Molecular Orbital Picture Research Articles

Linear and non-linear optical properties of some donor–acceptor oxadiazoles by ab initio Hartree-Fock calculations

The molecular hyperpolarizability of some donor–acceptor oxadiazoles was investigated using ab initio methods. Ab initio optimizations were performed at the Hartree-Fock level using different basis sets, starting with the minimal basis set, and then split valence sets. The first hyperpolarizabilities were calculated at the Hartree-Fock level employing the corresponding basis sets using Gaussian 98W. In general, the first hyperpolarizability is dependent on the choice of method and basis set. In order to understand this phenomenon in the context of molecular orbital picture, we examined the molecular HOMOs and molecular LUMOs generated via HF/6-31G level. It has also been calculated the polarizability, anisotropy of polarizability and ground state dipole moment of all the molecules. Several of these oxadiazoles display significant second-order molecular nonlinearity, β(8.57–195.05 × 10−30 esu) and provide the basis for future design of efficient nonlinear optical materials having the oxadiazole core.

Ab initio Hartree-Fock calculations on linear and second-order nonlinear optical properties of new acridine-benzothiazolylamine chromophores

The calculation of optimized molecular structure and molecular hyperpolarizability of four new acridine-benzothiazolylamine chromophores (1-4) [2-nitro-6-(piperid-1-yl) acridine (1), 6-(benzothiazol-2-yl-amino)-2-nitro-acridine (2), 6-(6-ethylcarboxylate-benzothiazol-2-yl-amino)-2-nitroacridine (3), 6-(6-(beta-hydroxyethyl-benzothiazol-2-yl-amino)-2-nitroacridine (4)] have been investigated using ab initio methods. Ab initio optimization were performed at the Hartree-Fock level using STO-3G basis set. The first hyperpolarizabilities have been calculated at the Hartree-Fock method with 6-31G and 6-311G basis sets using Gaussian 98W. In general, the first hyperpolarizability is dependent on the choice of method and basis set. To understand this phenomenon in the context of molecular orbital picture, we examined the frontier molecular orbital energies of all the molecules by using HF/6-31G, 6-311G levels. The polarizability, anisotropy of polarizability and ground state dipole moment of all the molecules have also been calculated. These acridine-benzothiazolylamine chromophores display significant second-order molecular nonlinearity, beta (60.2-137.0 x 10(-30) esu) and provide the basis for future design of efficient nonlinear optical materials having the acridine-benzothiazolylamine core.

Valence Bond Approach of Metal−Ligand Bonding in the Dewar−Chatt−Duncanson Model

Metal-ligand bonding, as considered in the Dewar-Chatt-Duncanson model, is described via an ab initio Valence Bond (VB) approach and applied to typical Pd-L complexes (L = NH3, PH3, CH2, SiH2). A progressive construction of the VB wave function is followed and leads to a very compact, though accurate, description of metal-ligand bonds. A description with the donation interaction only (ligand-metal) is first constructed and enriched so the back-donation interactions (metal-ligand) are also introduced. This latter VB wave function, although being extremely compact, provides bonding energies in agreement with standard (correlated) methods. A comparison between the two VB levels allows a quantification of adiabatic back-bonding energies and reveals very different trends between the ligands considered. A very faint back-donation in Pd-NH3 is found, which contrasts with a significant effect in Pd-PH3. Back-donation is, however, more important in Pd-XH2 complexes. In Pd-CH2, it is such that it even represents the major source of bonding. For Pd-SiH2, back-donation is slightly weaker than donation. The nature of the interaction in these metal-ligand complexes is revealed by the VB wave function analysis. Results are as well rationalized using the simple molecular orbital picture and compared to previous studies.

Van der Waals Complexes of Cu, Ag, and Au with Hydrogen Sulfide. The Bonding Character

The electronic and structural features of the Cu...SH2, Ag...SH2, and Au...SH2 complexes are investigated by using the spin-adapted restricted open-shell HF coupled cluster CCSD(T) method combined with the second-order spin-free Douglas-Kroll-Hess (DKH) relativistic approach. M...SH2 complexes are nonplanar with bonding energies -5.99, -1.99, and -9.08 mHartree, respectively. Comparison with analogous M...OH2 and M...NH3 complexes allows us to establish general features of the bonding between coinage metal atoms and ligand molecules with the participation of their lone electron pairs. Consistent interpretation of the interaction effects can be obtained by using the molecular orbital picture of the M...L region. The bonding character is explained by stressing the importance of the charge transfer from the lone pair of the ligand to the metal atom. Relativistic changes of the metal element electron affinity and polarizability facilitate the understanding of major trends in the pattern of interactions between the coinage metal atoms and different lone pair donating ligands.

