Henrik Ottosson

Aromaticity and antiaromaticity are twin concepts, where the first leads to compounds with exceptionally high stability relative to nonaromatic isomers while the second, in its original form by Breslow, leads to compounds with exceptionally low stability. Yet, the concepts are fuzzy and have been debated for decades. We find that analyzing the limitations in excited states is revealing. For the lowest pp* triplet states, Baird showed in 1972 that 4np-electron cycles are aromatic and those with 4n+2 are antiaromatic, opposite to Hückel’s rule for the closed-shell singlet ground state (S0). It has been found through computations that Baird’s rule in several cases also can be extended to the lowest pp* singlet state. Yet, how does this translate to the lowest excited states of species which are s-aromatic in their S0 states? We revealed that H3+ (Figure 1), which is s-aromatic in S0 and the most abundant aromatic species in the Universe, is s-antiaromatic in first 1ss* states. This leads to an extensive destabilization and a lowest vertical excitation energy of 19.3 eV. We have now also explored the nonclassical vinyl and ethyl cations, having similar 3-center 2-electron bonding as H3+, and find similarities in the aromaticity of the S0 state and antiaromaticity of some of the ss* states. Conversely, we will also discuss the tentative Baird s-aromaticity of the hypothetical H4 molecule in its first ss* states.