Guanidine and guanidinium cation in the excited state—theoretical investigation

摘要: Diverse ab initio and density-functional-theory methods were used to investigate geometries, energies, electronic absorption spectra of guanidine its protonated form, as well their photo-deactivation processes. It was shown that the is a weakly absorbing species with excitation spectrum consisting mostly transitions Rydberg excited states one valence n-π4 state. The lowest energy band has maximum at ca. 6.9 eV (∼180 nm). protonation affects substantially. A major shift higher energies clearly visible strongly from ground state π3-π4 π2-π4 appears 7.8 (∼160 Three low-lying conical intersections (two for guanidine) between first singlet located. They are accessible Franck–Condon region through amino N–H stretching out-of-plane deformations in guanidine, respectively. relaxation π3-3s via bond hindered by barrier. nondissociated intersection mediates radiationless deactivation compound after into This fact detrimental photostability since conjugate acid stable aqueous solution over wide pH range protein environment, where guanidinium moiety arginine expected be form.