In this review, we present the recent discovery and confirmation of a new negative molecular ion, the CH[Formula: see text] anion. The experimental identification of this high-spin exciplex was difficult because it overlaps with the negative oxygen ion commonly present as a contaminant in vacuum systems and with the same mass-spectrometric signature. Born–Oppenheimer molecular dynamics (BOMD) simulations finally reveal that this anion is a quartet ([Formula: see text]) metastable species, which leads to the formation of a molecular (CH[Formula: see text] excited complex.