title: Guanidine and guanidinium cation in the excited state--theoretical investigation.
abstract: Diverse ab initio and density-functional-theory methods were used to investigate geometries, energies, and electronic absorption spectra of guanidine and its protonated form, as well as their photo-deactivation processes. It was shown that the guanidine is a weakly absorbing species with the excitation spectrum consisting mostly of transitions to the Rydberg excited states and one valence n-π4 state. The lowest energy band has a maximum at ca. 6.9 eV (∼180 nm). The protonation of guanidine affects its excitation spectrum substantially. A major shift of the Rydberg states to higher energies is clearly visible and strongly absorbing transitions from the ground state to the π3-π4 and π2-π4 states appears at 7.8 eV (∼160 nm). Three low-lying conical intersections (two for guanidine and one for protonated guanidine) between the ground state and the first excited singlet state were located. They are accessible from the Franck-Condon region through amino N-H stretching and out-of-plane deformations in guanidine and protonated guanidine, respectively. The relaxation of the π3-3s Rydberg state via amino N-H bond stretching was hindered by a barrier. The nondissociated conical intersection in protonated guanidine mediates the radiationless deactivation of the compound after excitation into the π3-π4 state. This fact is detrimental for the photostability of guanidine, since its conjugate acid is stable in aqueous solution over a wide pH range and in protein environment, where guanidinium moiety in arginine is expected to be in a protonated form.
authors.affiliation: Division of Organic Chemistry and Biochemistry, Ruđer Bošković Institute, P.O. Box 180, HR-10002 Zagreb, Croatia.
authors.forename: Ivana
authors.initials: I
authors.lastname: Antol
authors.affiliation: Division of Organic Chemistry and Biochemistry, Ruđer Bošković Institute, P.O. Box 180, HR-10002 Zagreb, Croatia.
authors.forename: Zoran
authors.initials: Z
authors.lastname: Glasovac
authors.affiliation: Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany.
authors.forename: Rachel
authors.initials: R
authors.lastname: Crespo-Otero
authors.affiliation: Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany.
authors.forename: Mario
authors.initials: M
authors.lastname: Barbatti
mesh_terms: D019791:Guanidine / Q000737:chemistry*; D008027:Light; D008958:Models, Molecular; D008968:Molecular Conformation; D011522:Protons; D013816:Thermodynamics
chemical_list: D011522:Protons; D019791:Guanidine