Radical electron ionization

For a limited range of substances, negative radical anions (M ) can be formed rather than positive ions (Equation 3.3). Negative radical anions can be produced in abundance by methods other than electron ionization. However, since most El mass spectrometry is concerned with positive ions, only they are discussed here. 

There are different ways to ionize a molecule (M, Scheme 2.1) extraction of an electron from gas phase molecules (Mg), yielding radical cations [Equation (2.1)], as occurs in electron ionization, or addition of one [Equation (2.2) Cl, MALDI, etc.)] or more protons [Equation (2.3) ESI]. Similarly, molecules can be ionized by the formation of negative ions due to single [Equation (2.4)] or multiple proton abstraction [Equation (2.5)]. 

The electron ionization (El) mass spectra of TMS ethers and esters are generally characterised by weak or absent molecular ions. The [M—15]+ ion formed by loss of a methyl radical is generally abundant and in the case of alcoholic functions, the loss of a trimethylsilanol molecule [M—90]+ is also diagnostic. The peak at mJz 73, corresponding to the TMS group, is important in nearly all the TMS-derivative mass spectra. Figure 8.2 shows the fragmentation of TMS esters and ethers in mass spectrometric analyses. 

Electron ionization (earlier called electron impact) (see Chapter 2, Section 2.1.6) occupies a special position among ionization techniques. Historically it was the first method of ionization in mass spectrometry. Moreover it remains the most popular in mass spectrometry of organic compounds (not bioorganic). The main advantages of electron ionization are reliability and versatility. Besides that the existing computer libraries of mass spectra (Wiley/NIST, 2008) consist of electron ionization spectra. The fragmentation mles were also developed for the initial formation of a radical-cation as a result of electron ionization. 

A little recognized systematic error in the calculation of accurate masses of, for example, small radical cation molecular ions (as in electron ionization (El)) or protonated molecular ions (as seen in the soft ionization methods) is the fact that the electron has a small, but finite mass. The accurate masses of radical cations, in which a valence electron has been removed, of anions that have been created by capture of an electron, and of protonated species produced by soft ionization processes, should take into consideration this small mass difference [19]. For example, there is a small difference between the relative atomic mass of a neutral hydrogen atom and a proton. The accepted accurate mass of 1H° is 1.007825 Da. The accurate mass of 1H+ is 1.007276 Da. To be completely correct, expected accurate masses of protonated molecular ions, [M+H]+, produced by electrospray should be calculated using the mass of one H+, rather than all of neutral hydrogen atoms. Mamer and Lesimple do acknowledge, however, that, for large molecules, the error is of little consequence. 

In the rare case the neutral was a radical, the ion created by electron ionization would be even-electron, e.g., for nitric oxide ... 

Electron ionization mainly creates singly charged positive ions by ejection of one electron out of the neutral. If the precursor was a molecule, M, it will have an even number of electrons, i.e., an even-electron or closed-shell species. The molecular ion formed upon EI must then be a positive radical ion, M" , odd-electron or open-shell) ion. 

The El mass spectrum of acetone is comparatively simple. It basically shows three important peaks at m/z 58, 43, and 15. According to the formula C3H6O, the peak at m/z 58 corresponds to the molecular ion. The base peak at m/z 43 is related to this signal by a difference of 15 u, a neutral loss which can almost always be assigned to loss of a methyl radical, CH3. The m/z 15 peak may then be expected to correspond to the ionic counterpart of the methyl radical, i.e., to the CH3 carbe-nium ion (Fig. 6.3). The question remains, as to whether this mass spectrum can be rationalized in terms of ion chemistry. Let us therefore consider the steps of electron ionization and subsequent fragmentation in greater detail. 

The EPR spectra of electrolytically produced anion radicals of Q -aminoanthraquin-ones were measured in DME and DMSO. The isotropic hyperfine coupling constants were assigned by comparison with the EPS spectra of dihydroxy-substituted antraquinones and molecular-orbital calculations. Isomerically pure phenylcarbene anion (PhCH ) has been generated in the gas phase by dissociative electron ionization of phenyldiazirine. PhCH has strong base and nucleophilic character. It abstracts an S atom from and OCS, an N atom from N2O, and an H atom from... 

In the gas phase, homolytic bond dissociation enthalpies (D//) relate the thermochemical properties of molecules to those of radicals while ionization potentials (IP) and electron affinities (EA) tie the thermochemistry of neutral species to those of their corresponding ions. For example, Scheme 2.1 represents the relationships between RsSiH and its related radicals, ions, and radical ions. This representation does not define thermodynamic cycles (the H fragment is not explicitly considered) but it is rather a thermochemical mnemonic that affords a simple way of establishing the experimental data required to obtain a chosen thermochemical property. 

