Ionization, mechanisms, initiating species

A further consequence of the ionization mechanism is that if the same carbonium ion can be generated from more than one precursor, the subsequent reactions it undergoes should be independent of its origin. As in the just-mentioned case of stereochemistry, our faith in this prediction is tempered by the fact that the ionization produces an ion pair initially, rather than dissociated ions, and the leaving group is sufficiently close to the carbonium ion to influence further reaction if it occurs prior to diffusion apart of the two species. 

The mass spectrum of aniline has been known since the early days of mass spectrometry. [122] Initially, the observed [M-27] ion has been interpreted in terms of HCN loss (Fig. 6.56a). The mechanism for loss of the elements of [H, N, C] from aminoarenes is perfectly analogous to CO loss from phenols (Chap. 6.9.1). [231] More recently, it could be demonstrated that loss of hydrogen isocyanide, HNC, occurs rather than losing the more stable neutral species HCN, a behavior typical of ionized pyridine. [222]... 

Unrealistic ionizations it is often necessary to ionize one of the reagents to initiate a reaction, and this requires careful consideration if the mechanism is to be realistic. For example, we should not attempt to protonate substrates under basic conditions, and we are unlikely to generate anionic species under acidic conditions. These are fairly obvious limitations, but are frequent mistakes. 

The actual species responsible for cationic polymerizations initiated by ionizing radiation is not established. The most frequently described mechanism postulates reaction between radical-cation and monomer to form separate cationic and radical species subsequently, the cationic species propagates rapidly while the radical species propagates very slowly. The proposed mechanism for isobutylene involves transfer of a hydrogen radical from monomer to the radical-cation to form the r-butyl carbocation and an unreactive allyl-type radical ... 

The electron capture detector is the result of a series of developments which were initiated in 1951 by D. J. Pompeo and J. W. Otvos (14) of the Shell Company s development laboratory in California. The device they invented was a beta-ray ionization cross-section detector (Section 5.8). Deal et al. (15) at the Shell laboratory in California and Boer (16) in Amsterdam modified the detector, used originally to monitor effluents of a large scale plant process, for applications in GC. From the limited success of the detector. Lovelock (17) produced the beta-ray argon detector in 1958 (Section 5.8). This modification substituted argon as the carrier gas and placed a potential of 1000 V across the electrodes. Argon passing between the electrodes absorbed radiation and formed a metastable species with energy (11.6 eV) sufficient to ionize most substances. Proposed mechanisms for this process are ... 

Different techniques have been used to study the products of photoreactions of organometallic compounds for example, irradiation of the arene complexes [CpFe() -arene)]+ resulted in the substitution of the arene by solvent or other potential ligands present in solution. In solutions containing an epoxide monomer, this photochemical reaction generated a species that initiated polymerization. Ion cyclotron resonance Fourier transform mass spectrometry and electrospray ionization mass spectrometry were used to elucidate the mechanism of these photoinitiated polymerizations. 

Polymerization initiated by ionizing radiation such as y rays or high-energy electron beams is somewhat similar to plasma polymerization. Understanding of radiation polymerization is very helpful in elucidating mechanisms of plasma polymerization. In radiation polymerization, no initiator is employed, and the chain-carrying species are created by the ionization of monomer. In this respect, radiation polymerization... 

Alkaline earth metal atoms have fairly low ionization potentials, as have alkali metal atoms (e.g., 5.21 and 5.14 eV for barium and sodium, respectively [89]). Hence the reactions of alkaline earth metal atoms with oxidizing molecules are also expected to be initiated by an electron transfer and should follow the harpoon mechanism. However, alkali metal atoms are monovalent species, whereas alkaline earth metal atoms have two valence electrons. Hence peculiarities are to be expected in the alkaline earth metal reaction dynamics, especially when doubly charged products such as BaO are to be formed [90]. The second valence electron also opens up the possibility of chemiluminescent reactions, which are largely absent in alkali metal atom reactions [91, 92]. The second electron causes the existence of low-lying excited states in the product. 

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