Chemi-ionization

Chemi-ionization reactions producing negative ions also remain possible—e.g.,... 

Fite, W.L. and P. Irving, Chemi-ionization in Collisions of Uranium Atoms with Oxygen Atoms and Molecules, J. of Chem. Phvs. 56 4227-4428 (1972). 

A Fontijn, ed. Gas-Phase Chemiluminescence and Chemi-Ionization. New York Elsevier, 1985. 

DH Stedman, DL McElwee, GJ Wendel. In A Fontijn, ed. Gas-Phase Chemiluminescence and Chemi-ionization. New York Elsevier Science Publishing, 1985, pp 345-355. 

Theoretical Study of the Gas-Phase Chemi-Ionization Reaction between Uranium and Oxygen Atoms. 

Note The process M H- X -> MX" + e , i.e., ionization of internally excited molecules upon interaction with other neutrals is known as chemi-ionization. Chemi-ionization is different from Cl in that there is no ion-molecule reaction involved [8,15] (cf. Penning ionization. Chap. 2.2.1). 

Highly exothermic chemical reactions can give ions as products. Such reactions are called chemi-ionization reactions. When one or more of the reactants are formed by pulsed photolysis, and the ion product(s) are time-resolved with a mass spectrometer, this comprises a FPTRMS experiment. The ions can be extracted by a biased pair of electrodes, one of which has an aperture through which ions pass to enter a quadrupole mass spectrometer. The time resolution of this experiment is superior to FPTRMS experiments between neutrals, since the ion velocities are set by the electrode bias voltage, and the thermal molecular velocity distribution limitation is no longer a factor. With modest electrode bias voltages, the detection time can be as short as 2 /is. 

Chemi-ionization has been developed as a method for kinetics investigations of CH(a4 ) reactions by following the concentration of this species via chemi-ions formed in its reaction with O atoms [129] ... 

The mechanism of chemi-ionization and of CH( 4 2A) emission has been studied in considerable detail269,331 but it was not possible to decide on a unique mechanism for the emission process. Observed rotational distributions in the CH (A 2 A) state have been interpreted in terms of rotational relaxation270 but the uncertainty in the excitation mechanism renders such an enterprise somewhat ambiguous. 

Chemi-ionization the production of an ion through the reaction of the sample molecule with another molecule that was excited beforehand ... 

The clean flames show low levels of ionization close to the reaction zone because of chemi-ionization processes such as ... 

Chemi-ionization. Chemi-ionization is the main and probably the only mechanism responsible for the formation of primary ions in fuel-lean flames. This mechanism, first proposed by Calcote (IS), has received much attention, and there is general agreement that CHO" is the only primary chemi-ion formed in lean flames (see below). 

Ionization by Charge Transfer. Many ion-molecule reactions between small species (< 50 amu) occur in flames (24), As mentioned above, the only primary ion in fuel-lean flames (non-sooting) is CHO", formed by chemi-ionization (13, 24, 25). 

Pyrolysis reactions are undoubtedly occurring after combustion, and chemi-ionization may be occurring in addition to charge exchange reactions between primary ions, such as CsHs, and light hydrocarbons. Hence, the chemi-ion profile would not decay necessarily as it does in leaner combustion, where recombination begins to dominate ion formation. Instead, a different sequence of chemi-ion reactions may become important, with the precursor being C2, rather than CH. 

The reactions of alkaline earth metal atoms with halogen molecules, either forming alkaline earth metal monohalides or leading to chemi-ionization, represent an important case study in the reaction dynamics of divalent systems [91, 92, 95-100]. Let us take as an example the reaction... 

The reaction product BaCl has the strueture Ba CC where a single electron is transferred from barium to chlorine. This reaction channel thus correlates adia-batically to the single electron-transfer intermediate Ba Clj. Chemi-ionization is also observed as a minor channel in the Ba + CI2 collision ... 

Intuitively, suitable substituents will favor placing the spin on one set and the charge on the other. Indeed, solution phase, chemi-ionization via a photoexcited quinone of gem-diaryl-substituted methylenecyclopropane results in two, non-interconverting, different radical cations. For the 2,2-derivatives (11) the electron is lost from the ring. This results in... 

There is one substituted methylenecyclopropene for which the radical cation is well understood the 2,3-diphenyl, 4-hydroxy, 4-methoxy derivative (16). This ion is not formed by 1-electron oxidation or chemi-ionization from the methylenecyclopropene. Instead it arises from rearrangement of its tautomer, methyl 2,3-diphenylcyclopropene-l-carboxylate (17) in pro tic media . This represents another example of reversal of stability... 

The comparison of the ionization potentials of identically substituted cyclopropenones, cyclopropenes and cyclopropanes is interesting, if not yet particularly informative to date. The ionization potentials of cyclopropane, cyclopropene and cyclopropenone are much closer, 9.86, 9.67 and 9.47 eV, than for their diphenyl derivatives. Diphenylcyclopropene has an adiabatic ionization potential of 7.45 eV while those of the cis and trans isomers of 1,2-diphenylcyclopropane (18) are 8.20 and 8.05 eV respectively. These latter values for the saturated species correspond to ring-opening to l,3-diphenylprop-l-yl-3-ium (19) (equation 24) a result corroborated by both experiment via solution phase chemi-ionization and ab initio calculations on the analogous divinylcyclopropane. (The... 

One may alternatively remove an electron from one of the n bonds of 3,3 -bicyclopropenyl, although in fact this process produces the same radical cation as before because of extensive through-bond interactions. Solution phase, photoexcited quinone-induced chemi-ionization of 3,3 -dimethyl-3,3 -bicyclopropenyl (22) results in the... 

Interestingly, the syn isomer of the above-mentioned tricyclo[3.1.0.0 ]hexane-l-yl-4-ium (24) may be described alternatively as a radical cation derived from the highly strained cyclopropanoid dimethyltriprismane (25). As such, besides a preliminary and still ill-defined chemi-ionization result on the parent compound, the above represents the sole... 

In trying to make sense out of these diverse data on whether electron loss lessens strain, the simplest regularity arises from the observation of whether the electron is removed from the predominantly olefin or bicyclobutane fragment. This interesting ambiguity is resolved in the case of benzvalene to be from the former as shown by gas phase photo- and solution phase chemi-ionization . Moreover, to the extent that the electron is taken from a 2-butene type orbital, one may expect that ionization of naphthvalene (30) and pleiavalene (31) will also be from the 7r-system because the ionization potentials of o-xylene and 1,8-dimethylnaphthalene are still lower. This is confirmed experimentally . 

The third returns the reader to chemi-ionization of cyclopropane derivatives with excited state electron acceptors. In the particular, it has been observed that oxidation of l,6-dimethylbicyclo[4.1.0]hept-3-ene (46) and [4.4.l]propella-2,6-diene (47) result in 2,6-dimethylcycloheptatriene (48) and l,6-methano[10]annulene (49) (equations 28 and 29 respectively). The former compound is conceptually the deprotonation product of the dication wherein the C-C bond between the bridgehead carbons has been totally depleted of electrons. In this case, however, the dication is not expected to be as stable as in the above pyramidal or cyclobutadiene cases and indeed, the product is seemingly not formed via the dication route. The methanoannulene likewise does not arise from the tetracation of its precursor. Rather, proton and electron transfer reactions involving merely radical cations proceed to remove hydrogens sequentially. 

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