Cation radicals in mass spectrometry

Cation radicals in mass spectrometry, 526 Cellobiose, 991-992 Cellulose, 994 Cembrene, 1027 Center of symmetry, 264—265 in meso-2,3-butanediol, 280 Cephalexin, 803 Cephalosporins, 803 Cerebrosides, 1047 Chair conformation... 

Some of the target molecules gain so much excess internal energy in a short space of time that they lose an electron and become ions. These are the molecular cation-radicals found in mass spectrometry by the direct absorption of radiation. However, these initial ions may react with accompanying neutral molecules, as in chemical ionization, to produce protonated molecules. 

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. 

By analogy with their behavior in mass spectrometry, branched hydrocarbons are cleaved when oxidized in CH3 CN/TEABF4 at —45 °C. The resulting acetamides of the fragments (Table 6) are formed by cleavage of the initial radical cation at the C,C bond between the secondary and tertiary C atom, to afford after a second electron transfer, carbocations, which react in a Ritter reaction with acetonitrile [29]. 

Ionization of 1,5-hexadiene in fluorochloroalkane matrix (Scheme 2.43) represents cation-radical monomolecular reactions. The initially formed cation-radical collapses to the cyclohexane cation-radical, that is, spontaneous cyclization takes place (Williams 1994). Zhu et al. (1998) pointed out that the ring formation from the excited valence isomer in the center of Scheme 2.43 is easier than in the corresponding ground-state dienes. Notably, tandem mass spectrometry revealed the same transformation of 1,5-hexadiene in the gas phase too. This provides ns with a hint that mass spectrometry can serve as a method to express predictions of monomolecnlar transformation of cation-radicals in the condensed phase. A review by Lobodin and Lebedev (2005) discnsses this possibility in more detail. 

Varied Methane Cations. The methane molecular ion (methane radical cation, CH4+ ), the parent ion in mass spectrometry, and the methane dication (CH42+) are of great significance and have been studied both experimentally and theoretically.800 802 Recent advanced studies have shown that the methane radical cation, CH4+ has a fivecoordinate planar structure as suggested in early calculations by Olah and Klopman.800... 

Hydrogen and chlorine atoms have an odd number of electrons and are radicals. The methyl radical is the simplest organic radical. It has one more electron than a carbocat-ion and one fewer than a carbanion. The last example is a radical cation, which results from the loss of one electron from a normal molecule. Radical cations are important in mass spectrometry (see Chapter 15). 

Vinyl cations and radical cations can be generated from the corresponding alkenes (equation 30). The appearance potentials and fates of the species thus generated are more of interest in mass spectrometry than in carbonium ion chemistry, and have recently been reviewed by Loudon and Maccoll (1970). 

Charged radical cations and anions are often indicated by the symbol, formula, or structure with a superscript dot followed by a plus or minus sign. However, in mass spectrometry, the reverse is used. Therefore, use the order of dots and signs for charges that is appropriate for the context. 

In mass spectrometry, the molecular ion is a cation radical. Further fragmentations of the molecular ion can be of two types - those that produce a cation plus a radical, and those that produce a different cation radical plus a neutral atom. In all cases, the fragment bearing the charge - whether cation or cation radical - is the one that is detected. 

Oxidative cleavage may begin with a loss of an electron from a heteroatom or an anion. Removal of an electron creates a radical cation in the absence of a suitable nucleophile or the possibility of losing a proton, the system may be stabilized by bond cleavage and formation of a double bond in a way similar to that found in mass spectrometry [Eqs. (14) through (17)]. Sulfur is more easily oxidized that nitrogen, which loses an electron more easily than oxygen. 

In mass spectrometry, the main types of cations arising from ionization of a neutral molecule M are radical cations M+ (typically formed via El) and pro-tonated ions [M + H]+ (formed in several ionization processes, including Cl, FAB, ESI, and MALDI). The energies associated with the formation of these gas-phase ions are the ionization potential and the proton affinity, respectively. These quantities are defined in more detail below. 

In mass spectrometry, a small sample of a compound is introduced into an instrument called a mass spectrometer, where it is vaporized and then ionized as a result of an electron s being removed from each molecule. Ionization can be accomplished in several ways. The most common method bombards the vaporized molecules with a beam of high-energy electrons. The energy of the electron beam can be varied, but a beam of about 70 electron volts (eV) is commonly used. When the electron beam hits a molecule, it knocks out an electron, producing a molecular ion, which is a radical cation—a species with an unpaired electron and a positive charge. 

Mass spectrometry allows us to determine the molecular mass and the molecular formula of a compound, as well as certain structural features. In mass spectrometry, a small sample of the compound is vaporized and then ionized as a result of an electron s being removed from each molecule, producing a molecular ion—a radical cation. Many of the molecular ions break apart into cations, radicals, neutral molecules, and other radical cations. The bonds most likely to break are the weakest ones and those that result in the formation of the most stable products. The mass spectrometer records a mass spectrum—a graph of the relative abundance of each positively charged fragment, plotted against its mjz value. 

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