From the mobile proton to wandering hydride ion: mechanistic aspects of gas‐phase ion chemistry

Abstract

Structural characterization of molecular species by mass spectrometry supposes the knowledge of the type of ions generated and the mechanism by which they dissociate. In this context, a need for a rationalization of electrospray ionization(+)(-) mass spectra of small molecules has been recently expressed. Similarly, at the other end of the mass scale, efforts are currently made to interpret the major fragmentation processes of protonated and deprotonated peptides and their reduced forms produced in electron capture or electron transfer experiments. Most fragmentation processes of molecular and pseudo-molecular ions produced in the ion source of a mass spectrometer may be described by a combination of several key mechanistic steps: simple bond dissociation, formation of ion-neutral complex intermediates, hydrogen atom, hydride ion or proton migrations and nucleophilic attack. Selected crucial aspects of these elementary reactions, occurring inside positively charged ions, will be recalled and illustrated by examples taken in recent mass spectrometry literature. Emphasis will be given on the protonation process and its consequence in terms of structure and energetic.

Gas‐phase dissociation study of erythrinian alkaloids by electrospray ionization mass spectrometry and computational methods

“…Protonation at the methoxy and hydroxy groups leads to the dissociative channel, and the side chain is eliminated as a neutral fragment. Protonation must take place on the most basic site, but migration of the proton after activation or low protonation on this secondary place must be considered [37,38] because the latter processes could culminate in elimination of methanol or water during protonation. We will exploit these possibilities in the proposed mechanisms.…”

“…Protonation on the nitrogen atom generates ions that are more stable (by 12 kcal mol À1 in the case of compound 2b) than the ions that would emerge due to protonation on the methoxy group. However, proton could migrate during the CID experiments, [38] which is a possibility that must be explored.…”

Gas‐phase fragmentation of protonated piplartine and its fungal metabolites using tandem mass spectrometry and computational chemistry

“…[53] MS/MS fragmentations were suggested as being initiated by protonation at the most stable site (higher PA), but the possibility of a mobile proton must be evaluated. [54] Thus, we suggest fragmentation on the basis of the protonation and the possibility of proton migration to the most reactive sites.…”

Characterization of Wax Esters by Electrospray Ionization Tandem Mass Spectrometry: Double Bond Effect and Unusual Product Ions

“…It was difficult to unambiguously determine the double bond position from MS/MS analysis, likely due to the migration of a hydride ion. [34]…”

“…However, this observation may also result from ammonium adduct formation (or proton adduct after loss of ammonia) with the double bond via cation-π interaction [44, 45] and charge-remote dissociation involving a six-membered ring transition state [46, 47] followed by hydride ion transfer (or a series of sequential transfers) [34, 49] (Supplementary Scheme S1a). …”