Deciphering the astrophotochemical inertness of H3+ at the molecular level

The trihydrogen cation, H3+, is unique in the Universe. It serves as the primary proton reservoir, driving essential astrochemical reactions, and functions as a thermostat for giant gas planets. H3+ has also a remarkably low photodissociation rate, explained by its exceptionally high first electronic excitation energy (19.3 eV), which is well above the ionization energy of monohydrogen (13.6 eV). Herein we reveal the key factors behind the high excitation energy of H3+: (i) aromatic stabilization in its electronic ground state, (ii) antiaromatic destabilization in its first excited state, and (iii) a high nuclear-to-electronic charge ratio (+3 vs. -2). Through comparisons with analogous (isolobal) pi-conjugated carbocations, we find that ground state aromatic stabilization plus excited state antiaromatic destabilization raise the excitation energy of H3+ by 4.8 - 6.0 eV. This means that for H3+, the excited state antiaromatic character (which normally leads to high photoreacticity) contributes to its astrophotochemical inertness. Indeed, only with the increase in excitation energy due to ground state aromaticity plus excited antiaromaticity can H3+ fulfil its unique functions in space.