The charge-transfer process is accompanied by the appearance of a new absorption band. In 1967, Fulton and colleagues studied the wavelength of the absorption maximum of NAD+ charge-transfer complexes.
“We have shown that NAD+ of concentration 1.03E-2M, when mixed with fairly strong electron donors, such as substituted pteridines, uric acid, serotonin creatine sulphate, lysergic acid, and phenothiazines, gave solutions which were coloured yellow to orange because of the formation of charge-transfer complexes." (A. Fulton, 1967)
Some of the electron donor molecules studied were LSD, indole, uric acid, promazine, and promethazine, and different absorption maximums were obtained for each donor-NAD+ complex as shown in Table 1 below. Also the researchers measured the ionization potential energies of the electron donors, and found that LSD had an ionization potential of 7.8 electron Volts, indole (7.9), uric acid (7.5), promazine (7.2), promethazine (7.2), and chlorpromazine (7.3).
|From Hallucinogens HOMO, charge-transfer|
The authors found that the absorption maximum of the charge-transfer complex correlated reasonably with the ionization potential of the electron donor drug, but not so well with the energies of the HOMO, perhaps because there are many approximations involved in the theoretical studies of the HOMO energy. However other groups have found a correlation between the absorption maximum and kHOMO energies.
As listed in Table 1 above, the LSD-NAD+ charge-transfer complex had an absorption maximum at 340 nm, which is the absorption maximum of the native LSD molecule. This compares to the 520 nm absorption maximum that is characteristic of LSD-TCNE or LSD-Ehrlich complexes.
Fulton A. and L. E. Lyons. (1967). Electron-accepting strength of NAD+. Aust.J.Chem. 20, 2267-2268.