Iterative Extended Huckel Method Research Articles

Binding of O2, NO, and CO to model active sites in ferrous heme proteins: Ligand geometry, electronic structure, and quadrupole splittings

The electronic structures of model oxy-, nitrosyl-, and carbon monoxy ferroporphyrin compounds with N-methylimidazole as an axial ligand are calculated for a number of different conformations of the O2, NO, and CO ligands using the iterative extended Huckel method. For the dioxygen ligand a bent end-on (Pauling) geometry with a low-energy off-axis displacement is favored. The “anomalously large” electric field gradient at the iron nucleus can be accounted for by a paired iron(II)-dioxygen configuration. The carbon monoxide ligand is linear in model compounds but bent or tilted in protein. The reduced affinity observed for the intact protein is explained by these results. The nitrosyl ligand is bent both in model compounds and in the protein. The calculated electronic structure is consistent with the spin density observed in hyperfine splitting in electron spin resonance spectra and gives a quadrupole splitting in excellent agreement with experiment.

Molecular orbital studies of the mast cell zinc-histamine storage complex

A zinc-histamine-heparin complex is proposed as a storage form of histamine in rat mast cells. A ring nitrogen and the ethylamine nitrogen of two histamine molecules in a square planar arrangement about one divalent zinc ion may intercalate with a portion of the heparin molecule such that two sulfonic acid or carboxylate residues provide the fifth and sixth coordinating ligands above and below the plane of the zinc histamine complex. Semiempirical molecular orbital calculations employing an iterative extended Huckel method, Cusach's modification, indicate that there is little or no covalent bonding between the Zn atom and the donor nitrogen atoms. Thus the stability of the complex would appear to be owing to electrostatic interactions. These data correlate well with the demonstrated case of liberation of histamine from mast cell granules.

Studies of some iron dithiolate complexes by the ESCA, Mössbauer and extended Hückel methods

Abstract Iron dithiolate complexes have been studied by Mossbauer and ESCA spectroscopy. Atomic charges and iron atom populations have been evaluated. Molecular orbital calculations have been made using the iterative extended Huckel method. The theoretical results are compared with the experimentally obtained charge distributions.

Transition metal silicides: aspects of the chemical bond and trends in the electronic structure

Presents a theoretical investigation of the electronic properties of bulk silicides of near-noble metals (NiSi2, NiSi, Ni2,Si, PdSi, Pd2Si, PtSi and Pt2Si). The theoretical approach is provided by the iterative extended Huckel method. The densities of states of various compounds, differing in silicon content and in the crystal structure, are presented and the main features of the chemical bond are discussed. A detailed comparison with the available experimental data and with other theoretical work is presented. The relevance of the present results for the interpretation of the silicide/silicon interfaces is also discussed.

Numerical study of systems with diagonal and off-diagonal disorder in a non-orthogonal basis

Simple structurally disordered materials were studied, taking into account diagonal and off-diagonal disorder in a non-orthogonal basis. The disordered systems were represented by random two-dimensional distributions of identical atoms. Calculations were performed by the iterative extended Huckel method, which allows for the inclusion of intra-atomic electronic repulsion. Densities of states, localisation of the wavefunctions and redistribution of charges due to the disorder were evaluated as a function of the density (or the disorder, as these two variables are taken to be inversely proportional). It was found that a metal-non-metal transition due to the disorder also occurred in these calculations with a non-orthogonal basis, and for a value of the density in agreement with experiments. Although the system studied was one of identical atoms, the effect of the disorder was to produce a different field around each atom and consequently some atoms become positively charged and others negatively charged. The distribution of charges obtained was asymmetric, with most atoms having charges between plus and minus 0.2e, and it became wider as the density decreased.

