Molecular Radicals

Unstable species such as O, FI and N atoms, molecular radicals and vibrationally excited diatomics can be injected by passmg the appropriate gas tluough a microwave discharge. In a SIFT, the chemistry is usually straightforward since there is only one reactant ion and one neutral present in the flow tube. 

Irradiation of the molecular radical anion of DESO, which has a yellow color, with light of X = 350-400 nm partially restores the red color and the ESR spectrum of the radical-anion pair. Similarly to the case of DMSO-d6 a comparison of the energetics of the photodissociation of the radical anion and dissociative capture of an electron by a DESO molecule permits an estimation of the energy of the hot electrons which form the radical-anion pair of DESO. This energy is equal to 2eV, similarly to DMSO-d6. The spin density on the ethyl radical in the radical-anion pair of DESO can be estimated from the decrease in hfs in comparison with the free radical to be 0.81, smaller than DMSO-d6. 

The rate expressions have been written in generalized fashion with the terms fp, fb, fst, and fgt containing the reaction rate constants, stoichiometric coefficients, and concentrations of the various stable species present in the reaction mixture. If one also wished to consider bi-molecular radical processes, these could also be lumped into the / parameters. 

CH4/O2 and CF4 as the reactant gases and observed the formation of [M — 4 H] ions in the Cl plasma (Scheme 13)166. Thus, repeated deprotonation and electron transfer processes appear to offer an efficient access to more highly unsaturated and/or ring condensed trimethylenemethane radical anions. The [M — 4 H]- ion is considered identical to the molecular radical anion (42) of acepentalene (43), which was generated as a short-lived species from the former by neutralization-reionization mass spectrometry167. Efforts to apply Squires methodology to triquinacene 41 and the tribenzotriquinacenes 44 have been made168. 

In the early studies, the cycloalkane holes were viewed as molecular radical cations that undergo rapid resonant charge transfer. At any given time, the positive charge was assumed to reside on a single solvent molecule and, once in 0.5-2 psec, to hop to a neighboring molecule. The low activation energy was explained by the similarity between the shapes of cycloalkane molecules and their radical cations [60]. 

Rate constants for the self-reactions of a number of tertiary and secondary peroxy radicals have been determined by electron spin resonance spectroscopy. The pre-exponential factors for these reactions are in the normal range for bi-molecular radical-radical reactions (109 to 1011 M"1 sec 1). Differences in the rate constants for different peroxy radicals arise primarily from differences in the activation energies of their self reactions. These activation energies can be large for some tertiary peroxy radicals (—10 kcal. per mole). The significance of these results as they relate to the mechanism of the self reactions of tertiary and secondary peroxy radicals is discussed. Rate constants for chain termination in oxidizing hydrocarbons are summarized. 

Finally, it should be remembered that a surface bombarded by ions will be a source of neutral atoms and molecular radicals which can react chemically with the surrounding surfaces. 

It has been proposed that, in most cases, the ion of mass 91 is a tropylium rather than a benzylic cation. This explains the ready loss of a methyl group from xylenes, although toluene does not easily lose a methyl group. The incipient molecular radical ion of xylene rearranges to the methylcycloheptatriene radical ion, which then cleaves to the tropylium ion (QH/). 

The existence of the molecular radical ion 02 , of atomic O-, and of the regular ions in the lattice O2- has been firmly established. A review by Lunsford (33) presents a summary of the experimental evidence which led to the discovery of 02 and O-. The participation of these various forms of oxygen in hydrocarbon oxidation is discussed in a review by Sachtler (11). It seems clear that both adsorbed and lattice oxygen species play an important role in the selective oxidation of hydrocarbons. 

This is followed by attaching (accepting) the resulting low-molecular radical to the diamagnetic surface center containing two-coordinated silicon atom ... 

Therefore, by using the appropriate molecules, one can generate various low-molecular radicals r (H, D, OH, OD, CH3, etc.) and subsequently obtain the silicon-centered radicals containing chemically different substituents. The EPR parameters of some of the PCs obtained by this method are presented in Table 7.7. 

Low-molecular radicals OH and H were also detected under UV-irradiation of the surface (=Si-0)2Si -0H radicals (our unpublished data). Thus, in this case the decomposition with the production of the low-molecular radical is also typical for the electronically excited state of the... 

In both examples considered it was only possible to detect the primary products of photo decay of the surface defects because they represented low-molecular radicals and could easily leave the place of their birth and can be trapped by especially chosen acceptors. 

DOSGs are efficient acceptors of low-molecular free radicals [51,73] (reactions 2-4) in Table 7.12). The formation of the corresponding products occurs, most likely, in two steps (except the H atom). The low-molecular radical first adds to one of the oxygen atoms of the cycle, which leads to the cleavage of the 0-0 bond and the formation of the oxy radical ... 

The formation of paramagnetic sites in the reactions of DOSGs with saturated hydrocarbon molecules indicate that, in one of the steps, the system is transformed into the biradical state. The 0-0 bond of the cycle is the most probable precursor for formation reactive in such processes. It can be assumed that, in both cases, the process occurs through the biradical state. However, in the first case, the low-molecular radical escapes from the reaction zone through the gas phase, and paramagnetic sites are formed. In the second case, the spatial separation of free radical sites is impossible, and they combine or disproportionate to form non-paramagnetic reaction products. 

This method using grafting to one silicon atom of SC makes it possible to approach spatially two chemically different groups r and rt and obtain free radical structures of the (=Si-0-)2Si(r )(r) type. This leads to more diverse intermediates on the solid surface and allows one to study new chemical processes involving these intermediates, including intramolecular reactions between r and /y groups. The low-molecular radicals necessary for the first step of SC modification can conveniently be obtained from the saturated H-r molecules, and appropriate defects on the silica surface can be used as... 

Let us discuss several specific examples to demonstrate the possibility for obtaining various intermediates and study of their reactivity. The next example illustrates another important advantage of the method possibility for the control of the processes of formation of low-molecular radicals during chemical reactions and obtaining the data on their nature. It was mentioned in Section 4 that one can obtain samples that have diamagnetic groups... 

The singlet triplet energy gap, A/isx. in organic biradicals35 182 183 and (inter-molecular) radical ion pairs184-190 has been shown to be a useful indicator of electronic coupling in ET and excitation energy transfer (EET) processes.191-194... 

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