Studies of delocalized electron bonding

Valence Bond calculations are made using the methane-type models, CH ₂, CH ₃, and CH ₄. The calculated proton spin-spin coupling constants are found to depend on the complexity of the model. These coupling constants are related to the exchange order and bond order, which are useful as explicit measures of the extent of delocalized electron bonding.

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Zusammenfassung An den Modellen vom Methantyp: CH ₂, CH ₃, und CH ₄ werden Valenzstrukturrechnungen ausgeführt. Es ergibt sich, daß die errechneten Spin-Spin-Kopplungskonstanten der Protonen vom Umfang des Modells abhängen. Diese Kopplungskonstanten werden mit Austausch- und Bindungsordnung verknüpft, die als explizites Maß für die delokalisierte Elektronenbindung von Nutzen sind.

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Résumé Des calculs sur les modèles du type méthane: CH ₂, CH ₃, et CH ₄ sont faits dans le cadre de la méthode de la mésomérie. On trouve que les constantes de couplage spin-spin protonique dépendent de la complexité du modèle. Ces constantes de couplage sont liées aux indices d'échange et de liaison, quantités qui peuvent servir de mesures explicites de la délocalisation de la liaison électronique.

     

Studies of delocalized electron bonding

A calculation of the ethane barrier is made using a six-electron model in which all single exchange interactions are included. A barrier of 0.6 Kcal. per mole favoring the staggered configuration is calculated. The wave function is used to calculate the NMR proton spin-spin coupling constants, which are found to be at variance with the experimental values. The factors which influence these calculated results are discussed.

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Zusammenfassung Die Potentialschwelle für die innere Rotation des Äthans wird mit einem Sechselektronen-modell unter Einschluß aller Austauschintegrale berechnet. Man erhält eine Schwelle von 0,6 kcal/Mol zugunsten der trans-Konfiguration. Die mit der Eigenfunktion berechneten magnetischen Spin-Spin-Kopplungskonstanten der Protonen weichen von den experimentellen Werten ab. Die die berechneten Werte beeinflussenden Faktoren werden diskutiert.

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Résumé La barrière de potentiel de la rotation intramoléculaire de léthane est calculée à l'aide d'un modèle à 6 électrons et tenant compte de toutes les interactions d'échange. On la calcule à 0,6 kcal/mole en faveur de la configuration trans. Avec la fonction d'onde sont calculées les constantes de couplage magnétique des spins protoniques; elles sont en désaccord avec les valeurs expérimentales. Les facteurs influençant ces résultats sont discutés.

     

On obtaining localized bonding schemes from delocalized molecular-orbital wave functions

A straightforward procedure is proposed for expanding a molecular orbital determinantal wave function into a set of determinantal wave functions composed of atomic orbitals localized at the atoms of a molecule. By employing this method, atomic orbital determinants and their weights can be derived for a molecule from the computed molecular-orbital wave function. The procedure permits the interpretation of a molecular orbital determinantal wave function in terms of bonding schemes related to the classic resonance structures used by organic chemists. By using the unrestricted molecular orbital determinant, bonding schemes and their weights are obtained for butadiene, the butadiene radical cation and the acrylonitrile radical anion. Their dominant bonding schemes are in accord with the relevant resonance structures for these molecules. For the butadiene radical cation and the acrylonitrile anion they are shown to be compatible with the accepted mechanisms of the electrochemical coupling reactions of butadiene and acrylonitrile. Received: 7 August 1996 / Accepted: 18 March 1997     

Localized versus delocalized bonding in the K5Bi4 metallic salt

The electronic structure of one member of the A(5)Pn(4) family of metallic salts, K(5)Bi(4), has been studied by means of first-principles density functional calculations. It is shown that the alkali and pnictide orbitals mix considerably providing for a large covalent bonding contribution to the stability of the compound. It is suggested that solids of the A(5)Pn(4) family of compounds (A = K, Rb, Cs; Pn = As, Sb, Bi) should have better conductivity in the direction perpendicular to the plane of the tetrameric pnictide units and that the low-temperature transition in the antimonides is an order-disorder transition associated with one of the alkali positions.     

