Isomerization and dissociation processes of protonated β‐propiolactone and related C 3 H 5 O 2 + isomers: A combined experimental and theoretical study

Part of the C 3 H 5 O 2 + 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, CH 2 CHC(OH) 2 + , e , as claimed in a recent thermolysis study. Theory and experiment show that the lowest energy isomers are ions CH 2 CHC(OH) 2 + , e , Δ H f = 385 kJ mol −1 , , a , Δ H f = 408 kJ mol −1 , , b , Δ H f = 424 kJ mol −1 and HOCH 2 CH 2 CH 2 CO + , f , Δ H f = 447 kJ mol −1 . At the Hartree–Fock (HF) level of theory, the carboxyethylium ions CH 2 CH 2 COOH + , c , and CH 3 CHCOOH + , d , are minima lying much higher in energy (∼ 160 kJ mol −1 above e ). Loss of I ˙ from CH 3 CH(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 ICH 2 CH 2 COOH 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 + + C 2 H 4 and to CH 2 COH + + CH 2 O, but rather they spontaneously dissociate CH 3 CHOH + + CO, CH 3 CO + + CH 2 O, CH 2 CHCO + + H 2 O and CH 3 CH 2 + + CO 2 . 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: [CH 2 O…H…CH 2 CO] + , g , [CH 2 O…H…OCCH 2 ] + , h , CH 3 CH(OH)CO + , i , , j , CH 3 C(O)OCH 2 + , k , and CH 3 CH 2 OCO + , 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, d 1 . 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, c 1 , with a geometry strikingly similar to that of the bridged ethyl cation. Ions c 1 and d 1 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 CH 3 CH(I)COOH leads to (a) product(s) Δ H f of 145 kJ mol −1 and may therefore correspond to a mixture of ions c 1 , and d 1 .