Ionization pathway

Scheme 5). The latter process proved to be faster in the more polar CD3CN than in CDCI3, which is consistent with an ionization pathway for the rearrangement. The bromination of (39) in CH2CICH2CI followed second-order kinetics with a rate... 

Proceeding with the large-R analysis of the ionization matrix element, (f V i), we find that at large Kr2-X separations it decreases as 1/R r2 x. As a result, the contribution of the two-electron recombination - ionization pathway to the decay width depends on the cluster geometry as l/-R r Kr Kr2-x-Since the decomposition of the (Kr+)2X cluster along the Kr+-Kr+ coordinate automatically means elongation of the Kr2-X distance as well, the power law exponents are effectively summed, resulting in the l/R Kr dependence. A detailed analysis shows that this type of power law is characteristic of all the possible decay pathways. 

Application of the extended Grunwald-Winstein equation to solvolyses of propyl chloroformate, PrOCOCl, in a variety of pure and binary solvents indicated an addition-elimination pathway in the majority of the solvents but an ionization pathway in the solvents of highest ionizing power and lowest nucleophilicity. For methanolysis, a solvent deuterium isotope effect of 2.17 was compatible with the incorporation of general-base catalysis into the substitution process.21... 

Ionization reactions can occur under vacuum conditions at any time during this process but the origin of ions produced in MALDI is still not fully understood [27,28], Among the chemical and physical ionization pathways suggested for MALDI are gas-phase photoionization, excited state proton transfer, ion-molecule reactions, desorption of preformed ions, and so on. The most widely accepted ion formation mechanism involves proton transfer in the solid phase before desorption or gas-phase proton transfer in the expanding plume from photoionized matrix molecules. The ions in the gas phase are then accelerated by an electrostatic field towards the analyser. Figure 1.15 shows a diagram of the MALDI desorption ionization process. 

Fig. 7. Common ionization pathways (a) protonation ofa basic site (b) association of a cation to a basic site (c) halide loss (d) oxidation (e) deprotonation of an acidic site.

EUis, D. A., Mabury, S. A. Chemical ionization pathways of polyfluorinated chemicals - a connection to environmental atmospheric processes. J. Am. Soc. Mass Spectrom., 14 1177-1191 (2003). 

Phases in the System XeF2/XeF5Asp6 and Structural and Vibrational Evidence for the Following Ionization Pathway Xep2 XeF+ + F"... 

Scheme 6. (A) Top, classical SN1 ionization pathway. For most common glycosylation reactions the reactive species probably resemble SSIPs and oxacarbenium ions, although this is not proved. (B) Revised schematic ionization energy versus reaction-coordinate model (compare Fig.lC left side). Relative energies are arbitrarily chosen, and may vary from case to case.

Since experimentally and theoretically optimized pulses obtained from OCT and CCL are available for NaK [84], first, it was possible to show under which conditions the shaped pulses are reproducible, and second, the connection between the forms of the shaped pulses and different ionization pathways was established. This allowed one determination of the mechanism for the maximization of the ionization yield under the participation of several excited states. The agreement between experimentally and theoretically optimized pulses, which was independent from the initial guess, showed that the shapes of the pulses can be used to deduce the mechanism of the processes underlying the optimal control. In the case of optimization of the ionization process in NaK, this involved a direct two-photon resonant process followed by a sequential one-photon processes at later times. These findings obtained for the simple system are promising for the use of shapes of tailored pulses to reveal the nature of processes involved in the optimal control of more complex systems. This will be addressed below. 

Chemi-ionization studies53,54 of the K2 + XY reactions have revealed surprisingly large cross sections, Q 3-10 A2, for reaction leading to ions. Three chemi-ionization pathways are energetically accessible,... 

In the two-color ionization employed here, various ionization pathways can produce highly vibrationally excited ions on account of involvement of Rydberg states. It is particularly difficult to discern between the competing 3-photon ionization pathways involving the combination of photons fid + hco + Aa>2, and Aa>i + + h(02. In our work we were able to unambiguously identify the... 

A target may resist hydrolysis or chemical degradation, or the degradation products may not yield useful information. It is also common that insufficient material exists for proper analysis. In these cases, an alternative degradation technique is available that uses the ionizing electron beam of a mass spectrometer. The ionization pathways available from electron impact in the mass spectmm are bond fission processes that occur by known and predictable pathways. Indeed, each pathway usually follows analogous chemical reaction pathways in a retro-synthetic manner. It therefore follows that an examination of mass spectral ionization patterns can give clues for suitable disconnections and a synthetic tree. 

DFT studies of the hydrolysis of acetyl and chloroacetyl chloride and of variously substituted benzoyl chlorides supported an S 2 mechanism. An extended Grunwald-Winstein equation correlation for the specific rates of solvolysis of 3,4,5-trimethoxybenzoyl chloride gave sensitivities towards changes in solvent nucleophilic-ity (/-value) of 0.29 and solvent ionizing power (m-value) of 0.54. The low m-value allowed specific rates to be determined in highly ionizing fiuoroalcohol/H20 mixtures. A parallel correlation of the specific rates of solvolysis of 2,4,6-trichlorobenzoyl chloride revealed that solvolyses in 100% and 90% ethanol or methanol did not appreciably follow the ionization pathway indicated for solvolyses in the other solvents and it was proposed that, despite the two or//to-substituents, the addition-elimination pathway had become dominant. ... 

The ionization pathway depends on the type of intermediate ions generated from the reagent gas as well as the properties of the analyte molecules. Therefore, the choice of reagent gas is particularly important. Considering the ionization pathway of proton transfer, intermediates should be able to donate protons while the analyte (A) should exhibit a tendency to accept a proton. The latter property is referred to as proton affinity (PA). 

The development of new ionization strategies usually opens the way to new fields of application. In particular, soft ionization techniques such as atmospheric pressure chenaical ionization (APCI) are interesting analytical tools for inorganic/organic MS. The mechanisms for the generation of reagent ions and the ionization pathways are similar for the most common APCI sources. First, the buffer gas is ionized by a beam of electrons accelerated in a high electric field. Then, a series of reactions. 

The mechanisms of MetA-SIMS are still under investigation. Recent simulations explained the sputtering yield enhancement due to the presence of the metal upon atomic bombardment [336], but the effect of the metal on the ionization mechanisms is not well understood yet. Metallization creates or enhances different ionization pathways for molecules departing the surface, the most obvious being the cationization by a metal atom/ion [132,324,325]. The enhancement observed for bare fragment [322] and parent ions [322,323], both... 

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