Collisional neutralization

The 2-glycyl radical H2NCHCO2H has been generated by collisional neutralization... 

Formaldonitrone, CH2=N(H)—O (3), the elusive simplest organic nitrone, has been prepared transiently in the gas phase by femtosecond collisional neutralization of its cation radical, CH2—N(H)—0+". The latter was generated by dissociative ionization of 1,2-oxazolidine. Nitrone 3 showed negligible dissociation upon collisional neutralization and was distinguished from its tautomers formaldoxime 2 and nitrosomethane 1 that gave different NR mass spectra. The enthalpy of formation was calculated from enthalpies of atomization and two isodesmic reactions as Af//29s(3) = 58 1 kJmol . The calculated, large activation barriers for isomerization of 3 (179 and 212 kJmoH for 3 anti-2 and 3 1, respectivelyindicate that once 3 is formed and diluted in the gas phase it should not isomerize unimolecularly to either 1 or (syn/anti) 2. 

An interesting class of elusive neutral species is heterocyclic ylids, as represented by the so-called Hammick intermediate that was postulated 65 years ago to explain the accelerated decarboxylation of 2-picolinic acid [146, 147]. An analogous dissociation takes place in ionized 2-picolinic acid in the gas-phase and was employed to generate the pyridine ion isomer 35+ (Scheme 13) [148]. Collisional neutralization of 35+ with A/,AT-dimethylaniline produced neutral ylid 35, which can also be represented as a singlet a-carbene (Scheme 13). Ylid 35 showed a survivor ion in the +NR+ mass spectrum, which was further char-... 

The extraordinary variety of transient species pertinent to organic, inorganic, and organometallic chemistry and biochemistry that were generated by collisional neutralization of cations and anions and characterized by NRMS is perhaps the best evidence of the power and generality of this approach to solving difficult and tantalizing chemical problems. The future developments in this... 

Several studies have appeared recently that deserve a note. C3 carbon clusters produced by neutralization of anions were found by 13C labelling not to undergo full scrambling of carbon atoms [268]. This contrasts with the recent finding regarding NRMS of CJ [110]. Silicon dihydroxide,Si(OH)2 [269] and trihydroxysi-lyl radical, Si(OH) [270] are stable when formed by collisional neutralization of the corresponding cations. 

The observation or nonobservation of a recovery signal depends on several factors. First, collisional neutralization of ions may produce neutrals in their dissociative state(s). Electron transfer between the ion and the target is considered as a vertical Franck-Condon process. If the geometry of the ion is close to that of the neutral, the latter has a good chance of being formed as a stable species. When the geometry of the ion and the neutral are significantly different then the neutral is likely to be unstable. 

Negatively charged ions ean also be subjected to collisional neutralization. In these cases oxygen is the most commonly used target gas. 

Figure 7 Neutralization-photodissociation/photoreionization spectrum of ND . Top total reionization, bottom after background subtraction. ND was formed by ND neutralization with dimethyl disulfide. Reprinted with permission of Elsevier from Sadilek M and Turecek F (1996) Laser photolysis of ND and tri-methylamine formed by collisional neutralization of their cations in the gas phase. Chemical Physics Letters 263 203-308.

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.

Figure Bl.25.8. The principle of SIMS Primary ions with an energy between 0.5 and 10 keV cause a collisional cascade below the surface of the sample. Some of the branches end at the surface and stimulate the emission of neutrals and ions. In SIMS, the secondary ions are detected directly with a mass spectrometer.

The total collisional energy between an ion and a neutral gas atom can be calculated from Equation 49.2, in which m, m are the masses of the colliding neutral gas molecule and the ion, respectively. Elab is ths energy imparted to the ion, and Ecm is the collisional energy referred to the center-of-mass of the ion and molecule. 

Collision-induced dissociation (or decomposition), abbreviated CID. An ion/neutral process wherein the (fast) projectile ion is dissociated as a result of interaction with a target neutral species. This is brought about by conversion during the collision of part of the translational energy of the ion to internal energy in the ion. The term collisional-activated dissociation (or decomposition), abbreviated CAD, is also used. 

Neutral stabilized collisional-radiative recombination is also quite slow under typical afterglow conditions. In his review, Flannery4 gives the simple formula,... 

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