Unimolecular Decomposition—Gases

Numerous gas-phase decompositions follow first-order kinetics over a wide range of pressures. Typical examples are the reactions 

Both reactions have activation energy barriers suggesting the question How can one reconcile first-order kinetics with an activation process which must involve molecular collision Lindemann proposed the following mechanistic answer  

An A molecule is activated by collision with another A molecule. In the collision process the relative kinetic energy of the two colliding molecules is transformed, in part, to internal energy of one of the molecules, thus activating it. Such inelastic collisions (in which kinetic energy is not conserved) provide the mechanical basis for understanding the activation process.The excited molecule A is stabilized in two ways. Either it is deexcited by another inelastic collision with A or it decomposes to form products. The rate expressions deduced from (5.18) are 

It is reasonable to assume that the steady-state hypothesis is applicable to A since molecular collisions are constantly occurring. The rates of production and depletion of A greatly exceed its net rate of change. Hence 

Inelastic processes are precisely those ignored by the postulates of elementary kinetic theory see Section 1.1. 

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The desire to understand catalytic chemistry was one of the motivating forces underlying the development of surface science. In a catalytic reaction, the reactants first adsorb onto the surface and then react with each other to fonn volatile product(s). The substrate itself is not affected by the reaction, but the reaction would not occur without its presence. Types of catalytic reactions include exchange, recombination, unimolecular decomposition, and bimolecular reactions. A reaction would be considered to be of the Langmuir-Hinshelwood type if both reactants first adsorbed onto the surface, and then reacted to fonn the products. If one reactant first adsorbs, and the other then reacts with it directly from the gas phase, the reaction is of the Eley-Ridel type. Catalytic reactions are discussed in more detail in section A3.10 and section C2.8. 

The thermal decomposition of diacyl peroxides has been the most frequently employed process for the generation of alkyl radicals. The rate and products of the unimolecular decomposition of acetyl peroxide have been the subject of many studies. Acetyl peroxide decomposes at a convenient rate at 70-80°C both in the solution and in the gas... 

In addition to this reaction, many other reactions of hydroperoxide decay occur in solution and they will be discussed later. This is the reason why the unimolecular decomposition of hydroperoxides was studied preferentially in the gas phase. The rate constants of the unimolecular decomposition of some hydroperoxides in the gas phase and in solution are presented in Table 4.11. The decay of 1,1-dimethylethyl hydroperoxide in solution occurs more rapidly. This demonstrates the interaction of ROOH with the solvent. 

Expressing the stability of these gas phase radicals in terms of approximate half-lives is not meant to imply a unimolecular decomposition of the radicals, nor even a strictly first order disappearance. 

Dependent Rate Constants of Multichannel Unimolecular Decomposition of Gas-phase a-HMX An Ab Initio Dynamics Study. 

The quasi-equilibrium theory (QET) of mass spectra is a theoretical approach to describe the unimolecular decompositions of ions and hence their mass spectra. [12-14,14] QET has been developed as an adaptation of Rice-Ramsperger-Marcus-Kassel (RRKM) theory to fit the conditions of mass spectrometry and it represents a landmark in the theory of mass spectra. [11] In the mass spectrometer almost all processes occur under high vacuum conditions, i.e., in the highly diluted gas phase, and one has to become aware of the differences to chemical reactions in the condensed phase as they are usually carried out in the laboratory. [15,16] Consequently, bimolecular reactions are rare and the chemistry in a mass spectrometer is rather the chemistry of isolated ions in the gas phase. Isolated ions are not in thermal equilibrium with their surroundings as assumed by RRKM theory. Instead, to be isolated in the gas phase means for an ion that it may only internally redistribute energy and that it may only undergo unimolecular reactions such as isomerization or dissociation. This is why the theory of unimolecular reactions plays an important role in mass spectrometry. 

The following Lindemann mechanism for the unimolecular decomposition of a molecule A in the presence of a species Y (which may be any molecule such as inert gas like Helium or even A itself)y considered ... 

Steps (15b-d) represent the unimolecular decomposition of the energized complex, the rate of which will be reduced by added foreign gas. 

Since the ionic states formed by high-energy radiation seem to be the chemically important ones, let us consider their reactions. The reactions between ions and neutral molecules in the gas phase can be studied directly in a mass spectrometer. Under ordinary operating conditions the pressure in the ionizing chamber of the mass spectrometer is about 10 6 mm. and the ions formed have little chance to collide with a molecule during their brief lifetime (10-5 sec.) before collection. Therefore, mainly unimolecular decomposition reactions occur and it is the products of these that are detected. The intensity of these primary ions increases with the first power of the pressure in the ionization chamber. However, when the pressure becomes great enough so that ion molecule collisions can occur readily, additional secondary ions which are the products of these ion molecule Collisions appear. The intensity of these secondary product ions depends on the concentrations of both the molecules and the primary ions, and thus on the square of the pressure. 

