Unimolecular reaction product distribution

Examples of the use of chemical laser techniques to provide unique information concerning reaction product distributions are the studies of Pimentel and Berry and co-workers of unimolecular photoelimination reactions/ " A number of these reactions are listed in Table 3.2. A very detailed study of photoelimination of vibrationally excited HCl from chloro-ethylene was reported by Berry. > Here again, relative times-to-threshold were used to determine relative vibrational populations with a grating selection technique. Figure 3.14 illustrates measured and calculated vibrational distributions of HCl product molecules formed in the t < 4 levels by... 

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. 

Scheme 1 illustrates the design of an experiment that could be used to determine the rate constant for H-atom abstraction from a group 14 hydride. Radical A- reacts with the hydride to give product A-H. In competition with this reaction, radical A- gives radical B- in a unimolecular or bimolecular reaction with a known rate constant, and product radical B- also reacts with the hydride, giving B-H. The rate constant for reaction of A- with the metal hydride can be determined from the product distribution, the known rate constant for conversion of A- to B-, and the concentrations... 

Isomerizations are important unimolecular reactions that result in the intramolecular rearrangement of atoms, and their rate parameters are of the same order of magnitude as other unimolecular reactions. Consequently, they can have significant impact on product distributions in high-temperature processes. A large number of different types of isomerization reactions seem to be possible, in which stable as well as radical species serve as reactants (Benson, 1976). Unfortunately, with the exception of cis-trans isomerizations, accurate kinetic information is scarce for many of these reactions. This is, in part, caused by experimental difficulties associated with the detection of isomers and with the presence of parallel reactions. However, with computational quantum mechanics theoretical estimations of barrier heights in isomerizations are now possible. 

Many association reactions, as well as their reverse unimolecular decompositions, exhibit rate parameters that depend both on temperature and pressure, i.e., density, at process conditions. This is particularly the case for molecules with fewer than 10 atoms, because these small species do not have enough vibrational and rotational degrees of freedom to retain the energy imparted to or liberated within the species. Under these conditions, energy transfer rates affect product distributions. Consequently, the treatment of association reactions, in general, would be different than that of the fission reactions. 

Under ordinary mass spcctrometric conditions only unimolecular reactions of excited ions occur, but at higher ionization chamber pressures bimolecular ion molecule reactions are observed in which both the parent ions and their unimolecular dissociation product ions are reactants. Since it requires a time of 10 5 sec. to analyze and collect the ions after their formation all of the ions in the complete mass spectrum of the parent molecule are possible reactants. However, in radiation chemistry we are concerned with the ion distribution at the time between molecular collisions which is much shorter than 10 5 sec. For example, in the gas phase at 1 atm. the time between collisions is 10 10 sec. and in considering the ion molecule reactions that can occur one must know the amount of unimolecular decomposition within that time. By utilizing the quasi-equilibrium theory of mass spectra6 it is possible to calculate the ion distribution at any time. This has been done for propane at a time of 10 10 sec.,24 and although the parent ion is increased by a factor of 2 the relative ratios of the other ions are about the same as in the mass spectrum observed in 10 r> sec. Thus for gas phase radiolysis the observed mass spectrum is a fair first approximation to the ion distribution. In... 

The lifetimes of the BRs are of critical importance to any attempt at quantitative analysis of the factors which will determine quantum yields and product distributions (E/C and t/c ratios) in Type II reactions of ketones under various reaction conditions. Virtually all information about lifetimes is derived from study of triplet BRs and much of it has been provided, and reviewed, by Scaiano [261]. There are many interesting reactions, both bimolecular and unimolecular, which occur at only one of the radical centers but they have little relevance to this chapter and are not discussed here. BR triplets derived from alkanophenones have lifetimes of 25-50 ns in hydrocarbon solvents. They are lengthened several fold in t-butyl alcohol and other Lewis bases capable of hydrogen bonding to the OH groups of the BRs. The rates of decay are virtually temperature independent but are shortened by paramagnetic cosolutes such as 02 or NO. The quenchers react with the BRs... 

Trajectory calculations for proton transfer and ionization in water cluster,112 116 isomerization,117 and various types of unimolecular reactions6,118 128 have been carried out, and the analyses on time course of the reaction, product ratio, and product energy distribution were reported. 