Exchange interaction in polynuclear complexes. Part 3. Experimental study and theoretical interpretation of the antiferromagnetic coupling in di-µ-oxo-octaoxodichromate(III); comparison with tri-µ-chloro-hexachlorodichromate(III)

The temperature dependence of the magnetic susceptibility of the new material Cr2Te4O11 has been studied in the range 4.2–300 K. The compound contains the binuclear anion [Cr2O12]14– as isolated units, resulting from sharing of an edge between two CrO6 octahedra. The exchange-interaction parameter J of the H.D.V.V. Hamiltonian –JŜAŜB is equal to –6 ± 0.5 cm–1. In the framework of a previously described model for the exchange interaction and using the iterative extended-Huckel method, the antiferromagnetic contribution of the exchange interaction has been calculated. A slmllar calculation has been carried out for the binuclear anion [Cr2Cl9]3–. Comparison of the results for both compounds shows that our model leads to the relative magnitude of the exchange interaction in cases of antiferromagnetlc coupling.

Geometrical shapes of molecules by iterative extended H�ckel method

The iterative extended Huckel method proposed by Harriset al. has been employed to calculate the shapes of 21 molecules consisting of H, C, N, and O. It has given fairly reasonable results, especially when it has been combined with a new formula; the formula regards the orbital energies and the wave functions of the Huckel method as those of the Hartree-Fock method.

Structure and packing arrangement of molecular compounds

The calculation of electronic structures of TCNQ and several electron-donor molecules has been performed by the Iterative Extended Huckel Method, taking into account all valence electrons of various atoms and retaining the overlap integrals. Calculations of intermolecular overlaps involving the lowest vacant molecular orbital of TCNQ and one or more highest occupied orbitals of each of the donors have been correlated with the observed donor-TCNQ geometrical configurations. The results of this simple approach, which is based on Mulliken's “Overlap and Orientation Principle”, suggest that the charge-transfer interaction plays an important role in the particular observed packing arrangement in the solid state.

Inner-shell Eigenvalues from valence orbital only calculations

Methods are examined for calculating Inner-shell Eigenvalues from molecular orbital models which do not explicitly include core basis functions. If the “valence-orbital only” calculation is a good one, a rather straightforward method can be used to obtain core eigenvalues with a rms error of ±0.01 a.u. compared with ab-initio values. Even simpler methods can be used to reproduce trends among core eigenvalues. The AAMOM valence technique, and to a lesser extent, the INDO model, can be used to yield core eigenvalues for orbitals centered on carbon and nitrogen: for oxygen the agreement is poorer. Extended Huckel or Iterative Extended Huckel methods cannot be used for this purpose with any degree of confidence.

Semiempirical LCAO calculations of electronic and dynamical properties of α and γ nitrogen crystals and nitrogen aggregates

Semi-empirical LCAO methods (extended Huckel, iterative extended Huckel and INDO) are examined as a possible simple starting point for constructing solid state interaction potentials for non-bonded interactions from molecular wave functions. These methods are applied to calculate the cohesion energy and lattice equilibrium unit cell dimensions for α and γ nitrogen crystals, the lattice optical mode frequencies at = 0 of α - N2, the electronic Davydov splitting of the molecular 1Πg state in α - N2 and the stability of molecular-ion aggregates. Reasonable agreement is obtained with extended Huckel and iterative extended Huckel methods, while INDO usually yields poor results. The possibility of excimer formation in nitrogen is discussed in light of these calculations. It is concluded that semiquantitative results can be obtained with these methods when they include the gross features essential for description of crystal non-bonded interactions, such as full overlap interactions, approximate magnitude of mole...

Iterative extended Huckel calculation on hexagonal boron nitride

Some properties of two-dimensional boron nitride are treated in a truncated crystal approach, employing the iterative extended Huckel method in a charge self-consistent calculation. The values of the band width, band gap, charges, equilibrium distance, and sublimation energy are in satisfactory agreement with experiment.

Energy-conformation studies of hydrogen cyanide tetramer: A prebiotic precursor

Using the Iterative Extended Huckel Method (IEHT), we have made energy-conformation studies of the two isomers of hydrogen cyanide tetramer, a known purine precursor in prebiotic syntheses. Our results show that the cis isomer is more stable than thetrans and that, for both, a totally planar conformation is most favorable, differing appreciably from the crystal structure determined by X-ray analysis. The ionization potential, dipole moment,π electron distribution and optical spectra for the tetramer have also been calculated. For all of these properties we find good agreement with experimental results Thus encouraged by the reliability of the description obtained, we plan to use these results for further studies of the chemical reactivity of the tetramer relevant to its postulated role in prebiotic syntheses.

