D8-THC and D9-THC have the same molecular weight and numbers of carbon, hydrogen, and oxygen atoms and appear nearly the same when their chemical structures are drawn in two dimensions, as shown in the figures below.
The only chemical difference between D8-THC and D9-THC is the location of the double bond in the cyclohexene ring of each molecule. In D8-THC (Figure 1), the double bond is between carbon atoms 8 and 9, whereas in D9-THC (Figure 2), the double bond is between carbon atoms 9 and 10. The differences in the names D8-THC and D9-THC are solely due to this difference in the location of the double bond. The location of the double bond in the cyclohexene rings of D8-THC and D9-THC causes the ring to assume different conformations. This conformational change results in D8-THC binding less strongly to the CB1 receptor than D9-THC, thereby decreasing potency relative to D9-THC.