RESUMO
This work reports a theoretical investigation of the solvent polarity as well as the halogenation of benzimidazole derivatives during excited state intramolecular proton transfer (ESIPT). It details how the environment and halogen substitution may contribute to the efficiency of ESIPT upon keto-enol tautomerism and exploits this effect to design fluorescence sensing. For this purpose, we first examine the conformational equilibrium of benzimidazole derivatives containing different halogen atoms, which results in intramolecular proton transfer, using density-functional theory (DFT) combined with the polarizable continuum model (PCM). Then we evaluate the fluorescence of the benzimidazole derivatives in different dielectric constants within time-dependent DFT (TD-DFT) approaches. Our results quantitatively allow the determination of large Stokes shifts in nonpolar solvents around 100 nm. These theoretical results are in agreement with experimental solvatochromism studies of benzimidazoles. The effect of halogenation, with fluorine, chlorine, and bromine, is less important than solvent polarization when ESIPT takes place. Thus, halogenation can be properly chosen depending on the interest of the synthesis of benzimidazole-based turn-on fluorescence in appropriate solvents.
RESUMO
In this work, we have evaluated the influence of temperature and pressure on the mechanism of furan oxidation by the OH radical. The stationary points on the potential energy surface were described at the M06-2X/aug-cc-pVTZ level of theory. In the kinetic treatment at the high-pressure limit (HPL), we have combined the multistructural canonical variational theory with multidimensional small-curvature tunneling corrections and long-range transition state theory. The system-specific quantum Rice-Ramsperger-Kassel theory was employed to estimate the pressure-dependent rate. In the HPL, the OH addition on the α carbon is the dominant pathway in the mechanism, producing a product via the ring-opening process, also confirmed by the product branching ratio calculations. The overall rate constant, obtained by a kinetic Monte Carlo simulation, reads the form koverall=5.22×10-13T/3001.10â¡exp1247(K/T) and indicates that the furan oxidation by OH radicals is a pressure-independent reaction under tropospheric conditions.
RESUMO
In this study, we reinvestigated the straightforward nitration of eugenol using traditional reagents and bismuth nitrate. NMR analysis of the obtained products revealed that the regioselectivity of eugenol nitration was independent of the inorganic nitrating reagent used, consistently resulting in the formation of 6-nitroeugenol. This contradicts previous literature reports because the elusive synthesis of 5-nitroeugenol using Bi(NO3)3·5H2O was not achievable through straightforward methods; instead, this isomer could only be prepared via the well-established three-step synthesis. Theoretical investigations using DFT calculations, considering both the dielectric constant of the medium and explicit water molecules, substantiated this regioselectivity. It was found that hydration water played a critical role in the formation of a Zundel cation, shifting the thermodynamic equilibrium toward the exclusive production of 6-nitroeugenol. These results imply that all biological studies involving eugenol derivatives synthesized via direct nitration with Bi(NO3)3·5H2O should be reviewed, as they dealt with 6-substituted eugenol derivatives rather than the previously assumed 5-substituted eugenol.
RESUMO
We investigate the photoionization pathways of naphthalene, 1-cyanonaphthalene, and 2-cyanonaphthalene upon complexation with the water dimer, aiming to understand the photodissociation process under conditions of the interstellar medium (ISM). We analyze the intermolecular bonding pattern, equilibrium rotational properties, energy complexation, far-IR spectra, and ionic trends of the possible photoproducts using dispersion-corrected density functional theory (DFT-D) and time-dependent DFT (TD-DFT). For the different configurations, we evaluate the possible charge-transfer (CT) excitations near the photoionization limit. Our results indicate that, in high-radiation regions of the ISM (>8.0 eV), CT excitations occur from localized occupied molecular orbitals (MOs) in the aromatic molecules to mixed unoccupied MOs in the complexes, favoring cationic aromatic species in these conditions. We notice that the photoabsorption spectra depend on the type of intermolecular interaction (H-bonds or O-H···π bonds) in the complexes, as well as the presence and position (1 or 2) of the cyano-functional group in naphthalene. For hydrated naphthalene, the O-H···π complexes assume a more relevant role for photodissociation. In the case of the cyano-substituted derivatives, the H-bonded structures are more favorable to be considered as prereactive models. However, the cyano group at position 2 indicates that CT excitations toward the water dimer are more likely to occur.