The structural and electronic properties of ZnO (1010) and (1120) surfaces were investigated by means of density functional theory applied to periodic calculations at B3LYP level. The stability and relaxation effects for both surfaces were analyzed. The electronic and energy band properties were discussed on the basis of band structure as well as density of states. There is a significant relaxation in the (1010) as compared to the (1120) terminated surfaces. The calculated direct gap is 3.09, 2.85, and 3.09 eV for bulk, (1010), and (1120) surfaces, respectively. The band structures for both surfaces are very similar.

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In this work we use gap-fitting procedure to tune the long-range corrected functionals and accurately investigate the electronic and optical properties of the five main molecules composing Buriti oil (extracted from Mauritia flexuosa L.) in the framework of density functional theory (DFT) and time-dependent (TD) DFT. The characteristic length (1/ω) was observed to be entirely system dependent, though we concluded that its determination is of fundamental importance to rescue geometrical, electronic, and optical properties with accuracy. We demonstrate that our approach of tuning characteristic length for each system resulted in an absorbance spectra in better experimental agreement when compared to the traditional methodology. Therefore, this study indicates that the tuning of the range-separation parameter is crucial to improve the description of the optical properties of conjugated molecules when TDDFT is used. For example, the wavelength of maximum absorption, λmax, for the phytofluene, obtained using B3LYP, is 381 nm, while using the gap-fitting procedure for the tuned-LC-BLYP the estimated λmax changed to 358 nm. The latter estimate is in better agreement with the experimental value of 350 nm.

mWe investigated the adsorption and heterolytic dissociation of H 2 0 and H2 molecules on a (ZnO)*? cluster corresponding to ZnO (OOOl), (OOOl), and (lOi0) surfaces using MNDO, AM^, and PM3 semiempirical procedures. The geometry of the adsorbed molecule has been optimized in order to analyze binding energies, charge transfer, and preferential sites of interaction. The adsorbed species interact most strongly when it is bonded to the twofold coordinated zinc atom of the cluster surface. The interaction of the H 2 0 molecule with the surface of ZnO has a charge transfer from H 2 0 to the surface ranging between 0.17 and 0.27 au. The neighboring atoms of the surface are the main receptors during the process of charge transfer. Our results indicate that there is a weak bonding of the hydrogen atom from OH with the oxygen surface atom that could produce the O--H....O band. The interaction of the H2 molecule with the surface is generally weak and only the PM3 method yields a strong binding energy for this interaction. There is a charge transfer from the H2 molecule to the surface. The chemisorption of H on oxygen atom of the surface transfer charge from the surface to the H. We also calculated the vibrational analyses for these interactions on ZnO surface and compared our results with available experimental data.

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ABSTRACT:We have determined the band-gaps of the main carotenoids present in poly(methyl)methacrylate/buriti blends, namely, trans-␤-carotene, 13-cis-␤-carotene, 9-cis-␤-carotene, phytofluene, and zeaxanthin. Semiempirical, model Hamiltonian, and density functional calculations were carried out to study these structures. The geometries were fully optimized using AM1, PM3, and B3LYP/6-31G(d,p) methods. The TD-DFT and ZINDO/S methods were applied for the calculation of the electronic absorption spectra of the optimized B3LYP geometries. The calculated spectra using the polarizable continuum model for the solvent effects were compared with the available experimental.

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Understanding the antioxidant activity of flavonoids is important to investigate their biological activities as well as to design novel molecules with low toxicity and high activity. Aromaticity is a chemical property found in cyclic structures that plays an important role in their stability and reactivity, and its investigation can help us to understand the antioxidant activity of some heterocyclic compounds. In the present study, we applied the density functional theory (DFT) to investigate the properties of seven flavonoid structures with well-reported antioxidant activity: flavan, anthocyanidin, flavanone, flavonol, isoflavone, flavone, and flavan-3-ol. Conformational, structural, magnetic, and electronic analyses were performed using nuclear magnetic resonance, ionization potentials, electron affinity, bond dissociation energy, proton affinity, frontier molecular orbitals (highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO)), and aromaticity through nucleus-independent chemical shifts to analyze these seven flavonoid structures. We revised the influence of hydroxyl groups on the properties of flavonoids and also investigated the influence of the aromaticity of these seven flavonoids on the antioxidant activity.