Generation and characterization of distonic dehydrophenoxide radical anions under electrospray and atmospheric pressure chemical ionizations (original) (raw)
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European Journal of Organic Chemistry, 2007
The ion formation mechanism in electrospray MS is reviewed, with special focus on the electrochemical red/ox reactions responsible for the formation of radical molecular ions. Prerequisites influencing the likelihood of formation and observation of a particular compound as an open-shell molecular species in ESI-MS (i.e., the structure and the oxidation potential of the analyte, the solvent and additives) are evaluated. For illustration of the ESI phenomena governing radical cation formation, an ESI-MS study of tetra(aryl)benzidine compounds is presented. The facile formation of abundant radical molecular cations in ESI-MS demonstrates imposingly that the basicity of the analyte's nitrogen atoms is strongly overcompensated by the ability to stabilize unpaired electrons. ESI-MS n spectra of the tetra(aryl)benzidine molecular ions exhibit a characteristic feature in the loss of radicals. This process is the major fragmentation pathway of open-shell molecular precursor ions in their MS 2 spectra, and
Journal of the American Chemical Society, 1985
The distonic radical ions CH2CH2X+H, CH2CH2CH2X+H, .CH2CH(CH3)CH2X+H, and .CH2CH2CH2CH2X+H, where X = OH or NH2, and their molecular ion counterparts HRX'. have been studied with isotopic labeling and mass spectra produced by neutralization-reionization (NR) and collisionally activated dissociation (CAD). The isomerization HRX'.-+ .RX+H is more exothermic for X = OH, but has a lower activation energy for X = NH2. For this reaction a 1,Shydrogen rearrangement is favored over 1,4-H, and the 1,3-H and 1,2-H rearrangements were not observed. NR spectra are particularly valuable for characterizing the distonic .RX+H ions, as neutralization produces an unstable hypervalent species. CAD spectra of larger .RX+H and HRX*. ions are very similar; activation of .RN+H3 mainly causes isomerization to HRNH,'. which dissociates by a-cleavage, the lowest energy pathway. Removal of an electron from a gaseous molecule yields an odd-electron molecular ion whose charge and radical centers are ostensibly at the same site, e.g., ionized butanoic acid (1) and I-butanol (4). Distonic radical ions,'S2 in which the radical and
Journal of the American Chemical Society, 2003
We have regiospecifically generated the α,2-, α,3-, and α,4-dehydrophenoxide anions by collisional activation of o-, m-, and p-nitrobenzoate. The α,2 and α,4 isomers also were synthesized by reacting o-benzyne radical anion with carbon dioxide and electron ionization ofp-diazophenol. All three dehydrophenol radical anions were differentiated from each other and identified by probing their chemical reactivity with several reagents. Each isomer was converted to phenoxide and its corresponding quinone as well. Thermochemical measurements were carried out on all three radical anions and their hydrogen-atom affinities, proton affinities, and electron binding energies are reported. These measured quantities are combined in thermodynamic cycles to derive the heats of formation of each of the radical anions and their corresponding carbenes (i.e., α,2-, α,3-, and α,4-dehydrophenol). These results are compared to MCQDPT2, G3, G2+(MP2), and B3LYP calculations and experimental data for appropriate reference compounds.
Journal of Physical Chemistry A, 1997
Decarbonylation is commonly observed in the 70 eV electron impact mass spectra of a series of S-alkyl thioformates, H-C(dO)SR 1-7. Using a combination of collisional activation (CA) and neutralizationreionization (NR) mass spectra, it is shown that the spectra of the so-formed [M-CO] •+ ions differ from the isomeric alkane thiol ions (RSH •+), if R > CH 3. These fragment ions are therefore distonic radical cations, and given the fact that the R-distonic • CH 2-S + H 2 ions are not present under our experimental conditions, it is proposed that they actually are-distonic ions. Strong evidence for the distonic nature of the fragment ions derived from 2-7 •+ has been obtained owing to the use of a new hybrid tandem mass spectrometer presenting a sector-quadrupole-sector configuration. In the case of ethyl derivatives (R) C 2 H 5), both the distonic • CH 2 CH 2 S + H 2 (2b) and conventional CH 3 CH 2 SH •+ (2a) ions are observed. The nonclassical structure 2b reacts indeed with nitric oxide (NO •) giving the production of H 2 S +-NO ions (m/z 64), characterized by high-energy collisional activation. Distonic ions 2b have also been otherwise prepared in an ion-molecule reaction involving the transfer of ionized ethene from • CH 2 CH 2 O + dCH 2 to neutral hydrogen sulfide, H 2 S. Upon CA, the resulting ions 2b show the same behavior as the product of decarbonylation of the S-ethyl thioformate molecular ions. Upon neutralization-reionization, the distonic isomers of the alkane thiol ions give rise to intense peaks corresponding to H 2 S •+ and ionized olefins. Portions of the potential energy surface related to the rearrangement and dissociation processes in the [C 2 H 6 S] •+ system have also been constructed using ab initio molecular orbital calculations at the QCISD(T)/6-311++G(d,p)//UMP2/6-31G(d,p) level. Theoretical results provide further support for the observation of-distonic ions.