Localized and delocalized chemical bonding in the compounds CaNiGe2, SrNiGe2, and SrNiSn2

The new compounds CaNiGe2, SrNiGe2 and SrNiSn2 have been synthesized from the elements by arc melting techniques with subsequent annealing of the sample at 1270 K, and their structures have been determined by single-crystal X-ray diffraction methods. They crystallize in the CeNiSi2 structure (space group Cmcm). For CaNiGe2: a = 4.2213(7) A, b = 17.375(4) A, c = 4.0514(7) A, R(1) = 0.033 (all data); for SrNiGe2: a = 4.429(1) A, b = 17.420(4) A, c = 4.200(1) A, R(1) = 0.041 (all data); and for SrNiSn2: a = 4.5924(7) A, b = 18.710(3) A, c = 4.5228(6) A, R(1) = 0.021 (all data). The main structural motifs are two-dimensionally condensed Ni-centered Ge5 or Sn5 square pyramids. The crystal chemistry and chemical bonding are discussed. Analyses of the electronic structures of CaNiGe2, SrNiGe2, and SrNiSn2, with the help of the electron localization function (ELF), indicate the coexistence of localized covalent and delocalized bonding between the metal atoms involved.     

Magic electron counts and bonding in tubular boranes

Ring stacking in some closo-borane dianions and the hypothetical capped borane nanotubes, predicted to be stable earlier, is analyzed in a perturbation theoretic way. A "staggered" building up of rings to form nanotubes is explored for four- and five-membered B(n)H(n) rings. Arguments are given for the stacking of B(5)H(5) rings being energetically more favorable than the stacking of B(4)H(4) rings. Elongated B[bond]B distances in the central rings are predicted for some nanotubes, and the necessity to optimize ring-cap bonding is found to be responsible for this elongation. This effect reaches a maximum in B(17)H(17)(2-); the insertion of additional rings will reduce this elongation. These closo-borane nanotubes obey Wade's n + 1 rule, but the traditional explanation based on a partitioning into radial/tangential molecular orbitals is wanting.     

Noninnocence of the ligand atoms in iron‐porphine: Chemical consequences of the delocalized electron spin

It is well recognized that the electronic spin density in transition metal complexes in high‐spin states, tends to delocalize from the metal ion itself to the donor atoms of the ligand. In square planar iron‐porphine [PFe]⁺ the delocalization occurs even further and spin corresponding to roughly one electron is delocalized over a large part of the ligand. In this article, density functional theory is applied to explore the chemical consequences of the delocalized spin in four‐coordinate iron‐porphine. It is shown that the porphine ligand has a moderate affinity for radicals, and that covalent bonds can form through spin‐pairing of the unpaired delocalized electron on the porphine ligand and the unpaired electron of another radical species. The hydrogen atom is used as a probe to evaluate the radical affinity of the different nitrogen and carbon atoms that constitute the porphine ligand. It is computationally predicted that the porphine ligand of four‐coordinate iron‐porphine is kinetically capable of activating weak C? H bonds of, for example, unsaturated organic compounds. Hydrogen atom transfer becomes spontaneous via subsequent homo‐coupling of the organic radical created. Whether or not the radical affinity of the porphine ligand has any mechanistic implications for heme‐containing enzymes is left as an open question.     

Studies on water bonding in potassium paradodecatungstate

Investigations of crystalline potassium paratungstate B by means of IR and NMR spectroscopy indicate that 2 water molecules and 2 OH groups are coordinated in the anion and the formula of the polytungstate is: K₁₀[W₁₂/OH/₂/H₂O/₂O₄₀]·6H₂O.     

