What chemical properties make LSD so psychoactive?
It's the indole portion of the molecule. the lowest two rings...if you remove those, the compound has no such properties. Indoles, the side chains of tryptophan , an amino acid, are suitable for binding to receptors that affect emotion, perception, cognition, sleep/wake cycles etc. You can see how the indole looks similar to both DMT (dimethyltryptamine--another hallucinogen) and the benzene ring bears similarity to psilocin, the major constituent of psilocybin (the active principle in magic mushrooms). Tryptamines are common hallucinogens.