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|Type:||Artigo de periódico|
|Title:||Multiple stage pentaquadrupole mass spectrometry for generation and characterization of gas-phase ionic species. The case of the PyC(2)H(5)(+center dot) isomers|
|Abstract:||Eleven isomers with the PyC(2)H(5)(+.) composition, which include three conventional (1-3) and eight distonic radical cations (4-11), have been generated and in most cases successfully characterized in the gas phase via tandem-in-space multiple-stage pentaquadrupole MS(2) and MS(3) experiments, The three conventional radical cations,that is, the ionized ethylpyridines C2H5-C5H4N+.(1-3), were generated via direct 70-eV electron ionization of the neutrals, whereas sequences of chemical ionization and collision-induced dissociation (CID) or mass-selected ion-molecule reactions were used to generate the distonic ions H2C.-C5H4N+-CH3 (4-6), CH3-C5H4N+-CH2. (7-9), C5H5N+-CH2CH2. (10), and C5H5N+-CH.-CH3 (11). Unique features of the low-energy (15-eV) CID an ion-molecule reaction chemistry with the diradical oxygen molecule of the isomers were used for their structural characterization. All the ion-molecule reaction products of a mass-selected ion, each associated with its corresponding CID fragments, were collected in a single three-dimensional mass spectrum. Ab initio calculations at the ROMP2/6-31G(d, p)//6-31G(d, p) + ZPE level of theory were performed to estimate the energetics involved in interconversions within the PyC(2)H(5)(+.) system, which provided theoretical support for facile 4 reversible arrow 7 interconversion evidenced in both CID and ion-molecule reaction experiments. The ab initio spin densities for the alpha-distonic ions 4-9 and 11 were found to be largely on the methylene or methyne formal radical sites, which thus ruled out substantial odd-spin delocalization throughout the neighboring pyridine ring. However, only 8 and 9 (and 10) react extensively with oxygen by radical coupling, hence high spin densities on the radical site of the distonic ions do not necessarily lead to radical coupling reaction with oxygen. The very typical 'spatially separated' ab initio charge and spin densities of 4-11 were used to classify them as distonic ions, whereas 1-3 show, as expected, 'localized' electronic structures characteristic of conventional radical ions. (C) 1996 American Society for Mass Spectrometry|
|Editor:||Elsevier Science Inc|
|Citation:||Journal Of The American Society For Mass Spectrometry. Elsevier Science Inc, v. 7, n. 11, n. 1126, n. 1137, 1996.|
|Appears in Collections:||Unicamp - Artigos e Outros Documentos|
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