Chemical bonding effect in electron scattering by gaseous molecules

The experimental intensity of 30 keV electron small angle scattering by a gaseous molecule is much different from the calculation using usual independent atom model. This is due to the rearrangement of electron distribution in a molecule by the formation of chemical bonds, and is called chemical bonding effect (CBE). The molecules studied are mainly hydrocarbons such as methane, acetylene, ethane, etc. and some non-hydrocarbons. The measurement was carried out on both elastic and total scattering and the effect was found for not only elastic but also inelastic scattering. The effect is relatively large for hydrogen rich molecules as H₂O, NH₃ and hydrocarbons, but is essentially related to the number of atoms contained in molecules. The origin of CBE will attribute mainly to the concentration of inner atomic electrons resulting from chemical bonding.     

Constrained optimization in delocalized internal coordinates

Using the recently introduced delocalized internal coordinates, in conjunction with the classical method of Lagrange multipliers, an algorithm for constrained optimization is presented in which the desired constraints do not have to be satisfied in the starting geometry. The method used is related to a previous algorithm by the same author for constrained optimization in Cartesian coordinates [J. Comput. Chem., 13 , 240 (1992)], but is simpler and far more efficient. Any internal (distance or angle/torsion) constraint can be imposed between any atoms in the system whether or not the atoms involved are formally bonded. Imposed constraints can be satisfied exactly. © 1997 John Wiley & Sons, Inc. J Comput Chem 18 :1079–1095, 1997     

A mixed-valence compound with one unpaired electron delocalized over four molybdenum atoms in a cyclic tetranuclear ion

The first oxidation of a species derived from a compound having two linked, quadruply-bonded Mo2 units has been performed and [[cis-Mo2(DAniF)2]2(mu-Cl)4]PF6, 2, has been isolated and characterized in many ways; it has one unpaired electron and a fully delocalized structure where the Mo-Mo distances increase from 2.1191(4) A in the reduced species to 2.1453(3) A in 2 and the Kc of 1.3 x 10(9) is three orders of magnitude larger than that of the Creutz-Taube ion.     

Outer sphere perturbation of delocalized mixed-valence complexes

The complexes [[Ru(ttp)(bpy)](2)(micro-adpc)][PF(6)](2) and [[Ru(ttp)(bpy)](2)(micro-dicyd)][PF(6)](2), where ttp is 4-toluene-2,2':6',2' '-terpyridine, bpy is 2,2'-bipyridine, adpc(2)(-) is azodi(phenylcyanamide), and dicyd(2)(-) is 1,4-dicyanamidebenzene, were prepared and characterized by IR and NIR, vis spectroelectrochemistry, and cyclic voltammetry. The crystal structure of the complex, [[Ru(ttp)(bpy)](2)(micro-adpc)][PF(6)](2).6DMF, revealed a planar bridging adpc(2)(-) ligand with the cyanamide groups adopting an anti configuration. IR and comproportionation data are consistent with delocalized mixed-valence complexes, and a spectroscopic analysis assuming C(2)(h) microsymmetry leads to a prediction of multiple MMCT transitions with the lowest energy transition equal to the resonance exchange integral for the mixing of ruthenium donor and acceptor orbitals with a bridging ligand orbital (the preferred superexchange pathway). The solvent dependence of the MMCT band energy that is seen for [[Ru(ttp)(bpy)](2)(micro-adpc)](3+) is due to a ground state weakening of metal-metal coupling because of solvent donor interactions with the acceptor azo group of the bridging ligand.     

