THERMOCHEMICAL ASSESSMENT OF THE AROMATIC AND ANTIAROMATIC CHARACTERSOF 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 stabiliz ation energy (247.3 kJ mol(-1)) and by comparing the ionization energi es of the cyclopropenyl radical (6.06 eV) and the cyclopropyl radical (8.24 eV). These data indicate substantial stabilization of the two pi -electron system in what is considered the archetypal aromatic cation. The calculated enthalpy of formation of the cyclopropenyl cation is 1 074.0 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 a s aromatic, in contrast to earlier suggestions. Our G2-calculated enth alpy of formation of the cyclopropenyl radical (Delta H-f298 = 487.4 k J mol(-1)) and its ionization energy are different from experimental e stimates and suggest that the experimental values may need to be revis ed. The most stable structure for the cyclopropenyl anion is a nonplan ar C-s singlet structure containing a strongly pyramidalized carbon. T he open-chain isomers of (CH)(3)(-) as well as the nonplanar triplet c yclic structures are all found to be higher in energy. The nonplanar C -2 ''allylic-type'' cyclic structure of (CH)(3)(-) is 8.9 kJ mol(-1) h igher energy than the cyclic C-s structure and corresponds to a first- order saddle point. While the G2 stabilization energy of the cycloprop enyl anion estimated using the energy of the homodesmotic reaction cyc lopropenyl anion + cyclopropane --> cyclopropene + cyclopropyl anion i s negative (-17.3 kJ mol(-1)), its absolute value is substantially les s than the corresponding stabilization energy calculated for cyclobuta diene (-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 o f the cyclopropenyl radical is found to be negative (-0.18 eV), indica ting that the cyclopropenyl anion is not bound in the gas phase.