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.

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“…9.09 eV respectively 77 . As in the case of AAN, the reason for this discrepancy is found in the extraction methods.…”

“…Binding Energy (eV) m/z 58 This Work butane azomethane (Frost 1976) isobutane (Murrell 1972) 2-propenol (Mines 1973) acetone (Kimura 1975) cyclopropanol (Bombach 1983) methyloxirane (Stranges 2005) ethylvinylether (Mines 1973) oxetane (Roszak 1992) propanal (Kimura 1975 9.09 eV respectively 77 . As in the case of AAN, the reason for this discrepancy is found in the extraction methods.…”

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“…545,546 (Please refer to Figure 7.6.2.1) showing the guanidinium oxidation states. 485,550,551 The oxidation state is dependent on the pH, temperature or any redox reactions it undergoes. 550,552 The designation of the peak around 289 eV to guanidinium/Guanidine is corroborated by the presence of the nitrogen 1s peak in the range reported for guanidinium compounds (Figure 7.6.2.2).…”

“…485,550,551 The oxidation state is dependent on the pH, temperature or any redox reactions it undergoes. 550,552 The designation of the peak around 289 eV to guanidinium/Guanidine is corroborated by the presence of the nitrogen 1s peak in the range reported for guanidinium compounds (Figure 7.6.2.2). 546 Licence et al conducted a study to determine the charge distribution within different guanidinium ionic liquids.…”

“…Figure 7.6.2.1 depicting the various oxidation states of Guanidine in the ethylene glycol solution. (a) Depicts the guanidinium cation suspected to be the most common form due to its ionic interaction with Iodide in (CH6IN3) , meanwhile (b) depicting a deprotonated neutral Guanidine, and (c) represents the deprotonated Guanidinate anion 486,550. …”