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 thermal decomposition of ketene1>60 93 142 146 apparently does not occur by the simple unimolecular decomposition CH2CO CH2 + CO. Young146 suggests that CH2 actually results from breakdown of allene, which is an intermediate product. Ketene is known to dimerize readily, even in the gas phase on Pyrex surfaces to form the d-methylene cyclic lactone. 

Above 250°C. we approach, in the gas phase, what is known as the cool flame regime. This is characterized by induction periods and by the appearance of pressure peaks and luminescent phenomena in the oxygen-hydrocarbon system. The consensus of present data seems to support the contention that these cool flames arise from the secondary decomposition of the hydroperoxides produced by the low temperature chain. The unimolecular decomposition of the hydroperoxide yields active alkoxy and hydroxyl radicals ... 

In the previous problem we examined temperature profiles and reactant (SiH4) concentration profiles in a channel-flow chemical vapor deposition (CVD) reactor. At sufficiently high temperatures (and pressures) SM4 undergoes unimolecular decomposition into the species SiH2 and H2. This is followed by numerous reactions of the intermediate species [180]. One such intermediate species formed in the gas phase is Si (i.e., a gas-phase silicon atom). In this problem we consider the gas-phase formation and destruction reactions governing the spatial profiles of Si atoms in a rotating-disk CVD reactor. 

The unimolecular decomposition of the neopentyl radical has also been investigated. Neopentyl was generated by the reaction of Cl atoms with neopentane, and the dissociation to isobutane and methyl was followed by photoionization mass spectrometry over the temperature range 560 to 650 K, and as a function of bath gas [126] ... 

Rates of Gas-Phase Reactions. Reaction rates have been reported for only a few CVD gas-phase reactions, and most reports are primarily for the silane system. Because of the high temperatures and low pressures used in CVD, the direct use of reported gas-phase rate constants must be done with care. In addition to mass-transfer and wall effects, process pressure may be another factor affecting reaction rates. Process pressure affects major CVD processes, such as the deposition of Si from SiH4 and GaAs from Ga(CH3)3, reactions that involve unimolecular decomposition. The collisional activation, deactivation, and decomposition underlying these reactions can be summarized qualitatively by the following reactions (139, 140) ... 

IR spectroscopy is not confined to stable substances. In recent years, matrix isolation IR spectroscopy has become important in the investigation of short-lived, unstable molecular species. A gas containing such highly-reactive molecules - produced by photolysis of a reaction mixture, or in a high-temperature furnace - is suddenly cooled by contact with an inert solid (e.g. argon at c. 40 K). The matrix-isolated molecules are protected by the low temperature from unimolecular decomposition, and - by sheer isolation, if the dilution is sufficient - from bimolecular processes such as dimerisation or disproportionation. For example, the photolysis of Mn(CO)5H by a laser produces the otherwise unstable Mn(CO)5 and Mn(CO)4H molecules whose IR spectra can be measured in an argon matrix. Because of the low temperature, the lack of inter-molecular interactions and the rigidity with which the molecules are trapped in the matrix, such spectra are often very well resolved, better than can be achieved by conventional methods. Thus matrix isolation spectroscopy is widely used in the study of stable species, in preference to conventional techniques. 

It is likely that theoretical methods, both ab initio and MD simulations, will be needed to resolve the complicated chemical decomposition of energetic materials. There are species and steps in the branching, sequential reactions that cannot be studied by extant experimental techniques. Even when experiments can provide some information it is often inferred or incomplete. The fate of methylene nitramine, a primary product observed by Zhao et al. [33] in their IRMPD/molecular beam experiments on RDX, is a prime example. Rice et al. [99, 100] performed extensive classical dynamics simulations of the unimolecular decomposition of methylene nitramine in an effort to help clarify its role in the mechanism for the gas-phase decomposition of RDX. 

Consider an alkoxyl radical that undergoes unimolecular decomposition in the gas phase, by the following reaction, which is known to be endothermic ... 

There are very few cases where one can compare directly the same reaction taking place by the same mechanism in both gas phase and solution. If at all temperatures the reactions have equal velocities in the two phases the values of 5 and of E are the same, and it may be safely assumed that the reaction mechanisms are identical and the solvent has no effect. Undoubtedly the simplest comparison exists in the unimolecular decomposition of nitrogen pentoxide and in this reaction the solvent has little effect. The unimolecular racemization of pinene at 200° proceeds at the same rate in the gas phase, in liquid pinene and in a solution of petrolatum. 

The absorption of a quantum of light by a molecule in the gas phase may initiate a unimolecular decomposition or rearrangement process. The potentially intimate relationship between photochemistry and nonequilibrium unimolecular reaction theory has yet to be realized, since most of these photoprocesses take place on electronically excited poten-... 

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