Multiple stage pentaquadrupole mass spectrometry for generation and characterization of gas-phase ionic species. The case of the PyC₂H₅^+· isomers

Abstract

Eleven isomers with the PyC2H 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(+)-CH 2 (·) (7-9), C5H5N(+)-CH2CH 2 (·) (10), and C5H5N(+)-CH(·)-CH3 (11). Unique features of the low-energy (15-eV) CID and 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 PyC2H5 (+·) system, which provided theoretical support for facile 4⇌7 interconversion evidenced in both CID and ion-molecule reaction experiments. The ab initio spin densities for the a-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 derealization 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.

Mass spectrometric studies of elusive molecules that contain an N+-X? bond

“…These ions, by analogy with the behavior of CH 2 ±O± CH 2 Á (5 Á ) (Corderman et al, 1976;Bouma et al, 1980;Lange 1993), react with neutral pyridineÐboth by CH 2 Á and CH ± CH 3 Á transfers. As expected, 84 Á radical cations are generated during the interaction between 86 Á ions and pyridine (Sorrilha et al, 1996).…”

Properties and Reactivity of Gaseous Distonic Radical Ions with Aryl Radical Sites

“…Eberlin and co-workers later generated and differentiated the three conventional and a total of eight distonic ethylpyridine radical cation isomers by using multiple stage pentaquadrupole MS 2 and MS 3 experiments. 121 Examination of the reactivity of the isomeric ethylpyridine radical cations toward dioxygen confirmed the above finding 115 that the β-distonic N -ethylenepyridinium cation reacts extensively while the α-distonic ion is completely unreactive. Interestingly, the α-distonic meta - and para -methyl substituted N -methylenepyridinium ions were found to readily react with dioxygen while the ortho- analog was unreactive.…”

“…Ab initio calculations found the spin densities for the α-distonic ions to be largely localized on the N -methylene radical site. 121 Hence, high spin densities on the radical sites of distonic ions do not necessarily lead to radical coupling reaction with oxygen.…”

Gas-Phase Bimolecular Reactions between •CH2—(CH2)n—C+=O Distonic Ions and Pyridine: A Combined Experimental and Theoretical Study

“…These ions were prepared by Eberlin et al by reacting pyridine molecular ions with ethylene in their pentaquadrupole instrument. 19 They also demonstrated that the easy hydrogen loss from these m/z 107 ions probably yields the N-vinyl pyridinium cation (m/z 106).…”