Ion–neutral complexes in the unimolecular reactivity of organic cations in the gas phase

Tandem mass spectrum of a growth hormone secretagogue: Amide bond cleavage and resultant gas-phase rearrangement

“…However, a plausible explanation for this isotopic scrambling may be the existence of ion-neutral complexes in the rearrangement process. As is well documented, the most common reactions occurring in ion-neutral complexes are H transfers and H exchanges [14], which can lead to isomerization and isotopic scrambling.…”

“…Recently, ion-neutral complexes have been found to be involved in many gas-phase rearrangements [14]. Specifically, Harrison and co-workers have demonstrated that upon low-energy CID, fragmentations of protonated amide proceed by way of ion-neutral complexes [28].…”

“…The rearrangement cannot be well explained by the above-mentioned mechanisms as described in recent literature. In the past two decades, ion-neutral complexes have been noted in many gas-phase rearrangements of organic ions [13][14][15]. It is proposed that the rearrangements of protonated 1 and 2 also involve ion-neutral complexes, each of which contains a b-type ion and a neutral piperidine formed during the cleavage of the protonated piperidine amide bond.…”

Ion-neutral complexes resulting from dissociative protonation: Fragmentation of α-furanylmethyl benzyl ethers and 4-N,N-dimethylbenzyl benzyl ethers

“…Even before that time, approximately half a century ago, the concept of ion-neutral complexes was first proposed to explain the loss of identity in labeled benzyl produced from tert-butylbenzene [4]. Since then, mechanisms involving the complexes of "weakly coordinated cations" have been suggested to explain a number of otherwise puzzling reactions of ions in the gas-phase [3,[5][6][7][8][9]. Specifically, when an ion encounters a neutral molecule in the gas phase, the interaction may lead to the formation of a loose complex in which the ion and neutral molecule are held together by electrostatic forces, but still maintain their individual mobility [8].…”

“…Since then, mechanisms involving the complexes of "weakly coordinated cations" have been suggested to explain a number of otherwise puzzling reactions of ions in the gas-phase [3,[5][6][7][8][9]. Specifically, when an ion encounters a neutral molecule in the gas phase, the interaction may lead to the formation of a loose complex in which the ion and neutral molecule are held together by electrostatic forces, but still maintain their individual mobility [8]. These ion-neutral complexes may decompose into a product ion and a neutral species, may fragment and recombine into covalently bonded molecular ions, or may undergo another ion-molecule reaction for which the stabilization energy of the products is small.…”