Guanidine and guanidinium cation in the excited state—theoretical investigation

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

Insights on the Auxochromic Properties of the Guanidinium Group

The work was supported by the Croatian Science Foundation Grant No. 9310. The calculations were performed on the Isabella cluster (isabella.srce.hr) at the Zagreb University Computing Center (SRCE). Funds for Germany-Croatia bilateral collaboration from the German Academic Exchange Service (DAAD, Project 54368738) and Ministry of Science, Education and Sports (Project: Optimizing bidirectional “highway” for photoactive response through guanidine-chromophore junction) are also acknowledged. M.B. thanks the support of the A*MIDEX Grant (No. ANR-11-IDEX-0001-02) and of the project Equip@Meso (No. ANR-10-EQPX-29-01), both funded by the French Government “Investissements d’Avenir” program

Valence and Rydberg states of protonated formaldehyde

MR-CISD and MR-CISD + Q calculations have been performed for the vertical excitations of protonated formaldehyde in comparison to formaldehyde. Singlet and triplet states have been investigated. It is shown that the protonation causes the Rydberg states to be shifted to higher energies by several eV. This finding is discussed by means of the Rydberg formula in terms of quantum defects for the two lowest vertical ionization energies. For protonated formaldehyde the pi-pi(*) valence state is energetically the second lowest state at 9.80 eV, about 1.50 eV below the first Rydberg n-3s state. This finding is in strong contrast to the case of formaldehyde where the pi-pi(*) state is embedded within a series of Rydberg states. (C) 2003 Elsevier Science B.V. All rights reserved

A DFT study of pyramidalized alkenes : 7-oxasesquinorbornenes and 7,7'-dioxasesquinorbornenes

DFT calculations of 7¢–oxasesquinorbornenes and 7,7¢-dioxasesquinorbornenes using the B3LYP/6– 31G* method are reported. All the investigated structures (syn- and anti- derivatives) showed significant non-planarity of the central double bond, with the exception of those anti-derivatives possessing symmetrical structures. The influence of the replacement of the methylene groups at position 7- of the norbornene fragment with oxygen and the introduction of second and third (peripheral) double bonds and benzene rings on the molecular and electronic structures of these molecules have also been investigated

A DFT study of pyramidalized alkenes : 7-oxasesquinorbornenes and 7,7'-dioxasesquinorbornenes

DFT calculations of 7¢–oxasesquinorbornenes and 7,7¢-dioxasesquinorbornenes using the B3LYP/6– 31G* method are reported. All the investigated structures (syn- and anti- derivatives) showed significant non-planarity of the central double bond, with the exception of those anti-derivatives possessing symmetrical structures. The influence of the replacement of the methylene groups at position 7- of the norbornene fragment with oxygen and the introduction of second and third (peripheral) double bonds and benzene rings on the molecular and electronic structures of these molecules have also been investigated

Model Systems for Dynamics of π-Conjugated Biomolecules in Excited States

Mixed-quantum classical dynamics simulations have recently become an important tool for investigations of time-dependent properties of electronically excited molecules, including non-adiabatic effects occurring during internal conversion processes. The high computational costs involved in such simulations have often led to simulation of model compounds instead of the full biochemical system. This chapter reviews recent dynamics results obtained for models of three classes of biologically relevant systems: protonated Schiff base chains as models for the chromophore of rhodopsin proteins; nucleobases and heteroaromatic rings as models for UV-excited nucleic acids; and formamide as a model for photoexcited peptide bonds