On the dissociation and conformation of gas-phase methonium ions

Abstract

The dissociation pathways of both doubly and singly charged methonium ions, (CH(3))(3)N(+) -(CH(2))(n)-(+)N(CH(3))(3)·X(-) (n = 6,10; X = Br, I, and OAc), are measured using blackbody infrared radiative dissociation (BIRD) and SORI-CAD in a Fourier transform mass spectrometer. SORI-CAD of the doubly charged decamethonium ions results primarily in the formation of even electron ions by hydrogen rearrangements. In contrast, homolytic bond cleavage to form two odd electron ions is highly favored in the hexamethonium ion, presumably due to increased Coulomb repulsion in this ion. For BIRD of the singly charged salts, ions are mass selected and dissociated by heating the vacuum chamber to elevated temperatures. Under the low pressure conditions of our experiment, energy is transferred from the chamber walls to the ions by the absorption of blackbody radiation. From the temperature dependence of the unimolecular rate constants for dissociation, Arrhenius activation energies in the zero-pressure limit are obtained. The primary dissociation pathways correspond to counterion substitution reactions which result in loss of N(CH(3))(3) and CH(3)X. For hexamethonium and decamethonium with X = Br or I, the branching ratios for these pathways differ dramatically; the ratio of loss of N(CH(3))(3) and CH(3)Br is 3.8 and 0.4 for hexamethonium and decamethonium bromide, respectively. The hexamethonium acetate salt has a branching ratio of 0.1. The Arrhenius activation energies for hexamethonium (Br or I) and decamethonium (Br or I) are 0.9 and 1.0 eV, respectively. This value for hexamethonium acetate is 0.6 eV. Molecular dynamics simulations and Monte Carlo conformation searching are used to obtain the lowest energy structures of hexamethonium and decamethonium bromide. These calculations indicate that the methonium ion folds around the counterion to form a cyclic salt-bridge structure in which both quaternary nitrogens interact with the oppositely charged counterion. The significantly different branching ratios observed for these ions is attributed to the large change in orientation of the counterion with respect to the ammonium centers as the number of methylene groups in these ions increases. Similar ion conformational differences appear to explain the fragmentation for the OAc counter ion as well.

Ion/Ion Reactions with “Onium” Reagents: An Approach for the Gas-phase Transfer of Organic Cations to Multiply-Charged Anions

“…In many cases, a tetraalkylammonium reagent makes up the cation component of the complex and, upon collisional activation, transfers an alkyl cation to its counterion through an S N 2 reaction. This transfer results in both the alkylation of the anionic reagent and formation of a neutral amine as the remainder of the reagent [1]. Collisional activation of the cluster ion can also result in an elimination pathway (E2), wherein a proton, rather than a charged alkyl group, is transferred from the cation to the anion counterpart.…”

Cluster Phase Reactions: Alkylation of Triphosphate and DNA Anions with Alkylammonium Cations

“…[5][6][7] Williams and co-workers reported counterion substitution reactions in clusters of methonium cations and halide anions. 8 These studies of ionic complexes all used collisional activation to initiate reactions.…”

Activation Energies for Dissociation of Double Strand Oligonucleotide Anions: Evidence for Watson−Crick Base Pairing in Vacuo

“…35,36 BIRD has been used to measure Arrhenius parameters for the dissociation of a variety of small ions and clusters, [37][38][39][40][41][42] peptides, 43,44 and proteins. 27,35,45,46 Here, BIRD is used to provide the first quantitative measurements of gas-phase dissociation activation energies and entropies of anionic oligonucleotide duplexes.…”