Search results for " orbita"
showing 10 items of 447 documents
Theoretical study of the electronic spectrum of magnesium-porphyrin
1999
Multiconfigurational self-consistent field (SCF) and second order perturbation methods have been used to study the electronic spectrum of magnesium-porphyrin (MgP). An extended ANO-type basis set including polarization functions on all heavy atoms has been used. Four allowed singlet states of E1u symmetry have been computed and in addition a number of forbidden transitions and a few triplet states. The results lead to a consistent interpretation of the electronic spectrum, where the Q band contains one transition, the B band two, and the N band one. The computed transition energies are consistently between 0.1 and 0.5 too low compared to the measured band maxima. The source of the discrepan…
Structure, magnetizability, and nuclear magnetic shielding tensors of bis-heteropentalenes. IV. Dihydrophospholophosphole isomers
2005
The geometry of the heteropentalenes formed by two phosphole units has been determined at the DFT level. The magnetic susceptibility and the nuclear magnetic shielding at the nuclei of these systems have also been calculated using gauge-including atomic orbitals and a large Gaussian basis set to achieve near Hartree-Fock estimates. A comparative study of the various isomers, of their flattened analogs, and of the parent phosphole molecule, shows that the [3,4-c] isomer is the most aromatic system in the set considered, assuming diatropicity and degree of planarity as indicators, even if it is the less stable in terms of total molecular energy. Plots of magnetic field-induced current densiti…
An ab Initio MO Study of Silver Triflate Complexation in [2.2.1]Cyclophane π-Prismands
2002
Ab initio Hartree-Fock and DFT MO calculations have been used to study the conformations of six [2.2.1]cyclophane π-prismands and the formation of their π-complexes with silver triflate (AgSO 3 CF 3 ). The lowest energy cyclophane conformations and their silver triflate π-complexes have been calculated with HF/3-21G* and B3LYP/3-21G* levels of theory. The nature of bonding in silver triflate π-complexes has been studied with natural bond orbital analysis (NBO). Energies of the calculated cyclophanes and complexes, together with the formation energies of those complexes, have also been discussed. The results have been compared to available X-ray crystal structures and also to results of the …
Electronic nature of the emitting triplet in SF 5 -substituted cationic Ir(III) complexes
2018
Abstract A theoretical density functional theory study has been performed on a family of cationic iridium(III) complexes of the form [Ir(C^N)2(dtBubpy)]+ (dtBubpy = 4,4′-di-tert-butyl-2,2′-bipyridine), that incorporate 2-phenylpyridine (1, 2) and 1-phenylpyrazole (3, 4) cyclometallating C^N ligands functionalized with SF5 groups. The goal is to investigate the effect that the inclusion of SF5 groups in meta (1, 3) and para position (2, 4) with respect to the Ir–C bond has on the electronic nature of the emitting triplet state and the emission wavelength. The attachment of the electron-withdrawing groups induces the stabilization of the molecular orbitals localized on the C^N ligands and, in…
ChemInform Abstract: Spectroscopic and Theoretical Studies on the Internal Rotational Barrier of 1-Acetylcyclohexene
1987
Abstract MINDO-3 molecular orbital calculations with full geometry optimization have been carried out on 1-acetylcyclohexene (ACH) in order to estimate the rotational barrier for the acetyl group, and to localize the less energetic conformation. MNDO calculations have also been carried out for comparative purposes. Both methods point to the non-delocalized structure with the two π-clouds perpendicular to the less energetic conformation. A decomposition of the barrier energy by means of Fourier analysis is made in order to characterize the contributions to the barrier. The UV spectrum of ACH has been recorded both in vapor phase and in hexane solution. These spectra are related to the one ca…
Treatment of scalar-relativistic effects on nuclear magnetic shieldings using a spin-free exact-two-component approach.
2013
A cost-effective treatment of scalar-relativistic effects on nuclear magnetic shieldings based on the spin-free exact-two-component theory in its one-electron variant (SFX2C-1e) is presented. The SFX2C-1e scheme gains its computational efficiency, in comparison to the four-component approach, from a focus on spin-free contributions and from the elimination of the small component. For the calculation of nuclear magnetic shieldings, the separation of spin-free and spin-dependent terms in the parent four-component theory is carried out here for the matrix representation of the Dirac equation in terms of a restricted-magnetically balanced gauge-including atomic orbital basis. The resulting spin…
Atomic and electronic structure of both perfect and nanostructured Ni(111) surfaces: First-principles calculations
2011
article i nfo In this study, we perform first principles simulations on both atomically smooth and nanostructured Ni(111) slabs. The latter contains periodically distributed nickel nanoclusters atop a thin metal film gradually growing from adatoms and serving as a promising catalyst. Applying the generalized gradient approximation within the formalism of the density functional theory we compare the atomic and electronic structures of Ni bulk, as well as both perfect and nanostructured (111) surfaces obtained using two different ab initio approaches: (i) the linear combination of atomic orbitals and (ii) the projector augmented plane waves. The most essential inter-atomic forces between the …
Localized molecular orbitals for excited states of polyenals, polyendials, and polyenones
2003
The work is focused on the generation of localized molecular orbitals for excited states. A recently developed a priori method based in a CAS-SCF–type algorithm is applied. The method generates directly localized orbitals and can be applied to multireference wavefunctions. A detailed description of the performance of the method as well as the locality of the MOs for the example of the singlet nπ* (CO) excited state is given. It is in general possible to obtain local orbitals for the doubly occupied and virtual valence orbitals. The partial delocalization of the π* (CO) orbital is discussed, as is the effect of the use of different CAS spaces. The systems under study are polyenals, polyendia…
Anion ordering and defect structure in Ruddlesden-Popper strontium niobium oxynitrides
2004
The crystal structure of the n = 1 member of the Ruddlesden-Popper family (SrO)(SrNbO(2)N)(n) was refined by the Rietveld method using neutron powder diffraction data. This complex crystallizes in the I4/mmm space group with cell parameters a = 4.0506(2) and c = 12.5936(9) angstroms. The refined composition was Sr(2)NbO(3.28)N(0.72), which corresponds to a formal oxidation state for Nb of +4.72, meaning 72% Nb(V) and 28% Nb(IV). The nitrogen atoms order in the equatorial sites of the niobium octahedra according to Pauling's second crystal rule as the more charged anion occupies the site showing the larger bond strength sums. Pauling's second crystal rule is shown to be able to predict the d…
N-benzoylindole-2,3-quinodimethane: Diels-Alder reactivity and synthetic applications for [b]annellated indoles
1991
Abstract The Diels-Alder reactivity of in situ generated N -benzoylindole-2,3-quinodimethane has been expanded considerably to include reactions with carbon- and hetero-dienophiles which furnish a variety of [ b ]annellated indoles as well as functionalized and annellated carbazoles. The frontier molecular orbital theory was found to be a useful model for the prediction of the experimental results under consideration of reactivity aspects.