Intermediates in gas-phase reactions

The chemistry of silicon with unusual coordination numbers has developed in a remarkable progress. While short living silylenes (coordination number 2) were discussed as intermediates in gas phase reactions of suitable precursors already in the seventies, followed by investigations on derivatives with bulky substituents at silicon and thus stable in solution in the early eighties, n- and n-donor stabilized silylenes were synthesized in the solid state already in the late eighties. 

This concluding statement might be transferred without any restrictions to the history of the development of the silylene chemistry, the sila analogs of the carbenes CR2. Again, there are main milestones describing the efforts in this area first, silylenes were discussed as highly reactive intermediates in gas phase reactions of suitable silyl precursors, then followed by the spectroscopic characterization in the gas phase or in low temperature matrices. In a third period, these compounds were kinetically and/or thermodynamically stabilized and characterized in conden d phase and in the solid state, even by single crystal X-ray analysis. 

Airborne Free Radicals The widespread occurrence of free radical intermediates in gas-phase reactions has frequently been the subject of kinetics studies (refer to... 

A subsequent step in the sequence is the formation of Ti-hydroperoxo species from the reaction of H2O2 and tetrahedral Ti sites, more likely Ti tripodal sites. This Ti-hydroperoxo species has been proposed as an intermediate in the gas-phase epoxidation of propylene by analogy with the well-known chemistry for oxidations in the liquid phase with H2O2 and TS-1 [105,106,401,446]. In gas-phase reactions, hydroperoxide species have been inferred by D2 isotopic experiments [431] and detected by ex situ INS [432] and in situ UV-vis measurements [433]. Other species in this simplified sequence include adsorbed propylene on a Ti-hydroperoxo site and adsorbed PO on a Ti tripodal site. Desorption of PO and water results in the original Ti species, which closes the catalytic cycle. 

Ions play little part in gas-phase reactions, owing to the difficulty with which they are formed in the absence of an ionizing solvent. Atoms and free radicals are produced more easily in the gaseous phase and, because they enter readily into further reaction, they are important intermediates in reactions. For example, consider the reaction between hydrogen and bromide. If the product of reaction, HBr, is removed as it is formed, so as to eliminate its effect on the behavior, the rate equation is... 

A workable reaction mechanism must embrace all reactants, products and reaction intermediates produced and consumed in a catal3ftic reaction. In gas-phase reactions, two-molecule reaction is popular and three-molecule reaction is very less. For catalyst surfaces, multi-molecule steps become important as the concentration of chemical species is higher than that of gas-phase, and even in this case the reaction between more than three molecules is in general not common. 

In gas-phase reactions followed using the flowing afterglow technique, MesSiCl and Mel were found to react with anions (H2N , OH , F", HS, NC ) at each encounter, providing the anions were sufficiently basic. As the basicity of the nucleophile decreased, so did the rate of its reaction with Mel. In contrast, Me3SiCl reacted at each encounter with these nucleophiles until the net reaction ceased, when the displacement became endothermic. In the thermoneutral reaction between and Me3SiCl, the displacement of unlabeled Cl" was rapid with the proposed five-coordinate intermediate collisionally stabilized by the carrier gas and having a lifetime >10" s. 

Intense sodium D-line emission results from excited sodium atoms produced in a highly exothermic step (175). Many gas-phase reactions of the alkafl metals are chemiluminescent, in part because their low ioni2ation potentials favor electron transfer to produce intermediate charge-transfer complexes such as [Ck Na 2] (1 )- There appears to be an analogy with solution-phase electron-transfer chemiluminescence in such reactions. 

Either UV-VIS or IR spectroscopy can be combined with the technique of matrix isolation to detect and identify highly unstable intermediates. In this method, the intomediate is trapped in a solid inert matrix, usually one of the inert gases, at very low temperatures. Because each molecule is surrounded by inert gas atoms, there is no possiblity for intermolecular reactions and the rates of intramolecular reactions are slowed by the low temperature. Matrix isolation is a very useful method for characterizing intermediates in photochemical reactions. The method can also be used for gas-phase reactions which can be conducted in such a way that the intermediates can be rapidly condensed into the matrix. 

Other techniques for decompng nitric acid are photochemical flash photolysis. The photochemical decompn of nitric acid is not solely a gas-phase reaction X-rays have caused the evolution of 02 from nitric acid crystals. The use of flash photolysis has shown the nitrate radical to be an intermediate in the decompn (Ref 30)... 

Tetrhedral intermediate, 172 Thermodynamic cycles, 186 Thermolysin, zinc as cofactor for, 204 Thrombin, 170 Torsional potential, 111 Transition states, 41-42,44, 45,46, 88, 90-92 in amide hydrolysis, 219-221 oxyanion hole and, 181 stabilization of, 181,181 carbonium ion, 154,155,156-161, 167-169 for gas-phase reactions, 43... 

It also points out another advantage of isotopic tracers. The fact that a suspected intermediate reacts quantitatively to form the final product does not necessarily mean that it is an intermediate. Just because a compound is formed, or because it may give the desired final product, does not prove that the compound in question is a gas phase intermediate in the overall reaction. It must be shown that the compound behaves properly under dynamic reaction conditions, and isotopic tracers can test this behavior most effectively. 

Methyl radicals have heoi detected in the gas i iase over a Sr/LajO, catalyst during the reaction of CH4 with NO, provided Oj is present in the system. In the absence of O2 the concentration of CHj- radicals decreases almost to the background level. The results indicate that the enhanced effect of Oj on the reduction of NO by CH4 may be due to surface-generated gas-phase CH,- radicals, but in the absence of O2 another reaction pathway may be dominant. Evidence has been found for the presence of CHjNO, a likely intermediate in the radical reaction, at temperatures up to 800 °C. 

In a separate set of experiments designed to follow the gas phase reactions of CHj-radicals with NO, CHj- radicals were generated by the thermal decomposition of azomethane, CHjN NCHj, at 980 °C. The CH3- radicals were subsequently allowed to react with themselves and with NO in a Knudsen cell that has been described previously [12]. Analysis of intermediates and products was again done by mass spectrometry, using the VIEMS. Calibration of the mass spectrometer with respect to CH,- radicals was carried out by introducing the products of azomethane decomposition directly into the high vacuum region of the instrument. 

This type of reaction is involved as an intermediate step in few synthetically useful reactions, in the formation of polysulfones by copolymerization of an olefin with SO 2, as well as in aerosol formation in polluted atmospheres. We will discuss later in some detail the most important chain reactions involving step 11. However, Good and Thynne determined the Arrhenius parameters for the addition of methyl and ethyl radicals to SO2 in gas phase, the rate constants being 5 x 10 and 4 x 10 s respectively at ambient... 

In cyclohexane the same two ketones (12) and (13) are obtained from the photolysis of (11) but in aqueous dioxane two phenols are isolated as well as the bicyclic ketone (12). Swenton(10) suggested that the gas-phase reaction involves diradical species, whereas in polar solvents zwitterionic intermediates are favored ... 

Until recently, fast time-resolved IR spectroscopy has been a technique fraught with difficulty. Generally it has been easier to use low temperature techniques, particularly matrix isolation (2,4), to prolong the lifetime of the fragments so that conventional spectrometers can be used. In the last 5 years, however, there have been major advances in fast IR spectroscopy. It is now posssible to detect metal carbonyl intermediates at room temperature in both solution and gas phase reactions. In Section II of this article, we explain the principles of these new IR techniques and describe the apparatus involved in some detail. In Section III we give a self-contained summary of the organometallic chemistry that has already been unravelled by time-resolved IR spectroscopy. 

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