Satellite ions

Biemann further improved the nomenclature scheme by description of immonium and satellite ions [3]. 

At higher energies, fragmentation of the peptide molecule can also occur outside its backbone, that is, in the side chains. Ions formed in this way are termed satellite ions (Fig. 6.8). 

Figure 6.9. The analysis of satellite ions may help to differentiate between leucine and isoleucine, which have identical molecular weights and cannot be distinguished in most MS/ MS experiments. In high energy CID, Leu loses a 43 Da radical, while lie loses a 29 Da fragment.

The optical absorption spectra of the high mobility solvent holes resemble those for the radical cations isolated in freon matrices [20,22-25]. All of these spectra are bell-shaped featureless curves with maxima in the visible and/or near IR regions. In pulse radiolysis studies, the absorption signal from the solvent hole always overlaps with the signals from the fragment (and/or secondary) radical cations ("satellite ions"), even at the earliest observation times [22-25,57]. Therefore, complex deconvolutions are needed to extract the spectra of the solvent holes. This leaves large uncertainty as for the exact shape of the absorption spectra and the extinction coefficients. 

The formation of high-mobility holes and "satellite ions". As was briefly mentioned above, radiolysis of hydrocarbons results in the formation of several types of cationic species besides the solvent holes. Most of these "satellite ions" are generated within the first nanosecond after the radiolytic pulse. 

Transient absorption spectra of some "satellite ions" closely resemble the spectra of olefin radical cations. In cyclohexane, a band centered at 270 nm (at 2 ns [22]) is observed from 250 ps [25] after the ionization event (this band overlaps with the strong 240 nm band of cyclohexyl radicals [22]). The scavenging behavior and the decay kinetics of the UV-absorbing species suggest that they are normally-diffusing radical cations [25]. In the first few nanoseconds after the ionization event, the VIS absorbance is dominated by solvent excited states [22,57]. When the thermalized electrons are rapidly scavenged using a suitable electron acceptor (halocarbons or N2O), this... 

In some cases, the identity of paramagnetic "satellite ions" was established by ODMR [42,44,48]. For example, 9,10-octalin + was identified in decalins and their solutions [42]. ODMR spectra of "satellite ions" in cyclohexane were related to EPR spectra of matrix-isolated cyclohexene + (Note that in the liquid cyclohexane, cyclohexene + undergoes a fast ring-puckering motion that averages hyperfine coupling constants for equatorial and axial protons, so the the EPR spectra of cyclohexene + in liquid and solid matrices are different) [42,44,48]. In both of these cases, the olefin radical cations were formed in spurs rather than in a reaction of the solvent hole with the olefin in the solvent bulk [42] (octalins gradually accumulate as radiolytic products). Olefin "satellite ions" were also observed in squalane [24]. 

How do these "satellite ions" form in the early stages of radiolysis Two ideas were put forward [1,37,42]. First, the "satellite ions" could be generated by fragmentation of short-lived electronically- and/or vibronically-excited solvent holes formed upon the ionization of the solvent, for example... 

Second, the "satellite ions" could be generated in scavenging reactions of the solvent holes with radiolytic products in multiple-pair spurs [25,61-65]. The olefins are formed upon the fragmentation of excited solvent molecules generated in recombination of short-lived electron-hole pairs [1]... 

The prompt formation of the "satellite ions" introduces ambiguity in the measurement of the hole mobility in pulse radiolysis. Indeed, the conductivity is a product of the mobility and the yield [8]. The latter quantity is poorly defined since the branching ratio fh between the high-mobility solvent holes and the "satellite ions" is unknown. 

The way around this problem is to generate solvent holes by means other than radiolysis. Since the formation of "satellite ions" is partly due to spur chemistry, in the laser multiphoton ionization of neat hydrocarbons [14-18]... 

---