A Mass Spectrometric Study of Low Energy Rearrangement Reactions in Oxygen Containing Ions

Abstract

<p>Low energy rearrangement reactions in selected oxygen-containing ions have been studied by means of mass spectrometry-based techniques (metastable ion (MI), collision-induced dissociation (CID), neutralization-reionization (NR) mass spectrometry as well as multiple collision experiments (MSⁿ)) in conjuntion with ab initio molecular orbital calculations.</p> <p>The atom connectivity in the product ions has been investigated in detail by tandem mass spectrometry-based experiments on ²H-, ¹³C- and ¹⁸O-labelled isotopologues of some oxygen-containing ions. The thermochemical data (i.e. heat of formation (ΔHf) values) of the oxygen-containing ions have been obtained either experimentally from the appropriate measurement of ionization energy (IE), appearance energy (AE) and/or proton affinity (PA) or computationally.</p> <p>It has been proposed that the C-H∙∙O bonded counterparts of the O∙∙H∙∙O bonded species may, despite their lower thermodynamic stability, play an even more important role in the decay of oxygen-containing radical cations of the type HOCH(R₁)C(=O)R₂˙⁺. A case in point is ionized acetol (R₁=H,R₂=CH₃), methyl glycolate (R₁=H,R₂=OCH₃), methyl lactate (R₁=CH₃,R₂=OCH₃) and acetoin (R₁=R₂=CH₃). These species all dissociate by loss of R₁CO˙ by double hydrogen transfer (DHT). It is further proposed. from ab initio calculations, that the C-H∙∙O bonded intermediate R₁C(H)=O⋯H-C(=O)R₂˙⁺, I, does not lose R₁CO˙ via a hydrogen atom shift from neutral R₁C(H)=O to ionized H-C(=O)R₂˙⁺.</p> <p>Instead, charge transfer takes place in I so that the neutral R₁C(H)=O becomes charged and thus it can rotate and donate a proton to H-C(=O)R₂, after which dissociation follows.</p> <p>Keto-enol tautomerism in the dilute gas-phase has been studied in the unimolecular reactions of ionized ethyl glycolate, HOCH₂CO₂C₂H₅˙⁺, II, and its enol isomer, HOCH=C(OH)OC₂H₅˙⁺, III. It has been shown that the metastable ionized keto isomer II undergoes unidirectional isomerization to the enol ion III, via two consecutive 1,5-hydrogen shifts, from which C₂H₄ is lost to give the ionized trihydroxyethylene, (HO)₂C=CH(OH)˙⁺. NR experiments show that neutral trihydroxyethylene in the gas-phase is a remarkably stable species, which does not tautomerize to the keto isomer glycolic acid, HOCH₂CO₂H.</p> <p>Some members of the important homologous series of oxonium ions, CnH₂n₊₁O⁺, have been extensively investigated by ²H- and ¹³C-labelling experiments especially at low internal energies. Their intriguing unimolecular chemistry has been interpreted by means of mechanisms in which distonic ions and ion/neutral complexes play crucial roles in the hydrogen transfer and skeletal isomerization steps.</p> <p>Finally, part of the C₃H₅O₂+ potential energy surface was investigated by ab initio molecular orbital calculations and by mass spectrometry-based experiments to ascertain whether the carbonyl-protonated β-propiolactone ions CH₂CH₂OCOH⁺, IV, can interconvert in the dilute gas-phase with protonated acrylic acid, CH₂=CHC(OH)₂⁺, V, as suggested in a thermolysis study. It is shown that metastable ions IV do not communicate with ions V and the observed equilibrium IV ⇆ V in solution is due to an intermolecular process. Also, ions IV do not undergo cycloreversions to HOCO⁺ + C₂H₄ and to CH₂=COH⁺ + CH₂O, but rather they spontaneously dissociate CH₃CHOH⁺ + CO, CH₃CO⁺ + CH₂O, CH₂=CHCO⁺ + H₂O and C₂H₅⁺ + CO₂. The product ions of these dissociation reactions are characterized by multiple collision experiments and mechanisms for their formation are proposed. Analysis of appropriate isodesmic reactions indicate that the α-COOH group in 1-carboxyethylium ions, CH₃CHCOOH⁺, behaves as a hydrogen atom and therefore this group cannot be said to destabilize the adjacent positive charge.</p>