Collision stabilization

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. ... 

The (CjHsl ion can also function as a Lewis acid, forming collision-stabilized complexes or molecular ion adducts ... 

However, it was found by Haller and Srinivasan that the intervention of a collision stabilized the molecule so that at pressures of butadiene, which range from 4 to 90 mm., 1,2-butadiene is readily detected among the products. But at even higher pressures, butadiene itself is deactivated before it can undergo reaction (5). Thus, the yield of 1,2-butadiene vs. pressure shows a maximum (Table HI). 

Only one ion-molecule reaction of the molecular ion of 1,1-difluor-ethylene can be observed 845) formation of a collision-stabilized dimer ion, a third-order process with a very high rate constant which may arise from the high polarity of the molecule ... 

An important reaction of C2H5 ion is addition to the aromatic molecule in a process which probably involves collision stabilization, i.e.. 

HC = CH(OOH) isomer is stabilized after isomerization from chemically activated C2H3O2, this profile contains a maximum. At low pressures, a pressure increase leads to more collision stabilization of HC = CH(OOH) and prevents it from further reaction to C2H2 + HO2. This explains the initial increase of the production rate with pressure. At a certain pressure, a further pressure increase prevents the isomerization step from happening because the collisional stabilization of C2H3O2 becomes too fast. Therefore, at this point a pressure increase leads to a reduction of the HC = CH(OOH) formation rate. 

The key to sensitive detection of negative ions lies in the production of a sufficiently high population of thermal electrons. The extent of formation of M at sufficient electron density depends on the electron affinity of the sample molecule, the energy spectrum of the electron population and the frequency with which molecular anions collide with neutral particles and become stabilized (collision stabilization of the radical ion). Also with an ion trap analyser, using external ionization, NCI can be utilized analytically. The storage of electrons in the ion trap itself is not possible because of their low mass (internal ionization). 

Ion Trap Instruments Ion trap mass spectrometers with internal ionization can be used for Cl without hardware conversion. Because of their mode of operation as storage mass spectrometers, only a very low reagent gas pressure is necessary for instruments with internal ionization. The pressure is adjusted by means of a special needle valve which is operated at low leak rates and maintains a partial pressure of only about 10 Torr in the analyser. The overall pressure of the ion trap analyser of about 10 -10 Torr remains unaffected by it. Cl conditions thus set up give rise to the term low pressure CL Compared to the conventional ion source used in high pressure Cl, in protonation reactions, for example, a clear dependence of the Cl reaction on the proton affinities of the reaction partners is observed. Collision stabilization of the products formed does not occur with low pressure Cl. This explains why high pressure Cl-typical adduct ions are not formed here, which would confirm the identification of the (quasi)molecular ion (e.g., with methane besides (M + H), also M + 29 and M +41 are expected). The determination of ECD-active substances by electron capture (NCI) is not possible with low pressure Cl (Yost, 1988). 

Figure Bl.7.7. Summary of the other collision based experiments possible with magnetic sector instruments (a) collision-mduced dissociation ionization (CIDI) records the CID mass spectrum of the neutral fragments accompanying imimolecular dissociation (b) charge stripping (CS) of the incident ion beam can be observed (c) charge reversal (CR) requires the ESA polarity to be opposite that of the magnet (d) neutiiralization-reionization (NR) probes the stability of transient neutrals fonned when ions are neutralized by collisions in the first collision cell. Neutrals surviving to be collisionally reionized in the second cell are recorded as recovery ions in the NR mass spectrum.

Proton transfer from H3 + and CH5+ to cyclopropane yields a C3H7 + ion, which at atmospheric pressures is largely stabilized by collision (9). This ion reacts as a sec-propyl ion with an added interceptor molecule (9). Hence, the protonated cyclopropane ion undergoes ring opening to acquire the sec-propyl ion structure. Similarly, it has been shown that protonated cyclobutane rearranges to the sec-C4H9 + structure. 

Condensation reactions essentially include all reactions in which a strongly bound reaction complex, which may or may not be stabilized by collision, is produced. 

Such reactions are often exothermic and the role of the third body is to carry away some of the energy released and thus stabilize the product molecule. In the absence of a collision with a third body, the highly vibrationally excited product molecule would usually decompose to its reactant molecules in the timescale of one vibrational period. Almost any molecule can act as a third body, although the rate constant may depend on the nature of the third body. In the Earth s atmosphere the most important third-body molecules are N2 and O2. 

When the entering ligand, L, is uncharged, the stability of the outer-sphere complex M OH2 L2+ may be so low that its concentration does not differ significantly from that arising from diffusive collisions between M OH2m+ and L. Under these conditions, entry of L into the... 

Relatively few investigations involving palladium carbonyl clusters have been carried out, partly because palladium per se does not form stable, discrete homometallic carbonyl clusters at room temperature in either solid or solution states.114,917-922 Nevertheless, solution-phase palladium carbonyl complexes have been synthesized with other stabilizing ligands (e.g., phosphines),105,923 and carbon monoxide readily absorbs on palladium surfaces.924 Moreover, gas-phase [Pd3(CO)n]-anions (n = 1-6) have been generated and their binding energies determined via the collision-induced dissociation method.925... 

Association reactions, in particular, seem to present a severe problem for structural determination. In these reactions, an ion and a neutral species form a complex which is stabilized either by collision with a third body or, at especially low pressures, by the emission of radiation. The radiative mechanism, prominent in interstellar chemistry, is discussed below. Although some studies of radiative association have been performed in the laboratory,30,31 90 most association reactions studied are three-body in nature. It is customarily assumed that the product of three-body association is the same as that of radiative association, although this assumption need not be universally valid. 

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