Structure and Bonding of the Water−Hydroxyl Mixed Phase on Pt(111)

We combine low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and Auger electron spectroscopy (AES) with density functional theory (DFT) to reveal the structure and bonding of water−hydroxyl mixed layers adsorbed on Pt(111). We find that the stable water−hydroxyl adlayer forms a mixed phase of nearly coplanar hexamer structures resulting in (√3 × √3)R30° and (3 × 3) unit cells, respectively. In the asymmetric (3 × 3) structure the lateral O−O distances alternate between long and short bond lengths similar to the chemical bonding network for OH- ions in solution. The chemical driving force behind this similarity is discussed in a molecular orbital picture.

Effects of Hydrogen Bonding on the Ring Stretching Modes of Pyridine

The effects of hydrogen bonding on the ring stretching modes (both ring breathing and triangle) of pyridine are experimentally investigated using noisy light based coherent Raman scattering spectroscopy. Three systems, pyridine/formamide, pyridine/water, and pyridine/acetic acid, provide varying degrees of strength for the diluent-pyridine hydrogen bond complex. Formamide forms a relatively weaker hydrogen bond, while acetic acid essentially fully transfers a proton to pyridine. Both dilution studies and temperature studies are performed on the three systems. Together, these provide a broad context in which a very simple model for the electronic behavior of pyridine is formulated. This model is based on a molecular orbital picture and electrostatic arguments, and it well explains the observed experimental results. Additionally, a new mechanism for the line broadening of the ring breathing mode for the pyridine-water hydrogen bonded complex is proposed.

Donor−Acceptor Interactions and Electron Transfer in Cyano-Bridged Trinuclear Compounds

The NIR donor-acceptor charge transfer (DACT) bands of the series of trinuclear complexes trans-[(NC)5Fe(II/III)(mu-CN)RuIIL4(mu-NC)FeIII(CN)5](5/4-) (L= pyridine, 4-tert-butylpyridine, and 4-methoxypyridine) are analyzed in terms of a simplified molecular orbital picture that reflects the interaction between the donor and acceptor fragments. The degree of electronic coupling between the fragments is estimated by a full fit of the DACT band profiles according to a three-state model inspired in the Mulliken-Hush formalism. The information is complemented with determinations performed on the asymmetric heterotrinuclear species trans-[(NC)5CoIII(mu-CN)RuII(py)4(mu-NC)FeIII(CN)5]4-, whose preparation is reported here for the first time. The analysis of the NIR spectra of the symmetric trans-[(NC)5FeIII(mu-CN)RuIIL4(mu-NC)FeIII(CN)5]4- species reveals a low degree of mixing between the terminal acceptor fragments and the bridging moiety containing RuII, with H12 values between 1.0 x 10(3) and 1.5 x 10(3) cm-1. The reorganization energy contributions seem to be the same for the three species, even when the spectra were recorded in different media. This observation also applies for the CoIII-substituted compound. The computed potential energy surfaces (PES) of the ground state for these complexes show only one stationary point, suggesting that the FeII-RuIII-FeIII (or FeII-RuIII-CoIII) electronic isomers are not thermally accessible. One-electron reduction leads to asymmetric trans-[(NC)5FeII(mu-CN)RuIIL4(mu-NC)FeIII(CN)5]5- compounds with potentially two DACT bands involving the RuII and the FeII donor fragments. These species reveal a similar degree of electronic mixing but the PES shows three minima. We explore the role of the bridging fragment in the long-range thermally induced electron transfer between the distant iron centers. The results suggest that superexchange and hopping might become competitive paths, depending on the substituents in the bridging fragment.

