Catiomers and aniomers: unique classes of isomeric ions

Fragmentation pathways arising from protonation at different sites in aminoalkyl‐substituted 3‐hydroxy‐1,2,5‐oxadiazoles (3‐hydroxyfurazans)

“…29 In these instances, different types of product ions are obtained by fragmentation of isobaric ions differing in structure and/or the site of protonation. [8][9][10]14,30 Thus, structure elucidation depends on the recognition of the protonation site (protomers 31 ), 4,7,8,[15][16][17]19,22,23,32 the existence of tautomers, 22,33 and the identification of the exact ionized form undergoing fragmentation. 9,30,33 The latter is challenging because ion formation may be influenced by the wide range of properties associated with ions of diverse structure and experimental conditions.…”

A fixed‐charge model of the N‐protomer of 4‐aminobenzoic acid to facilitate the study of the unimolecular and bimolecular chemistry of its “neutral” carboxylic acid group

“…29,30 The low-energy CID spectrum of 7 is shown in Figure 1. The major fragmentation reaction observed is N-C bond homolysis to produce the radical cation of 4-dimethylamino benzoic acid, 8, at m/z 165 (Equation [1]). Whereas this bond homolysis is in violation of the "even-electron rule," 31 methyl radical loss has been reported for a number trimethylammonium aryls.…”

“…The DFT-calculated energy diagram for the three fragmentation reactions observed in the CID spectrum of 7 is shown in Figure 2. The methyl radical loss (Equation [1]), which appears not to have a transition state based on a scan in which the N-C Me distance is elongated to 3.4 Å (Figure S6 [supporting material]), has a ΔH 0 of 62.8 kcal/mol and is the lowest energy pathway, consistent with 9 being the major product ion. The loss of trimethylamine (Equation [2]), which appears not to have a transition state based on a scan in which the N-C Ar distance is elongated to 3.4 Å (Figure S7 [supporting material]), has the highest ΔH 0 at 101.2 kcal/mol, consistent with it being a minor product.…”

“…A relatively underappreciated “hidden” mixture of ions that can be produced via electrospray ionization (ESI) are those with identical m/z values but in which the site of the charge is located at different functional groups within the analyte. For [M + H] + these isomeric ions have been called protomers 1 . Under “mobile proton” conditions 2,3 these protomers can interconvert during low‐energy collision‐induced dissociation (CID), prior to fragmentation, so the resultant tandem mass spectrometry (MS/MS) spectra are reproducible 4 .…”

Automated and efficient quantum chemical determination and energetic ranking of molecular protonation sites

“…This has prompted researchers to investigate the protonated molecule equilibrium in the context of mass spectrometry, since the reactions of ionized molecules depend crucially on the location of the charge . In recent years, the terms protomer (isomers of protonated molecules) and catiomer (isomers of molecules with a positive molecular charge depending on the site of protonation) have appeared in the literature . Research has been conducted on the protomer equilibria for a wide range of molecules, from nicotine and its derivatives to peptides and drug molecules like taxol .…”