Electron ionization, mechanistic

Since dithio- and selenocarbamates and their derivatives are used so widely in the deposition of thin films and nanoparticles that a mechanistic study of their decomposition behavior was carried out by O Brien et al. [ 107]. Wold et al. [78] studied the decomposition products of Zn(S2CNEt2)2 using gas-chromatography mass spectrometry (GC-MS) and their reported deposition path shows clean ehmination of ZnS from the precursor (Eqs. 1 and 2 below). However, the proposed decomposition route is somewhat different to the step-by-step fragmentation observed in the electron-ionization mass spectrometry (EI-MS) of the compoimd, (Eq. 3). This difference can be attributed to inherent differences between the two techniques. 

Three types of ionization processes were distinguished ( 1) as contributing to the ions observed in molecular SIMS spectra direct desorption of precharged materials, cat ionization/anionization, and electron ionization (Equations 1-3, respectively). The equations illustrate overall reactions and do not attempt to explain detailed mechanistic steps. 

Combined electron ionization mass spectroscopy (EIMS) and matrix isolation FTIR spectroscopic data on vacuum pyrolysis of 1,1-dimethyl-l-germa-3-thietane assisted by theoretical calculations provide a reasonable foundation for mechanistic interpretation of its thermal decomposition (see Section 2.21.6.1, Equation 7) <1998JA5005>. 

The formal potentials ( ° ) of the three kinds of SODs were found to be dependent on solution pH as displayed in Fig. 6.6. As shown, the formal potential of bovine erythrocyte Cu, Zn-SOD decreases linearly with increasing solution pH with a slope of ca. -60mV/pH from pH 5.8 to pH 9.5 (curve b), indicating one proton and one electron are included in the electrode reaction of Cu, Zn-SOD, which is similar to previously proposed enzymatic catalytic mechanistic scheme of the Cu, Zn-SOD [139— 144], In contrast, the pH dependency of Fe-SOD from E. coli was complicated (curve a) the formal potential changes linearly with solution pH in a range from pH 5.8 to 8.5 with a slope of ca. -60mV/pH, and becomes pH-independent at above pH > 8.5. Previous studies have observed that the Fe (III) form of the protein ionizes with an apparent pKa of 9.0 0.3 and such ionization effect has been interpreted in terms of hydrolysis of a bound water molecule with p/<"a of ca. 8.5 [145], The C -pII profile of... 

Major emphasis has been on the isolation and identification of the main decomposition products arising from one electron oxidation reactions with the pyrimidine and purine bases of isolated DNA and related model compounds13,14D. In recent years, major interest has been devoted on the delineation of the mechanistic features of charge transfer within double stranded DNA. This is mostly achieved using defined-sequence oligonucleotides in which radical cations are generated in most cases by photo-ionization of selected nucleobases and 2-deoxyribose. For more information on these systems, the reader is encouraged to read the recent review article by Cadet et al.134 and other references mentioned there in. 

The inability of the electron-rich nucleophile molecules to approach phosphorus atoms of highly electronegative binary complexes (see Section III,D,5,a and Fig. 4) formed from the enzyme and the more extensively ionized species of phosphoanhydrides extant in the alkaline pH range is believed to explain this lack of reactivity of the latter compounds at the higher pH values (43). Support for this concept is found in the results of recent studies (43, 43), which indicate that carbamyl-P, which according to the above theory should be active at alkaline as well as acid pH values, displays potent phosphotransferase ability with the enzyme even at pH > 7. Mechanistic significance of these observations is considered further in Section III,D,5. 

On the basis of these data, it was proposed that precursors other than trapped sugar radicals are responsible for causing a significant fraction of strand breaks at low hydrations. A new mechanism was proposed in which two one-electron oxidations of a sugar at the same site lead to a diamagnetic product (which cannot be detected by ESR). Mechanistically, it was later proposed that the diamagnetic product is a carbocation formed by direct ionization followed by an oxidation by a mobile or nearby base cation radical. ... 

Pulse radiolysis is the radiation chemical analogue of flash photolysis. It is a fast-kinetics technique that enables transitory processes, initiated by the absorption of ionizing radiation, to be observed in time frames as short as the submicrosecond region. It permits the detection and characterization of short-lived intermediates, the determination of the kinetics of their decay, and a probing of reaction mechanisms. The technique finds use in the study of radiation effects on materials, and as a tool for the examination of mechanistic details. For inorganic systems, pulse radiolysis is used to characterize metal complexes in unusual oxidation states, to examine the kinetics and rates of ligand-labilization reactions and to elucidate the mechanism of electron transfer. 

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