Structure and stability of Be6, Be, and Be clusters

AbstractThe structure and stability of Be6, Be, and Be clusters have been investigated at the B3LYP, B3PW91, and second‐order Møller–Plesset (MP2) levels of theory, along with the 6‐311G* basis set for neutral and cationic clusters and the 6‐311+G* basis set for anion clusters. CCSD(T)/6‐311+G* has also been used to calculate some neutral structure to find the most stable structure. Twelve Be6, six Be, and eight Be isomers are identified. The distortion octahedron structure, pentagonal pyramids structure, and trapezoidal bipyramid structure are found to be the most stable structure on the neutral, cationic, and anionic surface, respectively. They are in agreement with the results reported previously. Natural bond orbital (NBO) analysis, molecular orbital (MO) pictures, and the nucleus independent chemical shift (NICS) values suggest aromatic of the neutral and cationic clusters and antiaromatic of the anionic cluster. The multi‐center σ bonds and delocalized π bonds play important role in the bonding of the beryllium clusters. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

Magneto-optical and optical spectroscopic ellipsometries of La2∕3Sr1∕3MnO3 thin films

Magneto-optical (MO) spectroscopy combined with spectroscopic ellipsometry has been used to learn about the electronic transitions in La2∕3Sr1∕3MnO3 (LSMO) film pulse laser deposited onto SrTiO3 (100) substrates. The dispersion function of LSMO films was parametrized by the sum of three damped Lorentz oscillators and adjusted together with the film thickness numerically. With the knowledge of film optical properties the off-diagonal elements of the LSMO permittivity tensor (magneto-optical constants) were calculated from the complex polar Kerr effect measured on the thickest (60nm) LSMO film (considered as optically thick). The considered approximation as well as the reliability of the obtained optical and MO constants was verified by the consistence of the calculated and experimental MO spectra for the thinner LSMO films (where the propagation effect plays an important role). The resolved electronic transition centered at about 4eV features the spectral dependences of diagonal and off-diagonal elements of LSMO permittivity in the studied spectral range. In the molecular orbital picture this could be assigned to the charge transfer electronic transition from the O 2p states to the Mn 3d t2g states, split by the spin-orbit interaction.

Theory of chemical bonds in metalloenzymes V: Hybrid‐DFT studies of the inorganic [8Fe–7S] core

AbstractThe electronic and spin structures of the [8Fe–7S] inorganic model complex (1) have been investigated using the density functional theory (DFT) UB3LYP and UBLYP methods. The experimental geometry of 1 was used to calculate the sets of exchange interactions (J values) between Fe‐spin centers within spin Hamiltonian models. The experimental temperature‐dependent paramagnetism is well reproduced on the basis of the calculated J values. The calculated low‐lying energy levels are also consistent with those of the experiments. This indicates that the ground spin state of 1 is singlet (S = 0) for the weak antiferromagnetic coupling between two [4Fe–3S] cubane units, which take the Fe(III)Fe(II)3 formal charge and S = 7/2 spin states. The first‐order density matrix of the most stable broken‐symmetry (BS) solution has been diagonalized to elucidate the natural molecular orbitals (MOs) and their occupation number of 1, which are hardly obtained by the symmetry‐adapted (SA) CASSCF approach. Several chemical indices such as the effective bond orders before and after spin projection are also calculated by using the occupation numbers to provide the SA MO picture of labile chemical bonds of 1. It is found that the BS MO calculation followed by spin‐projected SA analysis is an effective and practical alternative to the CASSCF MO approach for the multinuclear transition metal clusters. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

The hydrogen bond in ice probed by soft x-ray spectroscopy and density functional theory

We combine photoelectron and x-ray absorption spectroscopy with density functional theory to derive a molecular orbital picture of the hydrogen bond in ice. We find that the hydrogen bond involves donation and back-donation of charge between the oxygen lone pair and the O-H antibonding orbitals on neighboring molecules. Together with internal s-p rehybridization this minimizes the repulsive charge overlap of the connecting oxygen and hydrogen atoms, which is essential for a strong attractive electrostatic interaction. Our joint experimental and theoretical results demonstrate that an electrostatic model based on only charge induction from the surrounding medium fails to properly describe the internal charge redistributions upon hydrogen bonding.

Electronic structure and chemical reactivity of carbon nanotubes: a chemist's view.

A qualitative description of the electronic structure of single-wall carbon nanotubes from a chemical perspective is presented using real-space orbital representations and traditional concepts of aromaticity, orbital symmetry and frontier orbitals. This unusual view of carbon nanotubes allows us to merge the solid-state physics description of band structures with the molecular orbitals framework of reaction mechanisms used in organic chemistry and to predict intriguing chemical selectivity based on electronic structure.

