Tuning the Spin, Aromaticity, and Quantum Tunneling in Computationally Designed Fulvalenes.

Pentafulvalene is a symmetrical unsaturated hydrocarbon built from two five-membered rings connected by an exocyclic double bond, where each ring is one electron short of being a 6π-electron aromatic system. Here, we show computationally that by selectively introducing electron withdrawing and donating substituents, we can design pentafulvalene derivatives that exhibit tunable aromaticity properties. Pentafulvalene can be shaped into a species with connected aromatic-antiaromatic rings, which can also achieve π-bond shifting by carbon tunneling. We propose an NMR technique that can experimentally prove such tunneling mechanism. In addition, we devised a doubly aromatic fulvalene involving both Hückel and Baird aromaticities. These results can open possibilities to create novel molecules in terms of spin state, aromaticity, and reactivity by quantum tunneling.