Assigning structures to ions in the gas phase

Abstract

The purpose of this short review is critically to assess the important experiments in gas phase ion chemistq whose correct interpretation can lead to the assigning of structures to organic positive ions. The methods fall into two main categories, (i) the measurement of ion enthalpies and transition state energies for their fragmentations and ( i ) the detailed examination of the unimolecular and collision-induced fragmentation behaviour of cations, anions and neutral species. It is argued that in general, none of the above methods alone can suflice for an ion structure determination, but that in combination these techniques provide a powerful tool by means of which ion structures may confidently be assigned.

Intramolecular condensation reactions in protonated dipeptides: Carbon monoxide, water, and ammonia losses in competition

“…XxxYyy dipeptides carrying the more basic residue at the C-terminus ( Table 2). The formation of a 2 releases significant kinetic energy and produces non-Gaussian fragment ion peaks, indicating that it must overcome a barrier lying higher in energy than the products [31][32][33]. Such energetics has been reported for the loss of CO from b 2 ions with a protonated oxazolone structure [6].…”

“…In the low-energy regime, CAD spectra were measured as a function of collision energy (energy-resolved MS 2 spectra). For the fragments in the MI spectra of [M ϩ H] ϩ measured at high kinetic energy, the accompanying kinetic energy releases were calculated using fragment peak widths at half height (T 0.5 ) [31]; the quoted T 0.5 values were corrected for the main beam width using established procedures [32,33]. Approximately 100 -200 scans were summed per MI, CAD, or MS 3 experiment, depending on the intensity of the main beam.…”

“…Table 1 lists the MI spectra of the [M ϩ H] ϩ ions from the dipeptides (XxxYyy) studied. Metastable ions have a narrow range of usually low internal energies [31][32][33], which leads to the generation of relatively few fragments. At keV kinetic energies, metastable fragmentations take place within microseconds.…”

“…The kinetic energy releases measured lie in the 0.3-0.6 eV range, which is indicative of dissociation with a substantial reverse barrier, i.e., over a transition state that lies higher in energy than the products. Tight rearrangements are usually associated with such characteristics [31][32][33].…”

“…Some of the T 0.5 values observed are slightly higher than kinetic energy releases found upon dissociations with no reverse barrier (typically Յ0.03 eV) [31][32][33]. The excess could originate from kinetic shifts, which can be substantial for ions of the size of protonated dipeptides that must dissociate within a few microseconds [11,22]; alternatively, H 2 O loss may take place over an appreciable reverse barrier, but the excess energy of the products is dissipated into internal and not translational degrees of freedom.…”