Histogram of the DU, or alternatively the number of rings and π bonds (see text), for hydrocarbon molecules containing only H, O, N, C, Cl, S, or F. Only 10 (7%) interstellar hydrocarbons are fully saturated with no rings or π bonds. Most molecules contain one or two rings or π bonds (a double bond is one π bond; a triple bond is two π bonds). The fullerene molecules are not shown.