|From Domelsmith ionization potential|
Dihydro-harmine (harmaline) and harmine differ by 2 electrons, the saturation of one bond, which in this case changed the energy of the ionization potential by 0.5 eV, a very large quantity.
The trend continues that harmine and harmaline are the most psychoactive in this series of harmala alkaloids, and have the lowest energy ionization potential.
Domelsmith L. N. and K. N. Houk. (1978). Photoelectron spectroscopic studies of hallucinogens: the use of ionization potentials in QSAR. NIDA Res. Monogr. 22, 423-440.