1,721,283 research outputs found
A Quasi-Relativistic Density Functional Study of Structural and Electronic Properties of the Bis-Ketene Cis-[Pt{η3-C3H5}{η1-C(PPh3)CO}2]+
No full textQuasi-relativistic density functional calculations have been used to look into the molecular and electronic properties of cis-[Pt(η3-C3H5){η1-C(PPh3)(CO)}2]BF4. Numerical experiments have been run for both isomers compatible with IR and NMR outcomes. Binding energies and computed CCO stretching frequencies are very similar for both species, thus indicating their possible coexistence in solution. The comparison with homogeneous theoretical data pertaining to cis/trans-[Pt(η3-C3H5)2] and trans-[PtCl2{η1-C(PPh3)(CO)}2] provides a rationale of the [Pt(η3-C3H5){η1-C(PPh3)(CO)}2]BF4 low stability
A Theoretical Study of the Chemisorption of H2O and H2S on the Ti2O3(10-12) Non-Polar Surface
No full textDensity functional molecular cluster calculations have been used to investigate the interaction of two Brønsted acids (H2X, X = O, S) with the Ti2O3(10-12) non-polar surface. Adsorbate geometries, vibrational parameters and chemisorption enthalpies are computed and discussed. According to experimental outcomes [R. L. Kurtz and V. E. Henrich, Phys. Rev. B, 1982, 26, 6682], H2O is molecularly adsorbed, even if one of the O–H bonds is significantly lengthened as a consequence of a short hydrogen bonding between the H atom and a surface Lewis base site (Lbs). This interaction determines the peculiar arrangement of the molecule on the surface. At variance with water, H2S is partially deprotonated upon chemisorption giving rise to Las-SH (Las = surface Lewis acid site) and Lbs-H surface species. Independently of the adsorption character, molecular or dissociative, the valence band maximum of Ti2O3(10-12) is negligibly perturbed upon chemisorption, while the conduction band minimum extensively participates to the H2X–Ti2O3(10-12) interaction. The H2O–substrate bonding is dominated by a donation from the adsorbate in-plane and out-of-plane lone pairs into Las empty levels, whereas all the valence orbitals of the HS fragment participate in the Las-S bond to a similar extent
Ab Initio Modeling of TiO2 Nanosheets
No full textWe present density functional calculations on 1–6 monolayer (ML) thick TiO2 films peeled off from the main low-index surfaces of anatase. The structure of the films is optimized both by constraining the lattice constants to those of bulk anatase, and by allowing them to relax. It is found that the stability order of the films does not follow that of the surfaces from which they are derived, and does not increase monotonously with film thickness. Furthermore, relaxing the lattice constants can induce large modifications in the film structure. In particular, two anomalously stable films are found. One derives from the 2 ML (001) film, and rearranges to a lepidocrocite-TiO2 nanosheet. The other one derives from a 4 ML (101) film, and gives rise to a novel phase, where all the Ti ions are fivefold coordinated
A Comparative Study of CO Chemisorption on Al2O3 and Ti2O3 Nonpolar Surfaces
No full textDensity functional molecular cluster calculations have been used to investigate the interaction of CO with the M2O3(10-12) (M = Al and Ti) nonpolar surface. The electronic structure of the clean surface, the adsorbate geometry, vibrational parameters, and chemisorption enthalpies are computed and discussed. Theoretical results pertaining to the clean surface agree quite well with experimental measurements and other theoretical investigations. As far as the adsorbate−substrate interaction is concerned, our data indicate that the CO−M2O3(10-12) bonding is charaterized, in both Al2O3 and Ti2O3, by a two-way electron flow involving both donation from CO based σ levels into virtual orbitals of the unsaturated surface Lewis acid site and back-donation from surface states into the CO π* virtual levels. However, the nature of surface orbitals involved in back-donation and the concomitant effects on the adsorbate structure are very different in the two cases. CO is only slightly affected upon chemisorption on Al2O3(10-12), while perturbations induced into the CO electronic and molecular structure by the interaction with the Ti2O3(10-12) surface are very intense and, consistently with experimental data, the C−O bond becomes strongly weakened
A Theoretical Investigation of the Relaxation Effects Induced on the ZnO(10-10) Surface by the Chemisorption of H2 and CO