The electronic structure of the B2H 5 + ion

The B2H5+ ion has been studied using theab initio SCF method employing a basis set of Gaussian orbitals and various semi-empirical methods. The most stable structure appears to be a planar structure with two vacantp orbitals rather than a triply bridged structure suggested by analogy with acetylene with which it is isoelectronic. The semi-empirical results reinforce earlier conclusions that inclusion of charges from neighbouring atoms in iterative Extended Huckel methods is necessary to satisfactorily treat ionic molecules.

Self‐consistent pseudo‐eigenvalue equations in parametric charge‐dependent MO methods

Self-consistent pseudo-eigenvalue equations are explicitly derived for the iterative extended Huckel method with use of the minimum-energy criterion. It is shown that inclusion of the overlap matrix in the iterative procedure gives equations that are too complicated for the simple model under consideration. An expression for the total energy is derived, and it is shown that if the eigenvalues of the pseudo-eigenvalue equations are interpreted as ionization potentials and electron affinities, then the empirical parameters of the Hamiltonian matrix must be reinterpreted. The connection with the Hartree–Fock equations is mentioned. It is proposed that minimizing the energy in the VESCF method should be done in a manner similar to the one used in the iterative Huckel methods. Numerical applications to benzene and pyridine have been made.

Charge influence in semi-empirical calculations of heteroatomic ionic and neutral molecules

Iterative extended Hukel calculations for all valence electrons and iterative PPP calculations in the variable electronegativity formalism for π-electrons were performed on benzene, pyridine, fluorobenzene, and the pyrylium ion. The charge distributions for all compounds were found more uniform and plausible with the iteration procedures than without. Polarization effects from theσ-electrons were found to be of importance for the π-electrons. The lone-pair picture of the highest occupied MO in pyridine is preserved in the iterative extended Huckel method, and twoσ lone-pairs were obtained on the fluorine atom of fluorobenzene. The results indicate that this atom is not hybridized.

The Iterative Extended H�ckel Method (IEHM)

The iterative extended Huckel method for σ-electrons, analogous to the ω-technique for π-electrons, is used to calculate wave functions for BH4−, NH4+, CH4, C2H6, B2H6, C2H4 and C6H6. Results are compared with the simple extended Huckel method and with published selfconsistent field molecular orbital wave functions. It is found that iterating the extended Huckel method does not always lead to improvements in the wave function but that some improvements are found particular for ionic molecules. It is suggested that the reason for the failure of the iterated method is the neglect of terms due to charges on adjacent atoms.

Valence-shell calculations on polyatomic molecules

Calculations using the CNDO/2, the Extended Huckel (EH) method, and an iterative Extended Huckel (IEH) method are reported for HF, H2O, NH3, CO, H2CO, HCONH2, HCOOH, HCOF and sydnone. For the IEH method, it is shown that if the dipole moment is calculated by including the atomic dipole moment and the overlap moment (homopolar dipole) as well as the term from the Mulliken populations, then, except for carbon monoxide, the IEH method gives results in good agreement with experiment. The non-iterative EH method predicts dipole moments that are much too high. For molecules with dipole moments smaller than 3 Debyes, the IEH and CNDO/2 methods give similar results, but for molecules with higher dipole moments (formamide and sydnone), the CNDO/2 method gives better agreement with experiment. Comparison of the calculations on sydnone with those on other carbonyl compounds suggests that sydnone is best represented as a resonance stabilized azo-methine imine rather than as a “meso-ionic” or betaine type compound.

Modified huckel calculation of dipole moments in nitro- and nitrochloro-alkanes

An iterative extended Huckel method is used to calculate dipole moments in some vitro-alkanes and their chloroderivatives.