Experimental and theoretical study of the excited-state tautomerism of 6-azauracil in water surroundings

Experimental and theoretical study of the cytosine tautomerism through excited states

CONTEXT

The irradiation of water solution of cytosine with UV light (λmax = 254 nm) shows oxo-hydroxy tautomerism with a rate constant of 6.297 × 10-3 min-1. The order of the reaction implies a tautomeric conversion. After removing the UV light source, we observed a dark reaction with a rate constant of 1.473 × 10-3 min-1 which leads to a restoration of the initial tautomer as before the irradiation. The mechanism of oxo-hydroxy tautomerism of cytosine in water solution was studied in the excited state. It was found that the transformations occur along the 1πσ* excited-state reaction paths which link the Franck-Condon geometries of the tautomers and the conical intersections S0/S1 connected with the H-detachment processes of the corresponding bonds. Furthermore, we established that the conical intersections S0/S1 are also mutually accessible along the 1πσ* excited-state reaction paths.

METHODS

The ground-state equilibrium geometries were optimized at the B3LYP/aug-cc-pVDZ level of theory in water environment according to PCM as well as at the CC2/aug-cc-pVDZ level in the gas phase. The TD B3LYP and CC2 methods were applied for the study of the excited states. The tautomerization mechanisms were studied with the use of the linear interpolation in internal coordinates approach using the optimized geometries of tautomers minima and conical intersections S0/S1 at the CASSCF(6,6)/6-31G* level. All calculations were performed with the GAUSSIAN 16 commercial software.

Photoinduced phenomena in water solution of melamine explaining the photostability of the compound

Two tautomers of melamine (triamino and imino-diamino) were studied at the BLYP/aug-cc-pVDZ theoretical level. It was found that the two tautomers are bridged with the ¹πσ^* excited-state reaction path. The high photostability of melamine in water solution was explained with the mechanism of ring deformation which occurs along the ¹ππ^* excited-state reaction path. The two mechanisms are investigated at the TD BLYP level of theory using the linear interpolation in internal coordinates approach.

Influence of Varying Functionalization on the Peroxidase Activity of Nickel(II)–Pyridine Macrocycle Catalysts: Mechanistic Insights from Density Functional Theory

Nickel(II) complexes of mono-functionalized pyridine-tetraazamacrocycles (PyMACs) are a new class of catalysts that possess promising activity similar to biological peroxidases. Experimental studies with ABTS (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid), substrate) and H2O2 (oxidant) proposed that hydrogen-bonding and proton-transfer reactions facilitated by their pendant arm were responsible for their catalytic activity. In this work, density functional theory calculations were performed to unravel the influence of pendant arm functionalization on the catalytic performance of Ni(II)–PyMACs. Generated frontier orbitals suggested that Ni(II)–PyMACs activate H2O2 by satisfying two requirements: (1) the deprotonation of H2O2 to form the highly nucleophilic HOO−, and (2) the generation of low-spin, singlet state Ni(II)–PyMACs to allow the binding of HOO−. COSMO solvation-based energies revealed that the O–O Ni(II)–hydroperoxo bond, regardless of pendant arm type, ruptures favorably via heterolysis to produce high-spin (S = 1) [(L)Ni3+–O·]2+ and HO−. Aqueous solvation was found crucial in the stabilization of charged species, thereby favoring the heterolytic process over homolytic. The redox reaction of [(L)Ni3+–O·]2+ with ABTS obeyed a 1:2 stoichiometric ratio, followed by proton transfer to produce the final intermediate. The regeneration of Ni(II)–PyMACs at the final step involved the liberation of HO−, which was highly favorable when protons were readily available or when the pKa of the pendant arm was low.