Negative ion radical

The electron could also be captured by the neutral to form a negative radical ion. However, electron capture (EC) is rather unlikely to occur with electrons of 70 eV since EC is a resonance process because no electron is produced to carry away the excess energy. [10] Thus, EC only proceeds effectively with electrons of very low energy, i.e., from thermal electrons up to a few electronvolts (Chap. 7.4). However, the formation of negative ions from electron impact may occur with analytes containing highly electronegative elements. 

Definition The molecular ion has the same empirical formula as the corresponding neutral molecule. The neutral and its molecular ion only differ by one (or more) electron(s). A singly charged molecular ion can either be a positive radical ion, M" , or a negative radical ion, M" (not in case of El). The mass of this ion corresponds to the sum of the masses of the most abundant isotopes of the various atoms that make up the molecule (with a correction for the elec-tron(s) lost or gained. Chap. 3.1.4). [8,9]... 

Note The symbolism M" does not mean one added electron. The radical symbol is added to the molecular ion only to indicate a remaining unpaired electron after ionization. Addition of one electron to a neutral would make it a negative radical ion, M"" (electron capture). 

Any molecule that can at least formally be synthesized by a Diels-Alder reaction is a potential candidate for the mass spectral RDA reaction. In addition, the RDA reaction is not restricted to positive radical ions, but it may also occur from even-electron ions as well as from negative radical ions, NT. These findings remind us of the fact that ionic reactions are determined by the intrinsic properties and the internal energy of the ions, and thus only indirectly by the ionization technique used for their creation. 

Example The Bunte salt [ 3( 2)158-803] Na yields a very useful negative-ion FAB spectrum from NBA matrix (Fig. 9.11). NBA forms [Ma-H] and Ma " ions. The salt anion contributes the base peak at m/z 337.3. [Ch-2A] m/z 697.5, and [2C-I-3A] m/z 1057.6, cluster ions are observed in addition, their isotopic patterns being in good agreement with theoretical expectation. It is noteworthy that the matrix adduct at m/z 513.4 is a negative radical ion. 

Orbital diagrams of electroluminescence (or recombination luminescence) of positive and negative radical ions... 

The results characterizing MgO can be summarized as follows Adsorption of electron-acceptor molecules such as NB in vacuo onto a MgO powder prepared by thermal outgassing in vacuo exhibited an EPR signal that could be assigned to NB or the corresponding negative radical ions A. The concentration of the radicals was monitored as a function of the outgassing temperature of MgO in vacuo. Twm maxima were found in the... 

Some compounds may also form negative radical ions under electron bombardment. 

This is one of several reactions of this type in which an organic negative radical-ion and its parent molecule react in the presence of an alkali metal. It is found, rather interestingly, that the rate coefficients depend on the nature of the metal. To account for this, it has been postulated that the metal is involved in a bridging role in the activated complex, e.g., dipy.. K" ". . dipy for the case of 2,2 -dipy-ridyl (dipy) A more extreme case of this association between the radical-ion and the ion of the alkali metal used to form it occurs in the reaction of benzophenone with its negative ion. The spectrum of (benzophenone)" in dme has many hyper-fine lines caused by the interaction of the free electron with the and, when the metal is sodium, the Na nuclei. When benzophenone is added, the structure, due to the proton interaction, disappears and only the lines associated with the sodium interaction remain. To account for this, it has been suggested that the odd electron moves rapidly over all the proton positions too fast for the lines characteristic of the electron in the different proton environments to be seen), but relatively slowly from one sodium nucleus to another. Seen another way, this means that the transfer of an electron from molecule to molecule is associated with the transfer of the cation . ... 

Nitrobenzene (14) and nitrosobenzene (10) each reacts rapidly with e q to produce the corresponding negative radical ions. C0H.5NO2" absorbs strongly at 2900 A. with a subsidiary peak near 4600 A., while for C6Hr,N02, the maximum is at 2850 A. 

The species initially produced in the mass spectrometer are ions derived from normal molecules by loss of an electron. Such an ion will normally contain an odd number of electrons and is consequently described as a radical ion. The same is true of the negative radical ion formed from a neutral molecule by capture of an electron [equation (7.3)]. The relationships between a neutral molecule M, the derived radical ions and M", and the lowest singlet excited state M are shown in Fig. 7.5. 

Positive radical ions can be formed rather easily by bombarding neutral molecules with energetic electrons. The formation of negative radical ions, i.e., radical anions, is more difficult since radical anions have very low ionization potentials and easily lose their extra electrons to regenerate the parent molecules. Such a species can therefore be formed by electron capture, i.e., by the process... 

Mononegative ions of organic compounds and a few negative ions of inorganic compounds (in the following "negative radical ions"). 

Organic negative radical ions Reduction of organic compounds wth alkali metals or electrochemical reduction. 

Inorganic negative radical ions — Anorganische negative Radikalionen... 

Negative radical ion from Generation Matrix or solvent T K V kMo/s g-factor Splitting parameters gauss References for g and a Further references... 

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