A Mass Spectrometric Study of Organic Ions and Neutrals Using (Muttiple) Collision Experiments

Abstract

<p>The unimolecular chemistry of selected organic (radical) cations has been studied by mass spectrometry based experiments (metastable ion (MI), collisional activation (CA), collision-induced dissociative ionization (CIDI) and neutralization-reionization (NR) mass spectrometry as well as multiple collision experiments (e.g. CA/CA and NR/CA) and appearance energy (AE) measurements) and ²H, ¹³C, ¹⁵N and ¹⁸O isotope labelling experiments in conjunction with ab initio molecular orbital computations. Hydrogen-bridged (radical) cations, ion-molecule complexes and distonic ions are interesting species, partly because their neutral counterparts are usually unknown but, mostly because of their significance as key intermediates in the unimolecular dissociation reactions of gas phase ions. These types of species were found to play vital roles in the unimolecular chemistry of carbamate radical cations, RN(H)COOCH₃⁺ {R = H, CH₃}. When R=H, the metastable molecular ions dissociate to two sets of products, H₂N-CH-OH+HCO˙ and [H₂C=O�H�NH₂]⁺˙+CO, with the first steps of both reactions being: H₂NCOOCH₃⁺˙→H₂N-C(OH)OCH₂⁺˙ (distonic ion)→ [H₂N-C=O�H�C=CH₂]⁺˙ (hydrogen-bridged ion). Although it does not participate in this chemistry, the imidic acid (iminol) ion, H-N=C(OH)-OCH₃⁺˙, is also stable. However, attempts to generate it led instead to the more stable, but difficult to characterize, distonic ion. The surprising loss of a methyl radical from metastable N-methyl-O-methyl carbamate radical cations CH₃N(H)COOCH₃⁺˙, is proposed to involve equilibration with two distonic isomers, CH₃N(H)-⁺C(OH)OCH₂˙ and CH₂=⁺N(H)-˙C(OH)OCH₃, the latter serving as the immediate precursor to the products: ˙CH₃ and the N-carboxyiminium ion, CH₂=N(H)-COOH⁺. The heats of formation of the molecular ion and the product ion were estimated by additivity methods. A CA/CA experiment (CH₃N(H)COOCH₃⁺˙ → CH₃NHCOH˙⁺ → HNCOH⁺) was used to generate and characterize the elusive isoformylaminylium ion. The thermodynamic and kinetic stabilities of CH₅N₂⁺ isomers have been re-evaluated in light of the results obtained from high level ab initio MO calculations and (metastable) AE measurements. NR experiments showed that the neutral CH₂=NH-NH₂˙ is a stable species but the majority of neutralized CH₂=NH-NH₂⁺ ions dissociate into CH₂=NH and NH₂⁺, a finding that is rationalized in light of the different computed geometries of the ion and radical. Mechanisms are proposed for the metastable dissociation reactions of CH₅N₂⁺ ions to HCNH⁺ + NH₃ and to HC=N + NH₄⁺, the latter rationalized in terms of a dissociating [HC=N�NH₄]⁺ ion-dipole complex. A mechanism for the spontaneous deacelylation of N-acetylmethyliminium ions CH₃C(=O)・N(H)=CH₂⁺ to CH₂NH₂⁺ + CH₂=C=O was proposed. The role of the hydrogen-bridged complex [O=C=CH₂�H�N(H)=CH₂]⁺ is discussed with emphasis placed on the observation of large isotope effects and exceedingly small translational energy releases as indications of the intermediacy of such ion-molecule complexes. Several other isomeric C₃H₆NO⁺ ions, which were briefly examined and characterized, are also discussed. The second part of the thesis focusses on the generation and identification of elusive radicals and molecules by neutralization-reionization mass spectrometry and multiple collision experiments. It begins with a study of three isomeric [C₂NO] cations that were characterized by (multiple) collision experiments. NR and CIDI experiments were used to demonstrate the existence of the corresponding radicals NCCO˙, CNCO˙ and CCNO˙ as stable species in the gas phase. Appropriate dissociative ionizations were used to produce diamino- and dihydroxy-carbene radical cations. Reduction of the ionized carbenes by single-electron transfer in NR experiments yielded "survivor" signals but collision experiments on the reionized neutrals (NR/CA) were required to unambiguously estabIish that the prototype carbenes, H₂N-Ċ-NH₂ and HO-Ċ-OH, were generated as stable gaseous species, distinct from their isomers formamidine, H-N=C(H)NH₂ and formic acid, O=C(H)OH. Similarly, a combination of NR mass spectrometry and (multiple) collision experiments on D and ¹⁵N labelled isotopomers made it possible to assign structures to the prototype imidic acid, formimidic acid, H-N=C(H)-OH, and aminohydroxycarbene. H₂N-Ċ-OH, both isomeric with formamide. The ionic precursors H-N=C(H)-OH˙⁺ and H₂N-C-OH˙⁺ were produced by appropriate dissociative ionizations and direct ionization of formamide yielded H₂N-CHO˙⁺. NR experiments produced intense "survivor" signals. By combining information contained in CA mass spectra of the "survivor" ions with that from labelling experiments, the existence of formimidic acid and aminohydroxycarbene as stable molecules in the gas phase was established. Using the same experimental methodology, the existence of the imidic acid CH₃-N=C(H)-OH and the carbene CH₃-N(H)-Ċ-OH was also established.</p>