Energy Partitioning Studies of CO and NO Chemisorption on the Pd(111) Surface

A bond-by-bond partitioning of the electron energy within the p(2 × 2)-CO/Pd(111) and p(2 × 2)-NO/Pd(111) chemisorption systems has been used to demonstrate a preference for CO and NO chemisorption in hollow sites. The changes in bonding within the adsorbate and within the surface that accompany formation of the chemisorption bond are quantified using Hamilton population analysis: a partitioning of the electron energy among the atoms and bonds. In this way, the preference for CO and NO chemisorption in hollow sites is seen to result from the inability of the increased reduction in bonding within both the adsorbate and surface to counter the increase in surface−adsorbate bonding with increasing adsorbate coordination. By comparison with CO chemisorption, the chemisorption of NO is characterized by stronger surface−adsorbate bonding on all sites; principally the result of increased mixing between the NO(2π) orbitals and the surface d band. Increased mixing between the NO(2π) orbitals and the surface d band, in turn, results in increased back-donation to the NO(2π) orbitals on all sites and, correspondingly, a greater degree of bond weakening within NO on all sites. The increase in 2π-d mixing on chemisorbing NO does not, however, result in increased Pd−Pd bond weakening. Instead, increased 2π-d mixing on chemisorbing NO serves to depopulate a greater number of those surface states contributing to d−d antibonding interactions within Pd−Pd bonds about chemisorbed NO. In this way, the analysis of CO and NO chemisorption presented provides new insight into the mechanism by which chemisorbed CO and NO perturb the electronic structure of the surface and, potentially, influence the chemisorption of neighboring adsorbates at higher coverages. Detailed analysis of the adsorbate orbital contributions to Pd−CO and Pd−NO bonding also reveals that both the adsorbate σ and π orbitals mix primarily with the surface s and p bands. Within the context of the molecular orbital picture of CO and NO chemisorption presented, interaction of the adsorbate orbitals with the d band acts only to perturb the more substantial interaction between the adsorbate orbitals and the surface s and p bands. In this way, the molecular orbital picture of CO and NO chemisorption presented serves to validate the d-band model of chemisorption devised previously by Hammer et al.

Hepta- and octacoordinate boron in molecular wheels of eight- and nine-atom boron clusters: observation and confirmation.

Clusters of atoms can adopt different atomic arrangements from bulk materials. They often exhibit novel structures and properties, which provide opportunities for a new types of chemical bonding and stoichiometry. Herein we report experimental and theoretical evidence that 8- and 9-atom boron clusters are perfectly planar molecular wheels, with a hepta- or octacoordinated central boron atom, respectively, despite the predominance of three-dimensional structures normally found in bulk boron and its compounds.

Electronic structure of conducting polymers: Limitations of oligomer extrapolation approximations and effects of heteroatoms

Density-functional methods are used to analyze the scaling of discrete oligomeric π-electron conducting molecules towards idealized isolated polymer chains, treated in periodic boundary conditions. The band gaps of a series of conjugated oligomers of incrementally increasing lengths exactly fit a nearly-free-electron molecular-orbital picture and exhibit a smooth deviation from the classical empirical I/N trend for long oligomers and infinite polymers. The calculations also show a smooth convergence of bond lengths. The full band structures and densities of states of a polyacetylene, polypyrrole, polyfuran, and polythiophene show that band crossing, localized bands, and other effects cannot be accurately determined from simple extrapolation of oligomer electronic structures. Systematic comparisons of the electronic structure variations of the polymers investigated indicate that the electron affinity, rather than the electronegativity of the heteroatom or the bond-length alternation of the conjugated backbone, significantly affects the band gap of the resulting polymer as indicated by the presence of heteroatom states in the partial density of states of the conduction band, requiring revision of previous semiempirical analyses. Consequences for doping processes are also studied, along with a comparison of valence bandwidths, conduction bandwidths, and carrier effective masses as a function of heteroatom.

Electronic structure of the π-bonded Al–C2H4 complex: Characterization of the ground and low-lying excited states

The equilibrium properties of the π-bonded Al–ethylene complex in its ground state are calculated by coupled-cluster theory. Significant changes in the geometry of the ethylene molecule upon complexation (elongation of the CC bond, pyramidalization of the CH2 groups) are consistent with the formation of a chemical bond between fragments. The overall interaction is rather weak because bonding is derived from the overlap between: (i) a singly occupied p orbital of Al and the antibonding π* orbital of ethylene and (ii) a vacant Al sp hybrid and π of C2H4. Electronically excited states are studied by the equation-of-motion coupled-cluster method. The covalent nature of the interaction between fragments is reflected in the excited-state delocalization over both fragments (as opposed to the corresponding van der Waals complex). In the examined energy range (0–5.18 eV) both valence and Rydberg excited states are found. Bonding in the valence states is explained in terms of a simple molecular orbital picture. Two very intense transitions at 3.50 eV and at 3.79 eV can be used as a probe in experimental studies.

