Wiberg Indices Research Articles

The crystal and electron structure of O-(2-chloroethyl)-O-isobutyl-O-(2-phenyl-4-methylthio-3-oxo-2H-pyridazine-5-yl) thiophosphate

The crystal structure of the title fungicide was solved by direct methods, using Σ2 relationship and tangent formula. The structure refinement was made by full-matrix least-squares with anisotropic temperature factors to the final R = 0.096 for 1 486 significant reflexions. The compound crystallizes in the orthorhombic system, the space group Pccn. The lattice parameters are a = 1.8719(9), b = 3.0426(33), c = 0.7616(2) nm and Z = 8. The intermolecular interactions of types C-H···Cl and C-H···O are observed between the centrosymmetrically related molecules in the crystal structure. The coordination polyhedron around phosphorus is between trigonal pyramid and tetrahedron. The planes of the phenyl and pyridazine rings are rotated through 70° each other. The electron structure was calculated by semiempirical CNDO/2 method. The calculations revealed the most negative net charges on S(2) and O(1). The value of Wiberg index Iw = 1.08 and the bond distance 0.149 nm show that the bond N(1)-C(5) is simple and is not involved in the conjugation.

Strength of the metal-ligand bonds in some bicyclopentadienyl and dibenzene compounds of 3d transition elements

1. The mean values of the “internal” metal-ligand bond dissociation energies ¯D1(M-L) and Wiberg indices calculated by the INDO MO method W(M-L) are virtually invariant within the series of dicyclopentadienyl and dibenzene π-complexes of 3d transition metals. 2. The similarity in the behavior of the ¯D1(M-L) values and the W(M-L) values which characterize covalent bonding indicates that the contribution of the ionic component to the metal-ligand bond is either small or approximately identical in each of these series of compounds.

A quantum-chemical study of the electronic structure and absorption spectrum of 1,4-bis-diazocyclohexanetetraone

The authors have performed quantum-chemical calculations on the electronic structure parameters of 1,4-bis-diazocyclohexanetetraone by the SCF MO LCAO method. Two semiempirical forms of this method were used: CNDO/2 and CNDO/C2. The first has been widely used to calculate atomic charges, bond orders, Wiberg indices, and other parameters that are derived from the density matrix. The second enables one to interpret photoelectron spectra and electronic absorption spectra reliably. The molecule is characterized by the point symmetry group D/sub 2h/ and by geometrical parameters corresponding to the x-ray data.

A theoretical approach to the hydration of phosphodiesters in the presence of protons

Hydration of the dimethylphosphate anion (DMP–) and dimethylphosphoric acid (DMPH) and of DMP– in the presence of a proton has been studied by the CNDO/2 molecular-orbital method. The hydration energies of the (DMP–·H2O) and (DMPH·H2O) complexes and the stabilization energies of (DMP–·H+·H2O) systems of different structures have been found, and additionally Wiberg indices and bond energies (together with their three components) have been computed. Regardless of the type of structure considered, for the trihydrate of dimethylphosphate in the presence of a proton only one water molecule in the first hydration shell is characterized by the lowest bond energy, which is about twice as low as energies of the two remaining water molecules. For the trihydrate of DMP– two water molecules at two equivalent positions, E13 and E31, are characterized by the lowest hydrogen-bond energy and are ca. 20 kcal mol–1 lower in energy than the most stable sites in acid structures. The results of theoretical studies support the experimental data obtained in investigations of the hydration of synthetic poly(dialkylphosphate) by the pulsed n.m.r. method.

MO study of molecular nitrogen fixation: V. The role of equatorial ligands on the dinitrogen activation in aqua, sulphane and phosphane complexes of Ti, V, Cr, Mn and Fe

The electronic structures of 105 model complexes of dinitrogen activation was studied using the CNDO UHF method of MO LCAO SCF calculations. The systems under study covered various oxidation q and spin H states of H [M(L e) 4Cl(N 2)] q complexes where the central atom varied throughout the first transition metal row, namely M = Ti, V, Cr, Mn and Fe; in the role of equatorial ligands the L e = H 2O, H 2S and PH 3 were considered. Using the various criteria of dinitrogen activation (the dinitrogen π-acceptor indices, Wiberg indices, bicentric parts of the total molecular energy and the charge transfer onto dinitrogen), the studied complexes were classified into three groups depending on the degree of dinitrogen activation, i.e. systems with non-activated, one-electron activated and two-electron activated dinitrogen.

Quantumchemical interpretation of a typical fragmentation in the mass spectra of benzoquinolizine derivatives

The results of some quantumchemical calculations, performed by the use of the semi-empirical CND0/2 method, are compared with findings of the mass spectrometric investigations. The calculated charge distributions, force values and Wiberg indices, revealed useful informations on the properties and behaviour of the molecule in question and provided support for the suggested fragmentation pathway.

