Negative electron capture dissociation

While most peptide dissociation is carried out in the positive ion mode, the negative ion mode is often better suited for acidic peptides, particularity those carrying acidic modifications (e.g., phosphorylations). There are a number of equivalent ion activation methods for peptide anions, involving ion-electron and ion-ion reactions, such as electron detachment dissociation (EDD) [49], negative electron transfer dissociation (NETD) [50, 51], and negative electron capture dissociation (nECD) [52]. 

Quasiequilibrium statistical theory was applied to the negative ion mass spectra of diphenylisoxazoles. Electron capture by the isoxazole leads to molecular ions having excited vibrations of the ring and of bonds attached to it. The dissociation rate constants were also calculated (77MI41615, 75MI416U). 

Furlei and coworkers44 studied the negative ion mass spectra of several cyclic sulfones (82-98) upon dissociative electron capture and concluded that the negative molecular ions were notably stabilized by the introduction of electron-withdrawing substituents and/or unsaturation. Some difference was found in the negative ion mass spectra of configurational isomers (85 vs. 86 and 87 vs. 88) in contrast to the situation in their positive ion spectra. A strong S02 ion (m/z 64) was observed also for all the compounds studied. 

Resonance electron capture directly yields the negative molecular ion, M"", whereas even-electron fragment ions are formed by dissociative electron capture and ion-pair formation. Molecular ions are generated by capture of electrons with 0-2 eV kinetic energy, whereas fragment ions are generated by capture of electrons from 0 to 15 eV. Ion-pair formation tends to occur when electron energies exceed 10 eV. [77]... 

Capture may take place to bound states of the negative ion that undergo radiationless transitions to repulsive states of the negative ion resulting in dissociative electron capture. These radiationless intramolecular transitions (Auger transitions) are a result of overlap of the discrete states with a continuum of states of AB-. These types of processes are illustrated for diatomic molecules in Figure 2.2b. Electrons are first captured into the bound state represented by curve 1, and before autodetachment can take place an intramolecular radiationless transition occurs to curve 2 resulting in dissociation into A and B-. 

From the electron beam current variation experiments of Stuckey and Kiser, it is known that two electrons are involved in the overall process of the formation of a doubly-charged negative ion. One of these electrons must be involved in the initial formation of the singly-charged ion by either resonant capture, dissociative resonant... 

Our thermal functions Indicate that PF" is thermodynamically stable with respect to dissociation (P + F") below 2500 K. These predictions agree qualitatively with the results of MacNeil and Thynne ( ) who observed PF" in the negative ion mass spectrum of PF. Their reported appearance potential, AP(PF"/PFg) = 11.4 0.1 eV, gives EA(PF) = -0.47 eV assuming that the dissociative electron capture process is PF (g) + e" = PF"(g) + 2F(g). We believe this value is too low probably because of excess kinetic and/or excitation energies amounting to roughly 1.5 eV. 

A similar kinetic model has been developed for the measurement of ion complex formation kinetics and energies involving both dissociative and nondissociative electron capture, for example, the hydration of halide ions and of 02( ). The ratio of negative ions observed in NIMS can be used to determine energies of complex formation. In this case the sequential formation of the higher complexes must be added to the kinetic model. These studies are important because they demonstrate that the API mass spectrometer can be used to measure thermodynamic quantities. When we use the data for hydrates of 02(—) as an example, the kinetic expression is given by... 

Dissociative electron capture is observed with hyperthermal electrons in NIMS electron impact experiments. In order for dissociative electron capture to take place with thermal electrons, there must be a dissociative pathway that is accessible by the thermal activation of the neutral molecule or a low-lying negative-ion state. The quantity D(R — Le) — Ea(Le) must be less than about 1.0 eV. This limit has been established empirically. Two types of dissociative thermal electron attachment have been observed in NIMS and ECD. The first occurs by unimolecular dissociation in which there is only one temperature region for many compounds. In the original work a low-temperature low-slope region was observed but unexplained. We now believe this could represent the formation of a molecular ion with an electron affinity of about 0.1 eV. The exact nature of this ion is not known, but it could represent stabilization to an excited state. In Figure 4.8 ECD data are plotted for several... 

Compounds under dissociative electron capture in the ECD. DEC(l) refers to molecules that can dissociate unimolecularly via a single potential energy curve. DEC(2) refers to molecules that can dissociate via a negative-ion intermediate. 

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