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Solving the CH$_4^-$ riddle: the fundamental role of spin to explain metastable anionic methane

When atoms or molecules exist in the form of stable negative ions, they play a crucial role in the gas phase chemistry. Determining the existence of such an ion, its internal energy and its stability are necessary prerequisites to analyze the role of this ion in a particular medium. Experimental evidence of the existence of a negative methane ion CH$_4^-$ has been provided over a period of 50 years. However, quantum chemistry had not been able to explain its existence, and a detailed recent study has shown that the experimentally observed species cannot be described by the attachement of an electron in the ground state of CH$_4^-$. Here we describe CH$_4^-$ as being a metastable species in its lowest quartet spin state and we find that this species is a CH$_2^-$-:H$_2$ exciplex with three open shells, lying 5.8 eV above the methane singlet ground state but slightly below the dissociation fragments. The formation of charged exciplexes is a novel mechanism to explain small molecular anions with implications in a plethora of basic and applied research fields.