A CNDO/2 study of (μ 2-acetylene)hexacarbonyldicobalt ( Co Co) complexes

The electronic structures of Co 2L(CO) 6 complexes (L = ethyne, propyne and butyne) have been calculated by the CNDO/2 method using Clack's modifications. The nature of the bonding, charge-transfer processes and electronic rearrangement effects during complexation are discussed in terms of MOs and Wiberg indices. The results of the calculations have been correlated with IR and UV-VIS spectroscopic data and conclusions have been drawn on electronic grounds concerning the catalytic carbonylation reaction of acetylenes in the presence of cobalt carbonyls.

Molecular orbital study of coordinated dioxygen VII. Dioxygen activation on cyanate complexes of Mn, Fe, Co, Ni and Cu

Dioxygen complexes of the type κ[M(CN) 5(O 2)] q were investigated by the CNDO/3-UHF method of MO/3-LCAO/3-SCF calculations. Five central atoms M, three oxidation states q, and two spin states κ, yield 30 combinations under study. The dioxygen activation is described in terms of the local electron configuration of the dioxygen π antibonding orbitals. The Wiberg indices, charge distributions and energy partitioning complete this description. The complexes studied were classified into three groups depending on the degree of dioxygen activation. A limited geometry variation for the 2[Co(CN) 5(O 2)] 3− complex was also performed.

Theoretical study of annulenes structure [14], [16] and [18] annulenes

The quantum-chemical methods EHT, CNDO/2, MINDO/2 and the method of empirical potentials have been used for calculation of equilibrium geometry of six isomers of [14] annulene, four isomers of [16], and three isomers of [18] annulenes. The bond strengths and degree of delocalization are characterized by the overlap populations and the Wiberg indices. Relative stabilities have been determined by comparison of enthalpies of formation calculated by the MINDO/2 and empirical potential methods. Influence of internal strain of the molecules on their stability lowering is discussed. The degree of aromaticity has been determined by calculation of several aromaticity indexes.

Molecular orbital study of the dithiocyanate tetraamine nickel(II) complex

The equilibrium geometry and the electronic structure of the high-spin and low-spin forms of Ni(NH 3) 4(NCS) 2 have been studied using the CNDO-UHF type of MO-LCAO-SCF method. The adiabatic potential surface was considered as a function of equatorial and axial interatomic distances in the NiN′ 4N″ 2 chromophore. The equilibrium geometry, taking into account these degrees of freedom, and the harmonic force constants were obtained for this complex as well as for the dissociation limit cases, namely for Ni(NH 3) 4 2+ and Ni(NCS) 2 complexes. The electronic structure is discussed in terms of molecular orbitals, the values of Wiberg indices, charge distributions, spin densities and the energy partitioning.

MO study of molecular nitrogen fixation III. Dinitrogen activation on amine complexes of Ti, V, Cr, Mn and Fe

Abstract The electronic structures of 35 model complexes of dinitrogen fixation were investigated using MO—LCAO—SCF calculations in the CNDO/2 formalism. The systems studied cover various oxidation q and spin κ states of κ [M(NH 3 ) 4 Cl(N 2 )] q , where the central atom varies throughout the first transition metal row, namely M = Ti, V, Cr, Mn and Fe. The π acceptor index (a quantitative criterion of the dinitriogen π acceptor ability) was used to describe the dinitrogen activation. The Wiberg indices, the bicentric portions of the total energy and the charge distributions complete this description. The complexes studied were classified into three groups depending on the degree of dinitrogen activation.

Theoretical study of electron structure and stability of quasiaromatic analogues of azulene

Semiempirical CNDO/2 method was used in the study of 23 quasiaromatic azulene analogues containing a hydrogen bridge A..H-D, where A and D denote O, NH, or S. The values of the stabilization energy, interaction energy, and changes of Wiberg's indexes suggest that the stability of the studied compounds is caused by a strong hydrogen bond as well as by a strong delocalization of the π-electron system. Derivatives in which A and D are O and NH are especially stable. Derivatives containing an oxo or thio group are preferred in tautomeric equilibriums.

Molecular orbital study of coordinated dioxygen III. Electronic factors of dioxygen activation on Mn, Fe, Co, Ni and Cu complexes

The electronic factors of dioxygen activation have been investigated by the semiempirical CNDOUHF type of molecular orbital calculations. Model systems of the type [O 2MCl 4NH 3] a have been considered in various formal oxidation states of the central atom and spin multiplicity where M varies throughout the first transition metal series. A ‘softening’ of the OO bond is registered after coordination of the dioxygen; it was characterized by a decrease of the Wiberg index and an increase of the bicentric part of the total energy.

