The next most important feature derived directly from the structure is Ring Embedding Count, which quantifies the number of aromatic rings a particular carbon is embedded in. Specifically, the Ring Embedding Count feature is the number of aromatic rings sharing a particular carbon, which takes a value from 0 (if it is not in a ring) to 3. As seen in Figure , the carbon atoms most likely to be active in this dataset are members of exactly one aromatic ring. The carbons that are not members of aromatic rings are generally inactive in this dataset as they are usually in alkyl groups. The 1.1% of sites not in rings that still bind O2 are the terminal atoms of alkenes, which appear to bind oxygen quite strongly, whereas no alkanes bound oxygen. The relative activity of the sites at the edges of aromatic systems is likely due to the fact that the active carbon becomes sp 3 -hybridized as it binds oxygen. Active sites at the edge of an aromatic system have a non-aromatic neighbor that can shift out of the plane of the aromatic system to accommodate the new sp 3 hybridization without disrupting the geometry of the aromatic system. In contrast, active sites embedded in two or three aromatic rings must flex out of the plane of the aromatic system when they bind O2, which has more bond angle strain due to disrupting a larger region of the aromatic system than sp 3 hybridization at the edge sites causes