Integrating Computational Chemistry and Mass Spectrometry: A Study of Isomerization Reactions of Oxygen-Containing Ions in the Gas Phase

Abstract

<p>The unimolecular chemistry of several organic radical cations has been studied with ab initio molecular orbital calculations and tandem mass spectrometric experiments. The computational chemistry involves. Hartree-Fock (HF), density functional (DFT), Moeller-Plesset perturbation, (MP) and coupled cluster (CC) theories. The tandem mass spectrometry experiments involves metastable ion (MI), collision induced dissociative ionization (CID), and neutralization-reionization mass spectrometry (NRMS) in conjunction with isotopic labeling using ²H, ¹³C and ¹⁸O isotopes. The chemistry has been interpreted by consideration of several fascinating intermediates in the gas-phase which include hydrogen-bridged radical cations, distonic ions and ion-dipole complexes. Two new hydrogen-bridged radical cations have been identified by experiments and were characterized by calculations. The unimolecular chemistry of ionized 1,2-propanediol was re-examined with these methods. It was found that a considerable simplification of a previous mechanistic proposal could be brought about by invoking two different mechanistic concepts, a 1,2-proton shift catalyzed by a dipole and charge (electron) transfer taking place in ion-dipole complexes. This new mechanistic proposal was compared to a more conventional alternative which involves hydrogen atom shifts in distonic ions for both 1,2-propanediol and its lower involves hydrogen atom shifts in distonic ions for both 1,2-propanediol and its lower homologue, 1,2-ethanediol. It was found that a surprisingly large barrier exists for a 1,4-H atom shift in these stable distonic ions. Other low energy dissociation processes in ionized 1,2-propanediol (involving the loss of CH₃･, H₂O, H₂O + CH₃･, H₂O + CH₄) were interpreted by Bohme's 'methyl cation shuttle' mechanism taking place in ion-dipole complexes. Tandem mass spectrometry experiments confirmed previous indirect evidence that commercially available oxalacetic acid, HOOCCH₂C(=O)COOH (OAA) samples do not have the structure of the acid but rather that of its (Z)-enol, hydroxyfumaric acid, HOOCC(H)=C(OH)COOH. It was further established that: (i) the samples contained a minor impurity assigned as a dehydration product of 4-hydroxy-4-methyl-2-ketoglutaric acid; (ii) careful evaporation of OAA yielded mass spectra characteristics of the (Z)-enol form; partial ketonization of the neutral (Z)-enol takes place and these species are either ionized intact or decarboxylane giving a mixture of α-hydroxygen-bridged radical cation, CH₂=O•••H•••O=C-OH･⁺. The long-lived, low internal energy (metastable) ions enolize prior to the loss of C=O yielding an ion-dipole complex of ionized hydroxyketene and water, HOC(H)=C=O･⁺/H₂O. The high energy hydrogen-bridged radical cations also show an intriguing rearrangement ion, CH₂OH₂･⁺, resulting from a decarboxylation process. This reaction likely occurs via communication with ionized glycolic acid and the hydrogen-bridged radical cation CH₂OH₂･⁺•••O=C=O. The methyl ester of β-hydroxypyruvic acid, HOCH₂C(=O)COOCH₃, behaves analogously: upon decarbonylation the high and low internal energy ions are CH₂=O•••H•••O=C-OCH₃･⁺ and HOC(H)=C=O･⁺/CH₃OH respectively. The hydrogen-bridged radical cation formed in the high internal energy process shows an intriguing rearrangement as well. This ion rearranges extensively before dissociation into protonated methylformate, CH₃OC(H)OH⁺ and the formyl radical, HC=O･. This rearrangement occurs via sequential transfers of a proton, electron and another proton within ion-dipole complexes in the gas-phase, akin to that proposed for the diols mentioned above. In the EI mass spectrum of HPA an ion at m/z 59 is observed which was confirmed to have the expected structure HOCH₂-C⁺O. We found the calculated dissociation energy of ionized HPA into HOCH₂-C⁺=O and HOCO･ to be rather different from that derived from an earlier experimental and theoretical study. This observation led us to re-examine the enthalpies of formation for the m/z 59 C₂H₃O₂⁺ system of ions, which includes the methoxycarbonyl ion CH₃-O-C⁺=O, with high level G2 ab initio calculations. A variety of calculated reactions and reconsideration of previous and new experimental measurements resulted in an upwards revisal, by 9 kcal/mol, of the ∆Hf assigned to these ions.</p>