A molecular orbital study of the electronic structure and stereochemistry of dithiocyanate diamine and dicyanate diamine cupric complexes

CNDO/2—UHF molecular orbital calculations have been performed with the aim of studying the electronic structure and stereochemistry of the complexes Cu(NH 3) 2(NCX) 2 (X = O, S). Orbital energies, atomic charges, Wiberg indices, spin densities, total molecular energies and their partitioning into metal—ligand and ligand—ligand interactions have been calculated for three model structures of each complex. The role of axial interactions in stabilizing the planar arrangement of theCuN 4 chromophore is discussed. The harmonic force constants of some deformation vibrations of coordinated NCX groups have been estimated for the trans isomers and these estimates are compared with the values for the free ligands.

Theoretical study of [10] and [12]annulenes

Equilibrium geometries of various geometrical isomers of annulenes have been calculated by the quantum-chemical methods EHT, CNDO/2, MINDO/2, MINDO/3 and the method of empirical potentials. Automatic method of full optimization has been used for some [10]annulenes and all isomers of [12]annulene. The bond strengths and degree of delocalization have been determined from the bond overlap populations and the Wiberg indices. Relative stabilities of the isomers have been determined by comparison of the formation enthalpies calculated by MINDO/3, MINDO/2 and empirical potential methods. Strain energy has been calculated, and degree of aromaticity of the isomers has been characterized by several aromaticity indices.

CNDO/2 study of the isoelectronic series of complexes dicarbonyl(dinitrogen)- and dicarbonyl-η-benzenechromium, -η-cyclopentadienyl-manganese, -η-cyclobutadieneiron, and -trismethylenemethane-iron obtained by the matrix–isolation technique

The electronic structure of the isoelectronic series of [ML(CO)2(N2)] and [ML(CO)2]complexes [ML = Cr–(η-C6H6), Mn(η-C5H5), Fe(η-C4H4), or Fe{C(CH2)3}] has been investigated within a CNDO/2 formalism, using both experimental and standard geometries. The computed trends for CO and N2 bond strengths in the complexes, as measured by Wiberg indices, charge distributions, and orbital populations, are well correlated with the experimental CO and N2 stretching frequencies and energy-factored stretching force constants, i.e. Cr < Mn < Fe and [ML(CO)2] < [ML(CO)2(N2)] < [ML(CO)3]. The computed total energies of the [ML(CO)2] complexes suggest that in all cases a tilted geometry is preferred, i.e. the plane of the L ligand is not perpendicular to the M(CO)2 plane. New CNDO/2 calculations have been carried out for [Fe{C(CH2)3}(CO)3] which is shown to fit in well with other [ML(CO)3] complexes.

Automatic Geometry Optimization and Vibrational Analysis in External Electric Field: Ethylene

AbstractStationary points of the INDO energy hypersurface for various orientations of ethylene in external electric fields of the strength F=0, 2, 4, 6, 8 and 10 × 1010 V m−1 were found and their characteristics studied by the force constant matrix analysis. Energies, structural parameters, charges, Wiberg indices and dipole moments are presented. The only stable orientation of the ethylene molecule is that for which the CC bond is parallel to the field direction up to F=6 × 1010 V m−1 (orientation (a) in Fig. 1). Above this value the molecule is structurally unstable and it decomposes to the hydride anion and the C2H3+ cation. Rotational instability was found for two perpendicular orientations of the CC bond with respect to the field vector, in which the field vector was parallel and perpendicular to the molecular plane. Pseudorotations with negative eigenvalues of force constant matrices lead to the stable orientation (a). No stationary points were found when the angle between the CC bond and the field vector was between 0 and 90°. The five longest wavelength vibrational bands are presented for selected orientations and field strengths.

Extended CNDO/2 studies of the isoelectronic series AM(CO) 3 (AM  η 6-C 6H 6Cr,η 5-C 5H 5Mn,η 4-C 4H 4Fe,η 3-C 3H 5Co,η 2-C 2H 4Ni) and of (η 6-C 6H 6) 2Cr

The electronic structure of isoelectronic AM(C0) 3 (AM  η 6-C 6H 6Cr,η 5-C 5-H 5Mn,η 4-C 4H 44Fe,η 3-C 3H 5Co,η 2-C 2H 4Ni) and of (η 6-C 6H 6) 2Cr is investigated within an extended CNDO/2 formalism using both experimental and standard geometries. The computed trends for bond strengths and stretching frequencies (as measured by Wiberg indices or bicentric energy terms) and for reactivity (charge distributions) correlate with the experimental ones. For chromium complexes, the extended CNDO/2 results seem more reasonable, for example the charge and the orbital populations on chromium, than are the ab initio single > results. A general agreement is observed between CNDO/2 and SCCC electronic structures.