In summary, preliminary experiments have demonstrated that the efficiency and outcome of electron ionization is influenced by molecular orientation. That is, the magnitude of the electron impact ionization cross section depends on the spatial orientation of the molecule widi respect to the electron projectile. The ionization efficiency is lowest for electron impact on the negative end of the molecular dipole. In addition, the mass spectrum is orientation-dependent for example, in the ionization of CH3CI the ratio CHjCriCHj depends on the molecular orientation. There are both similarities in and differences between the effect of orientation on electron transfer (as an elementary step in the harpoon mechanism) and electron impact ionization, but there is a substantial effect in both cases. It seems likely that other types of particle interactions, for example, free-radical chemistry and ion-molecule chemistry, may also exhibit a dependence on relative spatial orientation. The information emerging from these studies should contribute one more perspective to our view of particle interactions and eventually to a deeper understanding of complex chemical and biological reaction mechanisms. 

The kinetics data of the geminate ion recombination in irradiated liquid hydrocarbons obtained by the subpicosecond pulse radiolysis was analyzed by Monte Carlo simulation based on the diffusion in an electric field [77,81,82], The simulation data were convoluted by the response function and fitted to the experimental data. By transforming the time-dependent behavior of cation radicals to the distribution function of cation radical-electron distance, the time-dependent distribution was obtained. Subsequently, the relationship between the space resolution and the space distribution of ionic species was discussed. The space distribution of reactive intermediates produced by radiation is very important for advanced science and technology using ionizing radiation such as nanolithography and nanotechnology [77,82]. 

In this connection investigations are to be mentioned in which a mass-spectrometric analysis has been made of neutral radicals, e.g., CHjCO, split off from acetone by u.v. photons in the ordinary range.27-28 In the first a flash lamp has been used and the radicals were ionized as usual by electron impact. In the second the same radical ionized at a field emission electrode. Recently, several alkyl radicals generated by pyrolysis have been studied. Their values of lv and of the photoionization cross sections could be obtained in the mass spectrometer under monochromatic vacuum u.v. irradiation.29... 

On exciting the MP anion radical with the second quantum of light, the anion radical is ionized and an electron is trapped by a CC14 particle... 

The gas-phase ionization of 2,4,6-tribromobenzene in the presence of m-fluoropyridine afforded the A -aryI -m-fluoropyridinc adduct from which the biradical cation was generated by loss of two bromine radicals.232 This biradical species was isolated and characterized using Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry and its chemical properties are discussed. FT-ICR was also used to isolate and characterize the products of electron ionization of fluorinated acetyl compounds, which included a biradical anion.233... 

The structure of diphosphallenic radical cations, generated from the allene ArP=C=PAr by electrochemical oxidation, has been examined using EPR spectroscopy. Ab initio calculations including correlation effects at the MP2 and MCSCF levels have determined that two rotamers exist compatible with Jahn-Teller distortion of the allene.146 Anodically generated radical cations of alkyl phosphites [(RO P] and silylphosphites [(RO)2POSiMe3] reacted with alkenes by initial attack at the C=C bond followed by electron transfer, deprotonation, and elimination of an alkyl or trimethylsilyl cation to form identical alkyl phosphate adducts.147 The electron ionization-induced McLafferty rearrangement of n-hexylphosphine afford the a-distonic radical cation CTEPH, the distinct reactivity of which suggests there is no... 

Recently, the distinction between electrophilic and ion radical (electron-transfer) mechanisms of addition reactions to the vinyl double bond of aryl vinyl sulfides and ethers has been achieved by studying substituent effects (Aplin Bauld 1997). Specifically, the effects of meta and para substituents on the rates of electrophilic addition correlate with Hammett cr values, while ionization of the substrates to the corresponding cation radicals correlates with cr+. The significance of the respective correlations was confirmed by statistical tests. The application of this criterion to the reaction of aryl vinyl sulfides and ethers with tetracyanoethylene revealed that formation of cyclobutanes occurs via direct electrophilic addition to the electron-rich alkene and not via an electron-transfer mechanism. 

However, applying extraction by solvent to the nuclear field is not an easy task for the solvent that undergoes multiple attacks—chemical, thermal, but especially radiolytic. This multiplicity is reinforced by the biphasic nature of the chemical system and the presence of numerous solutes, be it in aqueous or organic phase. Radiolysis of such a system thus leads to the formation of a multitude of radicals and ionized species (including the reactive species II-, OH-, solvated electrons, H2, or H202), which recombine in molecular products shared between the two phases. 

The first process involves electron ionization to form radical M-1"1 molecular ions. This process has been observed primarily for nonpolar molecules. The proposed mechanisms are charge-exchange transitions between sputtered ions and the neutral organic molecules or electron attachment of low-energy secondary electrons to neutral molecules. The fragmentation reactions of the M ions usually follow the dissociation pathways for odd-electron gas-phase ions. 

Very different and distinct ion chemistry has been observed in the reaction between the fragment ions obtained by electron ionization of tetramethoxygermane, Ge(OMe)4, and the parent neutral81. Reactions in this system proceed by nucleophilic addition followed by elimination of formaldehyde and/or elimination of methanol. An overview of the reactions of the different ions with Ge(OMe)4 is shown in Scheme 13 for the even electron ions, and in Scheme 14 for the radical ions originating from tetramethoxygermane. In these schemes, the neutral reagent of the ion/molecule reactions, Ge(OMe)4, is not shown for the sake of simplicity but the schemes include the neutral products that are eliminated upon addition of the reagent ion to the parent neutral molecule. 

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