Structures and stabilities of C3H4O+˙ isomers: A G2 theoretical study

Abstract

AbstractSix stable isomers andtwo transition structures were characterized on the C3H4O+˙ potential energy surface using the G2 procedure. Heat capacity corrections were made to allow the direct calculation of heats of formation at 298 K. The most stable isomer is the methylketene radical cation (1, ΔHf 298 = 797.0 kJ mol−1), followed by a distonic ion \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^ \cdot {\rm H}_2 {\rm CH}_2 \mathop {\rm C}\limits^ +=\!={\rm O} $\end{document} (2), only 46.5 kJ Mol−1 higher in energy (ΔHf 298 = 843.5 kJ mol−1). Ionized cyclopropanone (7) represents a transition structure for exchange of terminal methylene groups in the distonic ion with a corresponding barrier of 57.6 kJ mol−1. There is amuch larger barrier (106.7 kJ mol−1) for the 1,2‐hydrogen migration in the distonic ion (2) to form the mor estabe ionized methylketene (I), while the fragmentation of 2 to C2H4+˙ + CO is highlyendothermic (by 113.1 kJ mol−1). The predicted ease of formation of the distonic ion on ionization of neutral cyclopropanone (7n) and the higher barrier for further reaction would suggest that ionized cyclopropanone should show reactivity characteristic of a radical as observed for ionized cyclobutanone. Three additional C3H4O+˙ isomers, hydroxyallene radical cation (3), acrolein radical cation (4) and oxetene radical cation (5), lie within 15 kJ mol−1 of one another in energy, while a cyclic form (6) of the distonic ion is much less stable. G2 heats of formation and adiabatic ionization energies were also calculated for four of the neutral C3H4O analogs. WHile the ionization energies are in good agreement with available experimental values, as are the heats of formation for acrolein (4n) and cyclopropanone (7n), the G2 heat of formation of methylketene (−65.1 ± 10 kJ mol−1) is almost 40 kJ mol−1 higher than the ‘standard’ value, which it is considered shoul dbe revised. Calculations iwth the G2(MP2) method, which is less computationally intensive than G2, yields of formation for the neutral and ionic species within 24 kJ mol−1 of the G2 values.