No full textDensity functional theory coupled to the molecular cluster approach is used to investigate the chemisorption of CO on the clean and Cu doped ZnO(10-10) surface. Results pertaining to the dissociative interaction of H2 with ZnO(10-10) are also presented. Theoretical data include evaluation of surface relaxation upon chemisorption, optimized adsorbate geometries, adsorbate–substrate binding energies and adsorbate vibrational frequencies. According to experiment, the ZnO–CO interaction is found rather weak, while the Cu–CO bonding results definitely stronger. As far as the dissociative chemisorption of H2 on ZnO(10-10) is concerned, we find that the formation of strong Zn–H and O–H bonds implies the Zn–O bond breaking. Both vibrational parameters and adsorption energies reasonably agree with available experimental measurements
Molecular Chemisorption on TiO2(110): A Local Point of View
No full textThe molecular adsorption of some probe molecules (CO, H2O, and H2S) on a 5-fold-coordinated Ti Lewis acid site of the TiO2 rutile (110) surface is studied within the density functional theory. The substrate is modeled with a small Ti7O9 cluster terminated with pseudo-hydrogens. This is found to describe the electronic and structural properties of the clean surface in good agreement with both experimental and periodic slab calculations. Adsorption energies (6.7, 19.3, and 7.0 kcal/mol for CO, H2O, and H2S, respectively) and adsorbate stretching frequencies compare favorably with available experimental data. The agreement is particularly good for the C−O stretching frequency shift (+56 cm-1), as was found in previous investigations carried out with the same theoretical approach on other oxides. In contrast, the HOH scissoring mode is poorly reproduced, suggesting that molecularly adsorbed H2O is actually involved in weak bonds with other adsorbed species. This agrees with predictions of very recent slab calculations by Lindan et al. (Phys. Rev. Lett. 1998, 80, 762). Differences with respect to calculations carried out by embedding the cluster in point-charge arrays are discussed
Pairing of Hydrogen Atoms on the Si(100)-2×1 Surface: The Role of Interactions Among Dimers
No full textLocal-density-functional calculations of the pairing of H atoms on the (100)-2×1 surface of Si are presented. We find that significant interactions between neighboring dimers along a row are present, favoring configurations where H-unpaired dimers are in the same row and have the H-atoms all on the same side. We discuss how these effects can contribute to the effective H-pairing energies determined experimentally
Coordination Chemistry of CO and NH3 on ZnO(0001): a Molecular Cluster Study of the CO and NH3 Bonding Interaction with a d10 Ion
No full textA detailed investigation of the electronic structure of CO chemisorbed on the ZnO(0001) polar surface has been carried out by using the local-density-functional molecular-cluster approach. A Zn22O22/CO model cluster is used to evaluate internuclear distances, adsorption energies and vibrational frequencies of the surface-CO and C-O vibrational modes. The chaining to the rest of the solid has been simulated with pseudo-atoms carrying a not-integer nuclear charge. A realistic description of the electronic structure of the Lewis acid site is shown to be crucial to obtain a correct chemisorptive interaction. In particular, it appears that removing half an electron from each surface Zn allows an excellent agreement between experiment and theory as a consequence of the absence of occupied dangling bonds on ZnO(0001). Preliminary results relative to NH3 chemisorption over the same surface are also presented and discussed. The bonding mechanism of CO to ZnO(0001) is dominated by a covalent interaction involving a donation from the highest occupied molecular orbital of CO into the empty levels of the coordinatively unsaturated Zn surface ions. The same kind of mechanism is active in NH3 chemisorption, even though the electrostatic interaction between the nh3 dipole moment and the field at ZnO(0001) is at least as important as the covalent one. Slight variations of the electronic structure of the surface greatly affect the bonding of CO to the surface itself as a consequence of a consistent donation into the CO lowest unoccupied molecular orbital from the partially occupied dangling bonds of the Zn unsaturated ions nearest to the Lewis acid site. Transition state calculations seem to indicate that the assignments of the ultraviolet photoelectron spectra of ZnO(0001) after exposure to CO or NH3 need to be slightly modified
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