernels defined as responses of the electron density to changes in external or nuclear potential are irrelevant. For the study of unimolecular reactions, one needs only information about the total energy in the relevant configuration space of the molecule. 

Photochemical initiation has often been used as an excellent method of studying radical and chain reactions.1 2 The primary step in many systems is followed by a sequence of steps, which may include conventional unimolecular processes of species having known or calculable energy. Examples are numerous and well known. In order to understand such systems, whether reaction is initiated photochemical ly or thermally, the typical characteristics of unimolecular reactions and their dependence on the energy parameters of the systems and on molecular structure must be clarified. This is the purpose of the present chapter, which will deal principally with the smaller hydrocarbon species below C6. 

As soon as the reactant molecule includes numerous atoms (as is often the case in Organic Chemistry) one just cannot study the overall dynamics of the reaction. In particular, if one must renounce the investigation of the activation phase of the reaction, one must also renounce the attribution of statistical weights to individual trajectories. Then one must postulate, on the basis of either experimental information or physical intuition, initial activated states of the reactant system and study only its subsequent dynamical evolution. Thus the work is restricted to sample in a random way all the possible initial conditions with no attempt to obtain at the end theoretical values of experimental quantities. Nevertheless, this context is not too restrictive. The trajectory study of thermal unimolecular reactions allows one... 

Literature concerning the unimolecular reactions of oxygen containing compounds is very extensive. To cover all the kinetic studies in this field would be virtually impossible in a review of this kind. By necessity we have limited our coverage, in the main, to reactions for which Arrhenius or transition state parameters have been reported. Some relative rate and kinetic isotope studies judged to be reliable, and to contribute significantly to the elucidation of the kinetics, have also been included. Photochemical and irradiation induced reactions do not generally produce unimolecular reactions which can be studied quantitatively therefore, the vast majority of the reactions reviewed here are those induced thermally. 

In his postdoctoral study he got interested in non-equilibrium effects in thermal deeomposition of diatomic molecules. In his paper on thermal unimolecular reactions in 1959 [Pll] he provided a description for thermal aetivation by weak collisions. His earlier work on this subject is summarized in a book [Bl]. 

Thermal decomposition is ideally a unimolecular reaction with a first-order rate constant, kj, which is related to the half-life of the initiator, ti/2, by Eq. (6.29). For academic studies it is convenient to select an initiator whose concentration will not change significantly during the course of an experiment so that instantaneous kinetic expressions, such as Eq. (6.26), may be applicable. From experience it seems that an initiator with a ti/2 of about 10 h at the particular reaction temperature is a good choice in this regard. This corresponds to a kj of 2xl0 s from Eq. (6.29). For the peroxide initiators listed in Table 6.4 the required reaction temperatures for 10 h half-life (ti/2) are also shown. It should be noted, however, that the temperature-half-life relations given in Table 6.4 may vary with reaction conditions, because some peroxides are subject to accelerated decompositions by specific promoters and are also affected by solvents or monomers in the system. 

The study of unimolecular reactions was originally confined to thermal... 

Mies and Krauss have used a related method in their elegant theory of unimolecular reactions. They explicitly employ a generalization of the Fano theory of autoionization, but they have given less emphasis than we to internal energy redistribution since they study, primarily, thermally excited molecules. 

Fast transient studies are largely focused on elementary kinetic processes in atoms and molecules, i.e., on unimolecular and bimolecular reactions with first and second order kinetics, respectively (although conformational heterogeneity in macromolecules may lead to the observation of more complicated unimolecular kinetics). Examples of fast thermally activated unimolecular processes include dissociation reactions in molecules as simple as diatomics, and isomerization and tautomerization reactions in polyatomic molecules. A very rough estimate of the minimum time scale required for an elementary unimolecular reaction may be obtained from the Arrhenius expression for the reaction rate constant, = A. The quantity Hh from transition state theory provides... 

The thermal decomposition reaction of 1,2,4-trioxane, along with others, was studied in toluene solution over a wide temperature range <2000MOL360>. The reaction follows a first-order kinetic law up to ca. 50% peroxide conversion. Only the linear dependence of activation enthalpies and entropies of this unimolecular reaction is reported with a slope of 130.4 °C as the isokinetic temperature . 

Competitive a- and jS-dehydroffaiorination of 1,1-difluoroethane has been studied by chemical activation i and shock wave techniques olefin production via the former mechanism (carbene formation followed by a rapid 1 2-hydrogen shift) was own to contribute ca. 10% to the total elimination in the activation work and ca. 13% in the thermal system. Unimolecular reactions of chemically activated CHs CHF CDs, CHa CHF-CHg CHs, and (CH3)sCF have also been studied. Dehydrochlorination of hydrochlorofluoromethanes is considered later (p. 35). 

In these systems the critical energies for the secondary processes were known from studies of thermal unimolecular reaction rates, and RRKM theory was employed to estimate vibrational energy distributions in the excited fragments, similar techniques having been used in studies of ion fragmentation following electron impact. ... 

Additional information can be obtained from thermal unimolecular reactions when the effect of various collision partners on competing branching channels is studied near the low-pressure limit. - Recent experiments on the competing channels C H4 -> C2H2 + H2 and C2H4 -> C2H3 + H appear to be very promising,"... 

The idea of studying thermal unimolecular reactions at very low pressures under conditions where gas-gas collisions are negligible was introduced by Benson Spokes in 1967, and has been developed extensively both in the original and in variant forms since of interest to us here is the universal finding that gas-wall collisions are more efficient than gas-gas collisions in creating reactive molecules [67.B 73.G 79.KI 80.G2]. 

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