Interaction of Y, Y2, Mo, and Mo2 with NH3. A Density Functional Study

The interaction of Y, Y 2 , Mo, and Mo 2 with NH 3 is studied to understand the influence of the electronic configuration of the transition metal atoms and clusters on their reactions with ammonia. The interactions are investigated with the all-electron linear combination of Gaussian-type orbitals Kohn-Sham density functional theory (LCGTO-KS-DFT). The binding energies and harmonic frequencies characterize the equilibrium geometries. The reaction products investigated are MNH 3 , MNH, M 2 NH 3 , M 2 NH, and M 2 (NH) 2 . The binding energy indicates that the reaction of Y and Y 2 is possible. For Y, the stable products are YNH 3 and YNH with binding energies of 24.6 and 32.6 kcal/mol, respectively. For Y 2 , the stable products of the reaction are Y 2 NH 3 , Y 2 NH, and Y 2 (NH) 2 (binding energies of 13.9, 55.5, and 110.2 kcal/mol, respectively). For Mo, the stable product is MoNH 3 with a binding energy of 8.5 kcal/mol. For MoNH, the binding energies indicate that the reactants (Mo + NH 3 ) are more stable than the products (MoNH + H 2 ) by 9.8 kcal/mol. For the Mo 2 [NH 3 ] complex, the binding energy is 17.9 kcal/mol, in good agreement with the experimental value of 14 kcal/mol previously reported. For Mo 2 , there are no other stable products of the reaction because Mo 2 NH and Mo 2 (NH) 2 are less stable than the reactants (Mo 2 + NH 3 ) by 12.7 and 10.9 kcal/mol, respectively. The differences in the bonding are explained with molecular orbital pictures. For each metal, a relationship between the electronic configurations of the transition metals and the binding energies is reported.

A study of electronic and bonding properties of Sn doped Lin clusters and aluminum based binary clusters through electron localization function

We have carried out a thorough investigation of bonding and energetics in two different class of heteroatomic clusters viz. single impurity based LinSn (n⩽9) clusters and mixed aluminum-based binary clusters Al4X4 (X=Be, Mg, B, Si). We employ first principle Born–Oppenheimer molecular dynamics within the framework of density functional theory using generalized gradient approximation. The bonding characteristics have been analyzed using electron localization function (ELF) along with charge density and valence molecular orbital pictures. The analysis of bonding through ELF reveals that in LinSn (n⩽9) clusters there is a transition from ionic bond to a metallic bond through an intermediate ionic–metallic bond. On the other hand, it is found that Al4X4 (X=Be, Mg, B, Si) clusters show interesting characteristics of lone pairs on Al, polar covalent bonding, and a multicenter bonding. We demonstrate that ELF can play an important role in analyzing different types of bonding characteristics in heteroatomic clusters.

Non-linear optical properties of novel fluorenyl derivatives—ab initio quantum chemical calculations

We report accurate ab initio studies of the first static hyperpolarizabilities ( β) of fluorenyl derivatives in which electron donating (D) and electron accepting (A) groups were introduced either side of the fluorenyl ring system. Geometries of all molecules were optimized at the Hartree–Fock level in a series of steps, first with the STO-3G minimal basis set, then with the 3-21G split valence basis set and finally with the 6-31G basis set. The first static hyperpolarizabilities of these molecules were calculated using Hartree–Fock level using 6-31G basis set using gaussian98W. The calculated hyperpolarizabilities of these molecules were compared with biphenyl derivatives and other available data in the literature. To understand this phenomenon in the context of molecular orbital picture, we examined the molecular HOMOs and molecular LUMOs generated via gaussian98W. The study reveals that the fluorenyl derivatives have large β values hence in general may have potential applications in the development of non-linear optical materials.

Orbital picture of ionization and its breakdown in nanoarrays of quantum dots.

We present exact numerical results indicating that ionization could be a useful tool to study electron correlations in artificial molecules and nanoarrays of metallic quantum dots. For nanorings consisting of Ag quantum dots of the type already fabricated, we demonstrate that the molecular orbital picture breaks down even for lowest energy ionization processes, in contrast to ordinary molecules. Our ionization results yield a transition point between localization and delocalization regimes in good agreement with various experimental data.