Ion‐neutral complexes in unimolecular decompositions

Abstract

I. BACKGROUNDThe concept that some decompositions of ions in the gas phase take place through ion-neutral complexes has advanced sufficiently since Morton's classic review (1) to make timely a fresh survey of the subject. The reader is referred to Morton's article for discussion of relationships between complex-mediated reactions of ions in the gas phase, solvolysis reactions, unimolecular eliminations from neutrals, and conventional unimolecular reactions of ions. This review primarily addresses the role of ion-neutral complexes in unimolecular decompositions. The binding forces that maintain ion-neutral complexes and the transition states for their formation and reaction will be described first. This description is extended by discussing studies of ion-molecule reactions,'molecular orbital calculations, and decompositions of ionized steroids that illuminate the nature of ion-neutral complexes. Proposed criteria for distinguishing complex-mediated reactions from "conventional" reactions are then listed. Finally, ion-neutral eomplex-mediated reactions are surveyed. Although to keep the review to readable length the survey is not exhaustive, representative examples of complex-mediated reactions are presented. The term ion-neutral complex is applied to a species in which an ion and a neutral are held in association chiefly by electrostatic attractions. Species such as 'CH20H2 + , which have been referred to as ion-neutral complexes (2), but in which the partners are joined by a covalent bond (3), will not be considered.The suggestion of Rylander and Meyerson (4) that ionized alkyl benzenes decompose through cationated cyclopropanes was perhaps the first proposed ion-neutral complex-mediated dissociation of an ion. Although loss of water through [RCH==CHCH=N+=N-OH-] formed from an adduct of an ionmolecule reaction was proposed in 1977 (5), current investigations of ion-neutral complexes as intermediates in unimolecular dissociations were sparked by the

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

“…The CID spectrum of the MH ϩ ion of Compound 20 is shown in Figure 6b. The neutral loss observed from the spectrum is CH 2 O, not CD 2 O, which shows that the proton transfers to the ␣-carbon in the 4-N,N-dimethylphenyl group and rarely to the phenyl-d 5 . In light of all the above results and discussions, Scheme 2 proposes the probable fragmentation pathways.…”

“…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].…”

Dissociation of the N-Cα bond and competitive formation of the [zn−H]•+ and [cn+2H]+ product ions in radical peptide ions containing tyrosine and tryptophan: The influence of proton affinities on product formation

“…Rotations of large groups within ionneutral complexes are relatively commonplace and are achieved via low barriers [32][33][34].…”

A photoionization study of the ion-neutral complexes CH3CH +CH3·CH2CH3] and CH3CH2CH+CH3·CH3 in the gas phase: Formation, H-transfer and C—C bond formation between partners, and channeling of energy into dissociation

“…I on-neutral complexes (ionic and neutral fragments held together primarily by noncovalent attractions) are often intermediates in low energy dissociations of ions in the gas phase [1][2][3][4][5]. As the energy in an ion is increased toward the threshold for simple dissociation, partial dissociation to noncovalently bound fragments that can react with each other begins to occur [6][7][8].…”