硼烷分子中化学键性质的研究III. 锥型分子B3H6X的结构与成键特征

用MP2/6-31G方法, 对三角锥型分子B3H6X(X=B^2^-, C^-,N, O^+, BH^-, CH和NH^+)及其碎片B3H6和X的结构进行了abinitio研究。结果表明, 当X=NH^+, O^+和N时, B3H6基环上的端氢(Ht)朝着帽基X方向, 而当X=CH, BH^-, B^2^-和C^-时, Ht却转向帽基X的方向。这种特征可用配位原子的电负性和配位原子轨道的弥散性给以说明。我们还进一步研究了B3H6X系列化合物的结合能和稳定性。     

Characterization of bonding modes in metal complexes through electron density cross-sections

Qualitative inspection of molecular orbitals (MOs) remains one of the most popular analysis tools used to describe the electronic structure and bonding properties of transition metal complexes. In symmetric coordination complexes, the use of group theory and the symmetry-adapted linear combination (SALC) of fragment orbitals allows for a very accurate and informative interpretation of MOs, but the same procedure cannot be performed for asymmetric complexes, such as Schrock and Fischer carbenes. In this work, we present a straight-forward approach for classifying and quantifying MO contributions to a particular metal–ligand interaction. Our approach utilizes the topology of MO density contributions to a cross-section of an inter-nuclear region, and is computationally inexpensive and applicable to symmetric and asymmetric complexes alike. We also apply the same approach with similar decompositions using Natural Bond Orbitals (NBO) and the recently developed Fragment, Atomic, Localized, Delocalized and Interatomic (FALDI) density decomposition scheme. In particular, FALDI analysis provides additional insights regarding the multi-centric nature of metal-carbene bonds without resorting to expensive multi-reference calculations.     

Localized versus delocalized excitations just above the 3d threshold in krypton clusters studied by Auger electron spectroscopy

We present Auger spectroscopy studies of large krypton clusters excited by soft x-ray photons with energies on and just above the 3d(52) ionization threshold. The deexcitation spectra contain new features as compared to the spectra measured both below and far above threshold. Possible origins of these extra features, which stay at constant kinetic energies, are discussed: (1) normal Auger process with a postcollision interaction induced energy shift, (2) recapture of photoelectrons into high Rydberg orbitals after Auger decay, and (3) excitation into the conduction band (or "internal" ionization) followed by Auger decay. The first two schemes are ruled out, hence internal ionization remains the most probable explanation.     

Intervalence near-IR spectra of delocalized dinitroaromatic radical anions

The Class III (delocalized) intervalence radical anions of 1,4-dinitrobenzene, 2,6-dinitronaphthalene, 2,6-dinitroanthracene, 9,9-dimethyl-2,7-dinitrofluorene, 4,4'-dinitrobiphenyl, and 1,5-dinitronaphthalene show charge-transfer bands in their near-IR spectra. The dinitroaromatic radical anions have comparable but slightly larger electronic interactions (H(ab) values) through the same aromatic bridges as do the corresponding dianisylamino-substituted radical cations. H(ab) values range from 5410 cm(-)(1) (1,4- dinitrobenzene) to 3400 cm(-)(1) (9,9-dimethyl-2,7-dinitrofluorene), decreasing as the number of bonds between the nitro groups increases, except for the 1,5-dinitronaphthalene radical-anion, which has a coupling similar to that of 9,9-dimethyl-2,7-dinitrofluorene. All charge-transfer bands show vibrational fine structure. The vertical excitation energies (lambda(v)) were estimated from the vibrational components, obtained by simulation of the entire band. The large 2H(ab)/lambda(v) values confirm these radicals to be Class III delocalized mixed-valence species. Analysis using Cave and Newton's generalized Mulliken-Hush theory relating the transition dipole moment to the distance on the diabatic surfaces suggests that the electron-transfer distance on the diabatic surfaces, d(ab), is only 26-40% of the nitrogen-to-nitrogen distance, which implies that something may be wrong with our analysis.     

"Almost delocalized" intervalence compounds

Interest is high in +M-bridge-M systems that have very low barriers to intramolecular electron transfer giving M-bridge-M+ ("almost delocalized" systems). Hush showed how to evaluate the electronic coupling across the bridge (H) from the M-to-M charge-transfer optical absorption band, but did not point out that his classical model causes the extinction coefficient to suddenly drop to zero at a photon energy of 2 H. Ignoring this band cutoff leads to a low estimation of H.