Gas phase ionization potentials

Electrospray is viewed as the most versatile ionization technique for neutral compounds and ions in solution, and at the same time a general-purpose interface for LC-MS [14-24]. In electrospray ions are formed in solution and then transferred to the gas phase. This differs from APCI where neutral molecules are first transferred to the gas phase and then ionized by gas-phase ion-molecule reactions. The heart of the electrospray source is a metal capillary through which the sample solution flows. A potential of 3-6 kV is applied to the capillary forming a spray of fine droplets directed towards a counter electrode with a sampling orifice located about 1-3 cm from the capillary tip. A positive potential is applied to the capillary to generate positive ions, and a negative potential for negative ions. To accommodate different liquid flow rates droplet formation is assisted by optimization of the orifice diameter of the capillary sprayer, the use of a coaxial gas flow, and heat to increase the rate of solvent evaporation. 

A mixture of water/pyridine appears to be the solvent of choice to aid carbenium ion formation [246]. In the Hofer-Moest reaction the formation of alcohols is optimized by adding alkali bicarbonates, sulfates [39] or perchlorates. In methanol solution the presence of a small amount of sodium perchlorate shifts the decarboxylation totally to the carbenium ion pathway [31]. The structure of the carboxylate can also support non-Kolbe electrolysis. By comparing the products of the electrolysis of different carboxylates with the ionization potentials of the corresponding radicals one can draw the conclusion that alkyl radicals with gas phase ionization potentials smaller than 8 e V should be oxidized to carbenium ions [8 c] in the course of Kolbe electrolysis. This gives some indication in which cases preferential carbenium ion formation or radical dimerization is to be expected. Thus a-alkyl, cycloalkyl [, ... 

Radicals can be prepared from closed-shell systems by adding or removing one electron or by a dissociative fission. Generally speaking, the electron addition or abstraction can be performed with any system, the ionization potential and electron affinity being thermodynamic measures of the probability with which these processes should proceed. Thus, to accomplish this electron transfer, a sufficiently powerful electron donor or acceptor (low ionization potential and high electron affinity, respectively) is required. If the process does not proceed in the gas phase, a suitable solvent may succeed. 

We shall look more closely at this equation. On one hand, the standard chemical potentials of Ox and Red depend on their standard Gibbs solvation energies, AG ox and AG°Red, and, on the other hand, on the standard Gibbs energy of ionization of Red in the gas phase, AGlon Red. This quantity is connected with the ionization potential of Red, /Rcd, which is, however, a sort of enthalpy so that it must be supplemented by the entropy term, -TA5 on Red. Thus, Eq. (3.1.17) is converted to the form... 

GG and GGG sequences have been widely used in strand cleavage studies of hole migration in DNA [4-6]. According to conventional wisdom, GG and GGG sequences serve as hole traps . The calculated gas phase ionization potentials reported by Sugiyama and Saito [14] provide relative energies for G vs GG (0.47 eV) and G vs GGG (0.68 eV) that continue to be cited as evi-... 

The difference in stabilities of cation radicals located on G, GG, and GGG sequences was initially investigated by Sugiyama and Saito [14], who employed ab initio methods to calculate the gas phase ionization potentials of nucleobases stacked in B-DNA geometries. Their results indicated large differences in potential for holes on G vs GG (0.47 eV) and GGG (0.68 eV) sequences. A similar G vs GG difference was calculated by Prat et al. [62]. These values suggest that GG and GGG are, in fact, deep hole traps and they have been widely cited as evidence to that effect [54, 63]. 

Following Platzman (1967), Magee and Mozumder (1973) estimate the total ionization yield in water vapor as 3.48. The yield of superexcited states that do not autoionize in the gas phase is 0.92. Assuming that all of these did autoion-ize in the liquid, we would get 4.4 as the total ionization yield. This figure is within the experimental limits of eh yield at 100 ps, but it is less than the total experimental ionization yield by about 1. The assumption of lower ionization potential in the liquid does not remove this difficulty, as the total yield of excited states in the gas phase below the ionization limit is only 0.54. 

The potentially tautomeric side-chain thiol systems exist mainly in the thiol form in liquid solution and in the gas phase, as found by IR and NMR spectroscopy and by a study of ionization potentials.126 Upon alkylation using the ion-pair extraction method, only the S-alkylated compounds were obtained. The synthesis, reactions, and properties of some selenides of thiophene, furan, and selenophene have been reviewed.127... 

The electron-donor property of the enol form is indicated by the magnitude of its ionization potential (IP in the gas phase) or its oxidation potential ( °x in solution).4 8 Conversely, the relatively electron-poor keto form is a viable acceptor in measure with the magnitude of its electron affinity (EA in the gas phase) or its reduction potential in solution). 

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