Phenothiazine, N-methyl-phenothiazine and promazine have 4-5 ionization bands from pi orbitals, corresponding to 4 very clear peaks in the photoelectron spectra (Figure 3), and ranging in energy from 7.26-10.43 eV. The first two ionization potentials at 7.26 eV and 8.35 eV were assigned to central Nitrogen and Sulfur pi orbitals, which are significantly delocalized over the whole ring.
The photoelectron spectra of chlorpromazine, thioridazine, and trifluoperazine are shown below. Thioridazine, with 2-position methylthio, has the lowest ionization energy (7.00 eV) studied in Domelsmith's research, meaning that its electrons are very much on the fringe. Like promazine, the first and second ionization potential bands are assigned to the ring Sulfur and Nitrogen. In chlorpromazine, the 2-position Chlorine atom gives rise to a band at 11.24 eV. The trifluoromethyl groups in position-2 of trifluoperazine were not as electron withdrawing as predited by calculations. Trifluoperazine's first ionization potential energy of 7.31 eV was slightly higher than that of phenothiazine, 7.26 eV.
Nitrogen methylation tended to enhance the overall electron-donating ability of the molecule, as revealed by comparison between phenothiazine (first ionization potential energy, 7.26 eV) and N-methyl-phenothiazine (7.15 eV). A similar trend was observed in the tryptamine series; a Nitrogen side chain ionization potential was lowered from 9.25 eV in tryptamine, to 8.9 eV in N-methyl-tryptamine, to 8 eV in N,N-dimethyl-tryptamine.
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| From Domelsmith ionization potential |
Thioridazine (first ionization potential, 7.00 eV) surpassed chlorpromazine (7.16 eV), and LSD (7.25 eV) as an electron donating molecule.
Reference
Domelsmith L. N., L. L. Munchausen and K. N. Houk. (1977). Photoelectron spectra of psychotropic drugs. 2. Phenothiazine and related tranquilizers. J. Am. Chem. Soc. 99, 6506-6514. doi:10.1021/ja00462a007