Journal of the American Society for Mass Spectrometry
Different collision-activated dissociation (CAD) methods were evaluated for their effectiveness at distinguishing several ionized isomeric aromatic compounds by using a linear quadrupole ion trap/orbitrap mass spectrometer. The compounds were ionized by using atmospheric pressure chemical ionization (APCI) with carbon disulfide solvent in the positive ion mode to generate stable molecular ions with limited fragmentation. They were subjected to CAD in the linear quadrupole ion trap (ITCAD) and in an octupole collision cell (medium-energy collision-activated dissociation, MCAD; also known as HCD). Experiments conducted by attempting to vary ion activation times revealed that MCAD and ITCAD occur in the microsecond and millisecond time regimes, respectively. MCAD was found to impart substantially greater internal energies into the molecular ions compared to ITCAD. Accordingly, molecular ions subjected to MCAD favored dissociation via fast σ-bond cleavages, while molecular ions subjected to ITCAD tended to favor rearrangement reactions. MCAD used in the energy-resolved mode (ER-MCAD) enabled the distinction of six ionized isomeric compounds from each other based on modified crossing-point energies (collision energies where the molecular ions and selected fragment ions have an equal abundance). This was not true for ER-ITCAD. Overall, MCAD was superior over ITCAD at the differentiation of isomeric ions, and it provided more detailed structural information.
Evidence of neutral radical induced analyte ion transformations in APPI and Near-VUV APLI
Journal of the American Society for Mass Spectrometry, 2009
We report on the reactions of neutral radical species [OH, Cl, O( 3 P), H], generated in a typical atmospheric pressure ionization (API) source upon irradiation of the sample gases with either 193 nm laser radiation or 124 nm VUV light, the latter commonly used in atmospheric pressure photoionization (APPI). The present investigations focus on the polycyclic aromatic hydrocarbon pyrene as representative of the aromatic compound class. Experimental results are supported by computational methods: simple kinetic models are used to estimate the temporal evolution of the concentrations of reactants, intermediates, and final products, whereas density functional theory (DFT) energy calculations are carried out to further elucidate the proposed reaction pathways. The neutral radicals are generated upon photolysis of background water and oxygen always present in appreciable mixing ratios in typical API sources. Substantial amounts of oxygenated analyte product ions are observed using both techniques. In contrast, upon atmospheric pressure laser ionization (APLI) with 248 nm radiation, oxygenated products are virtually absent. In addition, kinetic data evaluation yielded a bimolecular rate constant of k ϭ (1.9 Ϯ 0.9) ϫ 10 Ϫ9 cm 3 molecule Ϫ1 s Ϫ1 for the reaction of the pyrene radical cation with OH radicals
Journal of Physical Chemistry A, 1997
Decarbonylation is commonly observed in the 70 eV electron impact mass spectra of a series of S-alkyl thioformates, H-C(dO)SR 1-7. Using a combination of collisional activation (CA) and neutralizationreionization (NR) mass spectra, it is shown that the spectra of the so-formed [M-CO] •+ ions differ from the isomeric alkane thiol ions (RSH •+ ), if R > CH 3 . These fragment ions are therefore distonic radical cations, and given the fact that the R-distonic • CH 2 -S + H 2 ions are not present under our experimental conditions, it is proposed that they actually are -distonic ions. Strong evidence for the distonic nature of the fragment ions derived from 2-7 •+ has been obtained owing to the use of a new hybrid tandem mass spectrometer presenting a sector-quadrupole-sector configuration. In the case of ethyl derivatives (R ) C 2 H 5 ), both the distonic • CH 2 CH 2 S + H 2 (2b) and conventional CH 3 CH 2 SH •+ (2a) ions are observed. The nonclassical structure 2b reacts indeed with nitric oxide (NO • ) giving the production of H 2 S + -NO ions (m/z 64), characterized by high-energy collisional activation. Distonic ions 2b have also been otherwise prepared in an ion-molecule reaction involving the transfer of ionized ethene from • CH 2 CH 2 O + dCH 2 to neutral hydrogen sulfide, H 2 S. Upon CA, the resulting ions 2b show the same behavior as the product of decarbonylation of the S-ethyl thioformate molecular ions. Upon neutralization-reionization, the distonic isomers of the alkane thiol ions give rise to intense peaks corresponding to H 2 S •+ and ionized olefins. Portions of the potential energy surface related to the rearrangement and dissociation processes in the [C 2 H 6 S] •+ system have also been constructed using ab initio molecular orbital calculations at the QCISD(T)/6-311++G(d,p)//UMP2/6-31G(d,p) level. Theoretical results provide further support for the observation of -distonic ions. † University of Mons-Hainaut. ‡ University of Leuven. X Abstract published in AdVance ACS Abstracts, December 1, 1997. Figure 1. Schematic representation of the intermediate region of the Micromass AutoSpec 6F mass spectrometer (hybrid configuration):
Organic Mass Spectrometry, 1990
Mass spectral fragmentations of four norbornaneleoe di-em-and di-endo-fused 1,3-oxazin-2(lH)-ones and four 1,3-oxazine-2(1H)-thiones were examined by means of metastable ion analysis, the collision-induced dissociation technique and exact mass measurement. Under electron impact (EI) conditions all the saturated compounds gave rise to complicated fragmentations, including several rearrangements. For the unsaturated compounds, a retro-Diels-Alder (RDA) process was the most favoured fragmentation pathway, which took place with hydrogen rearrangement to yield RDA + H fragments. The EI mass spectra did not permit isomeric differentiation. Under chemical ionization conditions, stereochemical effects were more perceptible, but isomeric differentiation was still difficult.