Mass spectrometry radical cation formation

The mass spectrometry of oxazole compounds has been reviewed by Traldi ef al. <1980H(14)847>. The main fragmentation pathway for most oxazole rings is shown in Scheme 1. Radical cation formation is followed by cleavage of the 0-C(2) bond, and then sequential loss of CO and HCN or nitrile <19920MS317>. 

Mass Spectrometry Aldehydes and ketones typically give a prominent molecular- ion peak in their mass spectra. Aldehydes also exhibit an M— 1 peak. A major fragmentation pathway for both aldehydes and ketones leads to formation of acyl cations (acyliurn ions) by cleavage of an alkyl group from the carbonyl. The most intense peak in the mass spectrum of diethyl ketone, for exanple, is m/z 57, conesponding to loss of ethyl radical from the molecular- ion. 

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. 

Formation of dihydrotropylium ions is a key feature of the C H9+ hypersurface. Currently, efforts in our laboratory276 have concentrated on the presence of different C H9+ isomers by probing their bimolecular reactivity. Thus, gas-phase titration in the FT-ICR mass spectrometer has revealed that mixtures of C7H9+ ions are formed by protonation of 1,3,5-cycloheptatriene, 6-methylfulvene and norbomadiene as the neutral precursors but that, in contrast to the results obtained by CS mass spectrometry, fragmentation of the radical cations of limonene yields almost exclusively toluenium ions275. 

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. 

This also proves an earlier conclusion on hyperconjugation in an 0CH20 fragment of the 1,3-dioxolane cation radical this conclusion was based on mass spectrometry (To-dres, Kukovitskii et al. 1981). As calculated, the carbon-hydrogen bonds corresponding to 0CH20 in the radical cation are weaker than those in the neutral molecule. For this reason, this site exhibits a maximal probability that deprotonation will result in the formation of the 2-yl radical (Belevskii et al. 1998). In experiments, photoirradiation of 1,3-dioxolane solutions in sulfur hexafluoride at 77 K really leads to formation of the cation radical of 1,3-dioxolane and the l,3-dioxolan-2-yl radical as a result of deprotonation. Consecutive ring... 

These fragmentations serve to illustrate many of the major types. The driving force behind all of them is the formation of stable cations and radicals. Fragmentations of functional groups that have not been covered here are often similar to those described earlier. Although this has been only a very brief introduction to mass spectrometry, the power and utility of this technique should be apparent. 

It was in 1990 that Kratschmer et al. [217,218] reported the first macroscopic preparation of in gram quantities by contact-arc vaporization of a graphite rod in a 100 Torr atmosphere of helium, followed by extraction of the resultant soot with toluene. Fullerene ions could also be detected by mass spectrometry in low-pressure hydrocarbon flames [219]. The door was opened by, Kratschmer and co-workers preparative success to extensive studies of the electrochemical behavior of the new materials. Cyclic voltammetry of molecular solutions of Ceo in aprotic electrolytes, e.g., methylene chloride/quatemary ammonium salts, revealed the reversible cathodic formation of anionic species, the radical anion, the dianion, etc. (cf. [220,221]). Finally, an uptake of six electrons in the potential range of 1-3.3 V vs. SHE in MeCN/toluene at — 10°C to form the hexavalent anion was reported by Xie et al. [222]. This was in full accordance with MO calculations. A parametric study of the electroreduction of Cgo in aprotic solvents was performed [223]. No reversible oxidation of C o was possible, not even to the radical cation. However, the stability of di- and trications with special counterions, in the Li/PEO/C 3 MoFf cell, was claimed later [224]. 

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. 

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