Isomerization and dissociation processes of protonated β‐propiolactone and related C3H5O2+ isomers: A combined experimental and theoretical study
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AbstractPart of the C3H5O2+ potential energy surfae was investiated by ab initio MO calculations executed at the MP3/6–31G*//4–31G + ZPVE and MP3/6–31G*//MP2/6–31G* + ZPVE levels of theory and by mass spectrometric experiments to ascetain whether carbonyl‐protonated β‐propiolactone ions , a, can interconvert with protonated acrylic acid, CH2CHC(OH)2+, e, as claimed in a recent thermolysis study. Theory and experiment show that the lowest energy isomers are ions CH2CHC(OH)2+, e, ΔHf = 385 kJ mol−1, , a, ΔHf = 408 kJ mol−1, , b, ΔHf = 424 kJ mol−1 and HOCH2CH2CH2CO+, f, ΔHf = 447 kJ mol−1. At the Hartree–Fock (HF) level of theory, the carboxyethylium ions CH2CH2COOH+, c, and CH3CHCOOH+, d, are minima lying much higher in energy (∼ 160 kJ mol−1 above e). Loss of I˙ from CH3CH(I)COOH produces ion d (or a structure akin to it) which displays characteristic collisional activation (CA) and neutralization–reionization (NR) spectra. Loss of I˙ from ICH2CH2COOH is proposed toyield ions a (via anchimeric assistance) rather than c. Metastable ions a, b, c, d and f freely interconvert, but ions e do not communicate with these ions. It is concluded that the observed equilibrium a ⇌ e in solution is due to an intermolecular process. Contrary to earlier suggestions, ions a do not undergo cycloreversion to HOCO+ + C2H4 and to CH2COH+ + CH2O, but rather they spontaneously dissociate CH3CHOH+ + CO, CH3CO+ + CH2O, CH2CHCO+ + H2O and CH3CH2+ + CO2. The product ions of these dissociation were characterized by double collision experiments andmechanisms for their formation are proposed. In this context, the dissociation behaviour of the following isomers was also examined: [CH2O…︁H…︁CH2CO]+, g, [CH2O…︁H…︁OCCH2]+, h, CH3CH(OH)CO+, i, , j, CH3C(O)OCH2+, k, and CH3CH2OCO+, l. NR spectra indicate that the radicals d and e are stable species, paalleling, in part, results from ESR Spectroscopy. Analysis of appropriate isodesmic reactions indicates that the α‐COOH group in ion d behaves as a hyudrogen atom and therefore this group cannot be said to destabilize the adjacet positive charge. This provides a rationalization for the observation that in solution α‐carbonyl cations can be formed at rates comparable to the unsubstituted analogues. On inclusion of electron correlation in the geometry optimization, the structure of ion d is transformed into that of a 2‐methyl‐1‐hydroxiratranyl cation, d1. The asymmetry in the OC bond lengths in the oxiranyl ring reflects a trade‐off between conjugative stabilization and ring strain energy. Ion c is found to adopt a bridged structure, c1, with a geometry strikingly similar to that of the bridged ethyl cation. Ions c1 and d1 have similar relative energies (148 and 135 kJ mol− above e) and are interconnected bya very low‐lying transition state and hence they may freely interconvert. The appearance energy for loss of I˙ from CH3CH(I)COOH leads to (a) product(s) ΔHf of 145 kJ mol−1 and may therefore correspond to a mixture of ions c1, and d1.
