Effect of excess electron on structure, bonding, and spectral properties of sulfur/selenium based dichalcogen systems

Abstract

First principle based quantum chemical methods are employed to characterize structure, bonding, and spectral properties of sulfur and selenium based dichalcogen systems in presence of an excess electron. Inter molecular two-center three-electron (2c-3e) bonding between two chalcogen (X) atoms is described in the systems of the type (R-X) 2•-(R = Ph, PhCH 2 X = S, Se). In addition, effect of electron withdrawing (-NO 2 ) and electron donating (-CH 3 ) groups in phenyl ring on the stability of these 2c-3e bonded systems is also studied in water medium applying a macroscopic hydration model. Molecular parameters and binding energy of the neutral, (R-X) 2 and reduced, (R-X) 2 •dichalcogen systems are compared. Search for minimum energy structures of these open shell doublet systems are carried out applying various density functionals with dispersion corrections and MP2 method considering 6-311++G (d,p) set of basis functions for all atoms. Effect of water medium is introduced through a macroscopic solvation model based on density (SMD). Frontier molecular orbitals based analysis is carried out for showing the definite presence of 2c-3e bond between two chalcogen atoms in these radical anions of sulfur and selenium based aromatic dichalcogen systems. Excited state calculations are performed on all these systems using Time Dependent Density Functional Theory (TDDFT). UV-Vis spectra are simulated and effect of solvent water on the absorption maximum of these radical anions is discussed. This study illustrates that the combination of electronic effect and geometrical flexibility decides the strength of two-center three-electron bond in these systems. K E Y W O R D S 2c-3e bond, antioxidant, chalcogen, density functional theory, excess electron, hemi bond, radical anion, selenium, sulfur 1 | INTRODUCTIONSulfur and selenium are present in amino acids, peptides, and proteins, essential for various biological redox reactions. Selenocystine and selenomethonine systems can act as an antioxidant by killing harmful peroxides in organisms, and these are also responsible for protection against inflammation and cancer. [1][2][3] Sulfur and selenium based diaryl dichalcogenes are eco-friendly oxidizing agents and demanding drugs in pharmaceutical chemistry mainly for their antioxidant properties. [4] One electron reduction of neutral disulfides and diselenides may lead to the formation of two-center three-electron (2c-3e) bonded radical anion systems. [5][6][7][8][9][10][11][12][13] The extra electron in such systems is accommodated in the lowest unoccupied molecular orbital which is often an antibonding sigma orbital. Due to the presence of this electron in a repulsive antibonding orbital, such three electron bonded species are not very stable and may dissociate easily. Having two electrons in the highest bonding sigma orbital and one electron in the lowest antibonding sigma orbital results in the formation of a half bond (bond order = 0.5) and hence these 2c-3e bond is also known as hemi bond or half bond. The typical symbol for s...