CONICET | Buscador de Institutos y Recursos Humanos

Abstract The UV-absorption, fluorescence excitation and emission spectra of the 6-chloro-, 8-chloro-, 6,8-dichloro-derivatives of nor- harmane, harmane and harmine and the 8-chloro-derivative of harmol were studied. These studies were performed in EtOH and in EtOH + 1% perchloric acid solutions (pa). Furthermore, fluorescence quantum yields (ff) in acetonitrile and acetonitrile + 1% perchloric acid solutions at 298 K were measured. As matrix assisted UV laser desorption/ionization (UV-MALDI; N2 laser, 337 nm) occurs in the solid state, the photon absorption at 337 nm was checked recording the UV-diffuse reflectance spectrum for each compound in solid sate. The HOMO and LUMO energy, the positions (lmax) and oscillator strength (f) of the 1S1<---1S0 band for all the neutral and protonated b-carbolines studied were calculated and compared with the experimental data. UV-MALDI analyte ionization occurs both in the solid analyte-matrix sample and in the plume (gas phase). Among other factors, matrix acid-base properties might be critical in both ionization processes. Thus, (a) pKa values in aqueous solution for for 6-chloro-, 8-chloro- and 6,8-dichloro-nor-harmane, harmane and harmine and 8-chloro-harmol were spectrophotometrycally measured (pKa(H2O)); (b) Change of the acid-base character of these compounds on going from the ground state (pKa) to the first electronic excited singlet state (pKa*) as DpKa = pKa* - pKa = 0.625 D`n / T, in ethanol solution at 298 K were calculated (DpKa(EtOH)); (c) Ground-state proton affinity (PA) for all the compounds studied defined as minus the enthalpy change of the reaction M + H+ ® MH+ (gas state) were also calculated. Keywords: chloro-b-carbolines; chloro-nor-harmane; chloro-harmane; chloro-harmine; chloro-harmol; UV-MALDI matrix.