At this time, no absolute rate constants have been determined for a reaction of an aminium cation radical. However, for synthetic utility, one needs to consider the relative rate constants for competing reactions. Competition between two unimolecular reactions depends only upon the relative rate constants for the processes. For competition between a unimolecular and a bimolecular reaction whose rate constants are comparable, product distributions can easily be controlled by the concentration of the second species in the ratio of rate laws. The ratio of reaction products from cyclization (unimolecular) versus hydrogen atom trapping before cyclization (bimolecular) can be expressed by the equation %(42 + 65)/%41 = Ar/(A H[Y - H]) (Scheme 20). Competition between two bimolecular reactions is dependent on the relative rate constants for each process and the effective, or mean, concentration of each reagent. The ratio of the products from H-atom transfer trapping of the cyclized radical versus self-trapping by the PTOC precursor can be expressed by the equation %42/%65 = (kH /kT) ([Y - H]/[PTOC]). 

The basic assumption in statistical theories is that the initially prepared state, in an indirect (true or apparent) unimolecular reaction A (E) —> products, prior to reaction has relaxed (via IVR) such that any distribution of the energy E over the internal degrees of freedom occurs with the same probability. This is illustrated in Fig. 7.3.1, where we have shown a constant energy surface in the phase space of a molecule. Note that the assumption is equivalent to the basic equal a priori probabilities postulate of statistical mechanics, for a microcanonical ensemble where every state within a narrow energy range is populated with the same probability. This uniform population of states describes the system regardless of where it is on the potential energy surface associated with the reaction. 

Non thermally Activated Unimolecular Reactions. Consider the steady-state system in which an excited species, M, is formed with an energy distribution/(e). The situation is represented by eq. (18), where D and S are the experimentally measured decomposition and stabilization products, respectively,... 

Unimolecular reaction rates and products quantum states distribution... 

The experiments discussed at the end of the previous section provided information about the translational excitation of the products of unimolecular fragmentation of energized species formed in association reactions. The distributions of vibrational energy in the products of some reactions of this, and related, types have been determined by chemical laser measurements and by observations of infrared chemiluminescence. Some of these studies were referred to in Section III.C, other reactions have been studied more recently [388-392], In all of these investigations, the product which has been observed is HF or HC1 formed in what is frequently termed a snap-out reaction. These processes require that, almost simultaneously, two bonds break, the HX bond forms, and the order of a bond in the other product is increased. The reverse reaction, a four-centre (bimolecular) one, has a high activation barrier, so in the snap-out process a considerable proportion of the total energy is released after the system passes through the activated state. Thus reaction (120)... 

Recent years have also witnessed exciting developments in the active control of unimolecular reactions [30,31]. Reactants can be prepared and their evolution interfered with on very short time scales, and coherent hght sources can be used to imprint information on molecular systems so as to produce more or less of specified products. Because a well-controlled unimolecular reaction is highly nonstatistical and presents an excellent example in which any statistical theory of the reaction dynamics would terribly fail, it is instmctive to comment on how to view the vast control possibihties, on the one hand, and various statistical theories of reaction rate, on the other hand. Note first that a controlled unimolecular reaction, most often subject to one or more external fields and manipulated within a very short time scale, undergoes nonequilibrium processes and is therefore not expected to be describable by any unimolecular reaction rate theory that assumes the existence of an equilibrium distribution of the internal energy of the molecule. Second, strong deviations Ifom statistical behavior in an uncontrolled unimolecular reaction can imply the existence of order in chaos and thus more possibilities for inexpensive active control of product formation. Third, most control scenarios rely on quantum interference effects that are neglected in classical reaction rate theory. Clearly, then, studies of controlled reaction dynamics and studies of statistical reaction rate theory complement each other. 

Basic Catalysis. The catalytic properties of alkali zeolites free of acidic sites have been investigated for the cracking of hexanes (25, 26). At 500 C K-Y zeolite cracks easily n-hexane and its isomers resulting in product distributions markedly different from those obtained over acidic zeolites or even by thermal cracking (pyrolysis). Free radical-type mechanism predominates on the zeolite surface. The relative rates of H atom abstraction (bimolecular) and B-scission (unimolecular) are greatly affected by the zeolite matrix. Zeolites also concentrate hydrocarbon reactants within the crystal, which enhances the rate of bimolecular reaction step. Comparison with silicalite (Al-free ZSM-5 zeolite) and quartz chips has been done in order to characterize the zeolitic effect. Silicalite behaves as inert quartz chips with no effect on the rate of H-abstraction step,... 

The radiolysis of the protiated and the deuterated form of n-hexane and mixtures thereof has been examined as a function of temperature and at one N20-concentration. A strong isotope effect can be observed for fragmentation giving hexene and hydrogen by a unimolecular process, whereas the other product-distribution is not much affected by deuteration. N20 does not decrease the primary formation of C6Dt 3-radicals in mixtures. A novel reaction type is proposed for these systems giving hexene and water in a unimolecular step. 

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