Thermochemical assessment of the aromatic and antiaromatic characters of the cyclopropenyl cation, cyclopropenyl anion, and cyclopropenyl radical: A high-level computational study

The aromatic stabilization energy of the cyclopropenyl cation (CH)₃⁺ is assessed with G2 theory by calculating its homodesmotic stabilization energy (247.3 kJ mol^-1) and by comparing the ionization energies of the cyclopropenyl radical (6.06 eV) and the cyclopropyl radical (8.24 eV). These data indicate substantial stabilization of the two π-electron system in what is considered the archetypal aromatic cation. The calculated enthalpy of formation of the cyclopropenyl cation is 1074.0 kJ mol^-1 and agrees with the experimental estimate of 1075 kJ mol^-1. The small stabilization energy of the cyclopropenyl radical (37.4 kJ mol^-1) suggests that this radical should not be classified as aromatic, in contrast to earlier suggestions. Our G2-calculated enthalpy of formation of the cyclopropenyl radical (ΔH_{f 298} = 487.4 kJ mol^-1) and its ionization energy are different from experimental estimates and suggest that the experimental values may need to be revised. The most stable structure for the cyclopropenyl anion is a nonplanar C_s singlet structure containing a strongly pyramidalized carbon. The open-chain isomers of (CH)₃^- as well as the nonplanar triplet cyclic structures are all found to be higher in energy. The nonplanar C² "allylic-type" cyclic structure of (CH)₃^- is 8.9 kJ mol^-1 higher energy than the cyclic C_s structure and corresponds to a first-order saddle point. While the G2 stabilization energy of the cyclopropenyl anion estimated using the energy of the homodesmotic reaction cyclopropenyl anion + cyclopropane → cyclopropene + cyclopropyl anion is negative (-17.3 kJ mol^-1), its absolute value is substantially less than the corresponding stabilization energy calculated for cyclobutadiene (-129.6 kJ mol^-1). A comparison of the G2-calculated gas-phase acidities of cyclopropene (1755.4 kJ mol^-1) and cyclopropane (1737.1 kJ mol^-1) also suggests the antiaromatic destabilization energy of the cyclopropenyl anion to be small. However, the electron affinity of the cyclopropenyl radical is found to be negative (-0.18 eV), indicating that the cyclopropenyl anion is not bound in the gas phase.