Gas-phase reaction pathways

The study of gas-phase reactions has obviously been of much assistance in understanding solution-phase reactions. Sometimes, however, this relationship breaks down because the reactants give a different reaction in the gas phase than in the solution phase. Caserio and Kim present such an example in Chapter 5, a thorough study of gas-phase reactions (ion cyclotron resonance) of alcohol nucleophiles with protonated carboxylates, carbonates, and phosphates. The goal of this work is to understand solvent and counterion effects on acid-catalyzed esterification. The gas-phase reaction pathways, however, turn out to be different from those in solution phase. 

Gas-phase reaction pathways of aluminum organometallic compounds with dimethylaluminum hydride and alane as model systems ... 

Compounds are transformed into each other by chemical reactions that can be run under a variety of conditions from gas-phase reactions in refineries that produce basic chemicals on a large scale, through parallel transformations of sets of compounds on well-plates in combinatorial chemistry, all the way to the transformation of a substrate by an enzyme in a biochemical pathway. This wide range of reaction conditions underlines the complicated task of imderstanding and predicting chemical reaction events. 

Compared with uncatalyzed reactions, catalysts introduce alternative pathways that, in nearly all cases, involve two nr more consecutive reaction steps. Each of these steps has a lower activation energy than does the uncatalyzed reaction. We can nse as an example the gas phase reaction of ozone and oxygen atoms. In the homogeneons uncatalyzed case, the reaction is represented to occur in a single irreversible step that has a high activation energy ... 

An ab initio study of elimination and substitution has been done for the gas-phase reaction of F with chlorocyclopropane. Among various findings it emerged that at the MP2/6-31(- -)G //HF/6-31(- -)G level, the 5 n2 pathway has a lower activation barrier by 7.3kcal moF compared with the E2 anti) pathway. 

A comparison is made between the gas phase and solution phase reaction pathways for a wide range of organic reactions. Examples are presented in which the gas phase and solution phase mechanisms are the same for a given set of reactants in which they differ, but attachment of the first molecule of solvent to the bare gas phase ionic reactant results in the solution phase products and in which the bare, monosolvated, and bulk-solvated reactions proceed by three different pathways for the same reactants. The various tools available to the gas phase ion chemist are discussed, and examples of their use in the probing of ionic structures and mechanisms are reported. 

Solution phase chemists have developed a tremendous variety of tools to elucidate mechanisms. Spectroscopy, kinetics, isotopic labeling, and many more are all in the chemical mechanic s tool kit, for use in mapping out reaction pathways. In contrast, the tool kit for the gas phase reaction mechanic is far more limited. The low concentration and short lifetime of gas phase reaction intermediates and products severely limits the use of many of the conventional tools. Gas phase ion-molecule chemists have therefore both adapted solution phase tools to their unique needs and developed many new ones. 

We can, however, form alkoxide ions that are monosolvated by a single alcohol group, via the Riveros reaction [Equation (7)]. When the monosolvated methoxide is reacted with acrylonitrile, the addition process reaction (8a), is the major pathway, because there is a molecule of solvent available to carry off the excess energy. The proton transfer pathway, reaction (8b), becomes endothermic, because the methoxide-methanol hydrogen bond, at about 29 kcal/mol, must be broken in order to yield the products. Thus, one can observe either the unique gas phase mechanism in the gas phase, reaction (6b), or the solution phase mechanism in the gas phase, reaction (8a), and the only difference is in the presence of the first molecule of solvent. 

To conclude this section on the effect of solvent on a-nucleophilicity, we refer to the current, rather controversial, situation pertaining to gas-phase smdies and the a-effect. As reported in our review on the a-effect and its modulation by solvent the gas-phase reaction of methyl formate with HOO and HO , which proceeds via three competitive pathways proton abstraction, nucleophilic addition to the carbonyl group and Sat2 displacement on the methyl group, showed no enhanced nucleophilic reactivity for HOO relative to This was consistent with gas-phase calculational work... 

A systematic study to identify solid oxide catalysts for the oxidation of methane to methanol resulted in the development of a Ga203—M0O3 mixed metal oxide catalyst showing an increased methanol yield compared with the homogeneous gas-phase reaction.1080,1081 Fe-ZSM-5 after proper activation (pretreatment under vacuum at 800-900°C and activation with N20 at 250°C) shows high activity in the formation of methanol at 20°C.1082 Density functional theory studies were conducted for the reaction pathway of the methane to methanol conversion by first-row transition-metal monoxide cations (MO+).1083 These are key to the mechanistic aspects in methane hydroxylation, and CuO+ was found to be a likely excellent mediator for the reaction. A mixture of vanadate ions and pyrazine-2-carboxylic acid efficiently catalyzes the oxidation of methane with 02 and H202 to give methyl hydroperoxide and, as consecutive products, methanol and formaldehyde.1084 1085... 

In addition to experiments, a range of theoretical techniques are available to calculate thermochemical information and reaction rates for homogeneous gas-phase reactions. These techniques include ab initio electronic structure calculations and semi-empirical approximations, transition state theory, RRKM theory, quantum mechanical reactive scattering, and the classical trajectory approach. Although still computationally intensive, such techniques have proved themselves useful in calculating gas-phase reaction energies, pathways, and rates. Some of the same approaches have been applied to surface kinetics and thermochemistry but with necessarily much less rigor. 

In every gas/solid catalytic cycle, at least one of the reactants must at some point be adsorbed on the catalyst surface. Let us consider the reaction A + B —> C. There are two options (Figure 4.2) In the first, both reactants A and B are first adsorbed on the catalyst, migrate to each other, and react on the surface, giving the product C, which is desorbed into the gas phase. This pathway, which we have already met in Chapter 2, is the Langmuir-Hinshelwood mechanism. The other option is that A is adsorbed on the catalyst surface, and B subsequently reacts with it from the gas phase to give C (the so-called Eley-Rideal mechanism [22]). The Langmuir-Hinshelwood mechanism is much more common, partly because many reactants are activated by the adsorption on the catalyst surface. 

Molecular dynamics simulations are attractive because they can provide not only quantitative information about rates and pathways of reactions, but also valuable insight into the details of ho y the chemistry occurs. Furthermore, a dynamical treatment is required if the statistical assumption is not valid. Yet another reason for interest in explicit atomic-level simulations of the gas-phase reactions is that they contribute to the formulation of condensed-phase models and, of course, are needed if one is to include the initial stages of the vapor-phase chemistry in the simulations of the decomposition of energetic materials. These and other motivations have lead to a lot of efforts to develop realistic atomic-level models that can be used in MD simulations of the decomposition of gas-phase energetic molecules. 

A brief sketch of what we know about the kinetics of RDX decomposition is needed for context for the discussion of the simulation studies. In most experiments the data are taken with little experimental control, with the observations complicated by the rapid release of large amounts of energy, and with the liquid or solid undergoing phase transitions and chemical reactions to form small gaseous molecules such as N2O, H20, H2CO, HCN, NO, N02, CO, and C02. The reaction mechanisms involve many sequential, branched pathways, which are strongly dependent on the experimental conditions. It is not our purpose here to try to sort out the mechanisms for the various conditions, but we do need, for foundation, to discuss the experimental observations relevant to the elementary gas-phase reactions. 

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