Gas-phase ionization

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 [, ... 

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]. 

Gas phase ionization techniques ionize the molecules that are in the gas phase. It is possible to bring molecules to the gas phase only of compounds that are volatile. To fulfil this requirement, a molecule must be nonpolar, not too big (molecular weight <1000 Da) and thermally stable, i.e. it must not decompose when heated. Although these are harsh... 

The most important gas phase ionization techniques are electron ionization and chemical ionization. The latter will be described in Section 2.6.2. 

Increasing molecular mass eventually limits the effectiveness of gas-phase ionization methods. What is the practical upper limit (approximately 1000 Da). 

Experimental and theoretical studies of the basicity and acidity of benzene-substituted indoles were performed169. In aqueous solution, the pK values are above —4 for the bases and above 15 for the acids. Gas-phase ionization enthalpies have been calculated using the AMI semiempirical method. 

The most commonly used LC/MS interfaces in pharmaceutical analysis are ESI and APCI. An ESI interface on the majority of commercial mass spectrometers utilizes both heat and nebulization to achieve conditions in favor of solvent evaporation over analyte decomposition. While ionization in APCI occurs in the gas phase, ionization using ESI occurs in solution. Attributes of a mobile phase such as surface tension, conductivity, viscosity, dielectric constant, flow rate and pFi, all determine the ionization efficiency. They therefore need to be taken into consideration and controlled. 

Atmospheric pressure chemical ionization (APCI) is a gas phase ionization process based on ion-molecule reactions between a neutral molecule and reactant ions [31]. The method is very similar to chemical ionization with the difference that ionization occurs at atmospheric pressure. APCI requires that the liquid sample is completely evaporated (Fig. 1.12). Typical flow rates are in the range 200-1000 xL min , but low flow APCI has also been described. First, an aerosol is formed with the help of a pneumatic nebulizer using nitrogen. The aerosol is directly formed in a heated quartz or ceramic tube (typical temperatures 200-500 °C) where the mobile phase and the analytes are evaporated. The temperature of the nebulized mobile phase itself remains in the range 120-150 °C due to evapo-... 

The above considerations need relativistic correction at v c, which may be performed in a straightforward manner. More importantly, Eq. (10) assumes that the ionization process is direct, i.e., once a state above the ionization potential is reached, ionization occurs with a certainty. Platzman [25] points out that in molecules, this is not necessarily so and superexcited states with energy exceeding the ionization potential may exist, which will dissociate into neutral fragments with a certain probability. For example, in water in the gas phase, ionization occurs with a sharp threshold at the ionization potential (I.P.) = 12.6 eV, but only with an efficiency of 0.4. Beyond the I.P., the ionization... 

Two main liminations have, however, become evident the hrst is that molecules whose gas-phase ionization energy exceeds > 9.5 eV cannot be oxidized by ionized solid Ar. The reason for this limitation is unclear, because the process is exothermic (the ionization energy of solid Ar is 13.9 eV, that of organic molecules in Ar is typically lowered by 1 eV in solid Ar relative to the gas phase ). Perhaps the localization of the spin and charge onto the substrate entails a Frank-Condon barrier that cannot easily be surmounted at 12 K. 

Despite our earlier enunciated electronegativity and bond polarity logic, we must forego nearly all comparison with the free (uncomplexed) carbanions. Unlike the rather stable cyclopropyl anion, the cyclobutyl and cyclopentyl ions are unbound with regard to loss of their extra electron. That is, the gas phase ionization process to form the radical from the carbanion, Ru R" -E e, is energetically favorable. 

The choice of the ionization method depends on both the nature of the sample and the type of information required from the analysis (Table 23.2). A great variety of ionization methods exists that can be classified into six major categories gas-phase ionization, field desorption and ionization, particle bombardment, atmospheric pressure ionization, and the laser desorption. 

Ionization methods such as electron impact, chemical ionization, desorption chemical ionization, and negative-ion chemical ionization are all based on ionization of gas-phase samples and, thus, fall within the first category of gas-phase ionization. 

In atmospheric pressure chemical ionization (APCI) a similar interface to that used for ESI is used. A corona discharge is used to ionize the analyte in the atmospheric pressure region. The gas-phase ionization in APCI is more effective than ESI for analyzing less polar species. Both ESI and APCI are complementary methods that are well-suited for LC/MS techniques. 

Correlations have been made between gas-phase ionization potentials of free ions and the redox potentials of isostructural [MXe]"" complexes of the elements of the same row of the periodic table (476). Despite the observation of such correlations, caution must be taken, because they ignore both cr and tt ligand field effects. The latter are often more important in influencing the relative oxidizing or reducing strength of complexes. 

Ionization interference can be a problem in the analysis of alkali metals at relatively low temperature and in the analyses of other elements at higher temperature. For any element, we can write a gas-phase ionization reaction ... 

Methylselenophene-2-thiol and 2,5-dimethylselenophene-3-thiol have been shown to exist as thiols by NMR and IR spectroscopy as well as by comparison of gas phase ionization potentials with those of appropriate model compounds (77ACS(B)198). Dipole moment studies have supported formulation of selenophene-2- and -3-thiols as such (73BSF1924) rather than the 2-thione structure originally proposed for the former compound (71BSF3547). 

Correlation of gas-phase ionization potential data with solution redox potentials for series of organic, organometallic and coordination compounds21 23 underlines the relationship between F(E°q- ° ) and orbital energies. For example, Cr 3d binding energy data for the first ionization process of complexes [Cr(CO)5L] correlate reasonably linearly with the (E°0 E°n) parameter, J°L,23 as is illustrated by Figure 3. 

Redox potential data frequently correlate with parameters obtained by other spectroscopic measurements. The correlation of E° potentials with gas-phase ionization potentials has already been briefly discussed. Electronic transitions observed by UV-visible spectroscopy involve the promotion of an electron from one orbital to another and this can be viewed as an intramolecular redox reaction. If the promotion involves the displacement of an electron from the HOMO to the LUMO, then the redox potentials for the reduction of the compound, °REd, and for its oxidation, °ox, are of importance. For a closely related series of compounds, trends in oxidation and reduction potentials can be related to shifts in the absorption frequency, v. If the structural perturbation causes the HOMO and the LUMO to rise or fall in energy in tandem, then (E°RED — E°ox) will remain constant in such cases the HOMO—LUMO frequency (energy) will be essentially independent of the structural perturbation. Where there is a differential influence of the perturbation on the HOMO and the LUMO, then ( °red E°ox) will vary as will the energy of the electronic transition. In such cases a linear correlation of °red or E°0x may result. In the limit the energy of the HOMO, or more usually the LUMO, will be unaffected by structural perturbation where the acceptor orbital is pinned, direct linear correlation of E°Gx with v should be apparent. With E°ox and v in a common energy unit, the plot E°0x versus v should have a slope close to one.33-36... 

For the trifluoroacetylation of 2-substituted thiophenes, furans, and pyrroles in C2H4C12, 75°, the p values are —7.4, —10.3, and ca. —4.5, respectively.259 The value for substituted benzenes is not known. In the gas phase ionization of substituted furans, thiophenes, selenophenes, and pyrroles,264 a reaction proceeding through a positively charged molecular ion taken to be analogous to the Wheland intermediate for electrophilic substitution, the p values are reported to be —20.2, —16.5,... 

The major chemical processes in radiation chemistry are reduction and oxidation reactions, according to the following examples. In the gas phase, ionization predominates... 

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