Collisional stabilization reaction

Photolysis of O3 yields O2 and electronically excited 0( D), which can either be collisionally stabilized (reaction 6.18) or react with a water molecule to yield two hydroxyl radicals (reaction 6.19). Atmospheric concentrations of hydroxyl radical on a 24-h seasonal average basis are estimated at 1 X 10 molecules cm while peak daytime concentrations of 46 X 10 molecules cm have been observed. ... 

It was experimentally found that the overall process of ion-molecule association reactions can more descriptively be schemed [1] as follows collisional stabilization reaction (Eq. 2.1), radiative stabilization reaction (Eq. 2.2), and elimination reaction (Eq. 2.3) from ion-molecnle complexes. 

To detect tlie initial apparent non-RRKM decay, one has to monitor the reaction at short times. This can be perfomied by studying the unimolecular decomposition at high pressures, where collisional stabilization competes with the rate of IVR. The first successful detection of apparent non-RRKM behaviour was accomplished by Rabinovitch and co-workers [115], who used chemical activation to prepare vibrationally excited hexafluorobicyclopropyl-d2 ... 

Although collision-stabilized reaction complexes take part in chain propagation, the complex spectra of ions observed for ethylene and acetylene suggest that this mechanism undoubtedly must compete with consecutive reactions of species produced by unimolecular dissociation of the complexes and by collisional dissociation of other ions. ... 

Reactants AB+ + CD are considered to associate to form a weakly bonded intermediate complex, AB+ CD, the ground vibrational state of which has a barrier to the formation of the more strongly bound form, ABCD+. The reactants, of course, have access to both of these isomeric forms, although the presence of the barrier will affect the rate of unimolecular isomerization between them. Note that the minimum energy barrier may not be accessed in a particular interaction of AB+ with CD since the dynamics, i.e. initial trajectories and the detailed nature of the potential surface, control the reaction coordinate followed. Even in the absence (left hand dashed line in Figure 1) of a formal barrier (i.e. of a local potential maximum), the intermediate will resonate between the conformations having AB+ CD or ABCD+ character. These complexes only have the possibilities of unimolecular decomposition back to AB+ + CD or collisional stabilization. In the stabilization process,... 

Notes The data clearly illustrate that at the low pressures in the ICR binary channels dominate, but that the intermediate complex is sufficiently long lived to be collisionally stabilized in the higher pressure SIFT experiments. The cyclic isomer, c-C,H(, was unreactive with these reactant neutrals. bAn isomerization reaction to yield c-C,H( also occurs.515-1... 

To identify the isomeric form of the product of the collisionally stabilized analog of reaction 44 experimentally, Scott et al.92 studied reactions of C3H30+ (C2Hj/CO) produced in that reaction and compared it with that produced directly from propynal in an electron impact ion source or by proton transfer from HCO+ to propynal in the flow tube (these latter two production methods yielded ions with... 

The intermediate reaction complexes (after formation with rate constant, fc,), can undergo unimolecular dissociation ( , ) back to the original reactants, collisional stabilization (ks) via a third body, and intermolecular reaction (kT) to form stable products HC0j(H20)m with the concomitant displacement of water molecules. The experimentally measured rate constant, kexp, can be related to the rate constants of the elementary steps by the following equation, through the use of a steady-state approximation on 0H (H20)nC02 ... 

Studies of kinetic energy release distributions have implications for the reverse reactions. Notice that on a Type II surface, the association reaction of ground state MB+ and C to form MA+ cannot occur. In contrast, on a Type I potential energy surface the reverse reaction can occur to give the adduct MA+. Unless another exothermic pathway is available to this species, the reaction will be nonproductive. However, it is possible in certain cases to determine that adduct formation did occur by observation of isotopic exchange processes or collisional stabilization at high pressures. 

In condensation reactions, one eventually obtains a product ion of greater molecular weight than the reactant ion through partial stabilization. In the extreme case, complete stabilization may even occur (collisional stabilization). An example is... 

There is increasing evidence that for some reactions, the R02 + NO reaction produces a fraction of the alkoxy radicals with sufficient energy that they decompose immediately. For example, Orlando et al. (1998) observed that in the reaction of OH with C2H4, approximately 25% of the H0CH2CH20 radicals generated in the reaction of H0CH2CH200 with NO decomposed before they could be collisionally stabilized. Similar observations have been made for R02 from the reactions of alternate CFCs (see Chapter 13). 

Stilbenes and associated molecules provide very good examples of the formation of intermediate unstable isomers which give a chemical route for internal conversion. Upon irradiation, stilbenes undergo a cis-trans isomerization as the predominant reaction. However, under oxidative conditions phenanthrene is also formed.12 It was shown that the phenanthrene came only from c/s-stilbene (13),61 and that an intermediate unstable isomer, nms-dihydrophenanthrene (14), was the precursor of the phenanthrene.62-64 The dihydrophenarithrene was in its ground state, but vibrationally excited, and was formed by a process calculated to be endothermic by 33 10 kcal/mole-1.02 Oxygen or other oxidants converted it to phenanthrene (15), but in the absence of oxidants it was either collisionally stabilized or reverted to m-stilbene. 

The intermediate species indicated by the asterisks are in chemically activated states which might be collisionally stabilized, in part, by atmospheric pressure. Reference spectra generated for the expected CH3C(0)-containing products in the 03-TME reaction are illustrated in Figure 21. 

A comparison of various calculations revealed <1997PCA9421> that an accurate description of the ozonolysis of ethene is obtained at the CCSD(T) level with a TZ+2P basis set, while other methods, which cover less correlation effects, fail to provide a consistent description of all reaction steps. It was shown that the primary ozonides (1,2,3-trioxolanes) are not collisionally stabilized under atmosphere conditions <1997PCA9421>. 

A potentially important question in association reactions is the temperature dependence of the collisional stabilization step. While this dependence is usually small, it is not always negligible. The primary evidence for this temperature dependence is that results obtained with different buffers show appreciably different temperature dependences. This problem has received considerable theoretical attention (Bates 1979a Bohringer et al. 1983 Herbst 1982 Moet-Ner 1979 Patrick and Golden 1985 Smith et al. 1984 Viggiano 1986 Viggiano et al. 1985). 

Criegee biradicals formed in the reaction of ozone with alkenes carry significant internal excitation and the majority of them decompose before they can be collisionally stabilized. The biradicals are sufficiently excited that many different rearrangements and decomposition pathways are energetically allowed. For example, in the case of the [CH3C( )H00( )] species formed during propene oxidation the IUPAC panel has recommended the following channels (and yields) [24] ... 

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