An analysis of the through-bond interaction using the localized molecular orbitals with ab initio calculations—I : Lone-pair orbital interactions in cis- and trans-hydrazines

Ab initio SCF MO calculations using STO-3G basis set were performed on the cis- and trans- hydrazines. The cannonical MOs obtained by these calculations were then transformed into the localized MOs. With the use of the localized MOs thus obtained, the variation in the lone-pair orbital energies of the molecules were pursued in the light of the through-space and/or the through-bond interactions between the specified localized MOs. As a result of this analysis, it was found that ; (a) the effect of the inner shell orbitais, l s electrons of N atoms, is not negligibly small, (b) the effect of the through-bond interaction is not so larger than the through-space interaction, and (c) the large contribution of the through-space interaction is caused from the indirect as well as direct interactions between two lone-pairs.     

ESR spectra of biradicals with weak exchange interaction

ESR spectra are reported for a new class of aliphatic iminoxyl biradicals. The additional hyperfine-structure lines are explained in terms of a weak exchange interaction between the unpaired electrons in the two centers. This occurs during intramolecular motion, which leads to collision between the centers, not as a result of deloalization of the unpaired electrons over the bonds.     

Structure and properties of non-classical polymers. IV. Magnetic properties of polymers with superexchange interaction

It is shown that, for a new class of polymers, ferromagnetic superexchange may arise. The model polymers of the new class have specific electronic structure. In addition to the delocalized system of coupled π electrons, these polymers have singly occupied molecular orbitals localized within each monomer unit. The localized electrons are indirectly exchange coupled, mediated via delocalized orbitals. The resultant exchange interaction is ferromagnetically signed. It depends strongly on the energy gap of the delocalized π-electron system. The suggested model is close to the superexchange of some rare earth magnetics where magnetic f electrons interact indirectly due to delocalized s electron system. The theory of Ruderman, Kittel, Kasuya and Yosida is used in the quantitative treatment of the exchange interaction.     

Pressure-induced cis to trans isomerization of aromatic polyacetylenes prepared using a Rh complex catalyst: A control of π-conjugation length

This study reports that stereospecific polymerization of aromatic acetylenes, e.g., p-methoxyphenylacetylene (pMOPA) and p-ethoxyphenylacetylene (pEOPA)was successfully performed to give polyacetylene selectively bearing cis-transoid forms in high yield when a Rh complex catalyst, [Rh(norbornadiene)Cl]₂ was used in the presence of triethylamine as the polymerization solvent together with a detailed characterization of the resulting polymers, before and after compression. Compression of these polymers induced a cis-trans isomerization at room temperature under vacuum even in the solid state. Based on data collected before and after compression it is estimated that the trans conjugated length, (CC)_n, produced as a result of the compression is n = 26 for PpMOPA and n = 40 for PpEOPA polymers, respectively. We further found that g values in the ESR spectra of the pristine polymer were shifted to higher magnetic field by compression, indicating that unpaired electrons called solitons are stabilized in the trans conjugation length as mobile electrons, although in the pristine polymers the unpaired electrons are stabilized in the less conjugated chain, showing large g value, suggesting a magnetic interaction between oxygen in the alkoxy group of phenyl moiety and unpaired electrons in the cis form. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 217–223, 1998     

Structure,energy spectra and magnetic properties of poly(m-aniline)s

The energy spectra and magnetic properties of a large class of one-dimensional poly(m-aniline)s (PMA) and further model polymers are investigated theoretically. The band structure of those PMA's which are aza-analogs of the alternant non-classical (non-Kekulé) hydrocarbons (polymers) is characterized by a wide gap in which there is a narrow half-filled band (HFB). The different contributions to the effective spin exchange between the unpaired electrons in the HFB: Coulomb (Hund), kinetic and indirect exchange interactions are calculated. While it has been shown earlier that PMA exhibits a net spin exchange of ferromagnetic nature, this approach enables a detailed understanding of the influence of substituents on the nitrogen centers and changes in the aromatic bridging unit. The ferromagnetic nature of the spin coupling in the singly bridged PMA models therebye prevails independent of the structural changes. The HFB width of those PMA's which are derivatives (aza-analogs) of ladder-type non-alternant hydrocarbons is very large and the Wannier functions are delocalized. For this group of polymers the delocalized, non-magnetic state is favored and, therefore, they may be good candidates for testing electrical conduction.     

Multiferroic: A Polymer Composite Based on Organometallic Cobalt Dimers with Dzyaloshinskii–Moriya Effect

A magnetoelectric effect was discovered in polystyrene/organoelemental binuclear cobalt complexes with ligands of spatially hindered phenols at room temperature. The Dzyaloshinskii–Moriya interaction (dipole–dipole and anisotropic exchange interaction of unpaired electrons on cobalt ions at 77 K and room temperature) is fulfilled in such complexes. A novel class of high-temperature multiferroics—polymer composites—can be produced on the basis of such molecular structures.     

Correlation between electron delocalization and structural planarization in small water rings

The discovery of the covalent‐like character of the hydrogen bonding (H‐bonding) system [Science 342 , 611(2013)] has promoted a renewal of our understanding of the electronic and geometric structures of water clusters. In this work, based on density functional theory calculations, we show that the preferential formation of a stable quasiplanar structure of (H₂O)_n(n = 3–6) is closely related to three kinds of delocalized molecular orbitals (MOs; denoted as MO‐I, II, and III) of water rings. These originate from the 2p lone pair electrons of oxygen (O), the 2p bond electrons of O and the 1s electrons of H and the 2s electrons of O and 1s electrons of H, respectively. To maximize the orbital overlaps of the three MOs, geometric planarization is needed. The contribution of the orbital interaction is more than 30% in all the water rings according to our energy decomposition analysis, highlighting the considerable covalent‐like characters of H‐bonds. © 2015 Wiley Periodicals, Inc.     

Synthesis and Magnetic Property of Polyacetylene Bearing πConjugated Bis(Diphenylene)Phenylallyl Radical

Abstract

m- and p Bis(diphenylene)propenylphenylacetylene (m-, p-8) were synthesized and polymerized with WCI₆, MoCl₅, and Rh catalyst, yielding solvent-soluble poly(phenylacetylene)s bearing a π-conjugated bis(di-phenylene)propenyl groups (m-, p-7a). The polymers gave their polyanion derivatives, which were electrolytically and chemically oxidized to yield the corresponding polyradicals (m-, p-7b). The polyradicals were chemically very stable due to the resonance stabilization of an unpaired electron whose spin concentration could be increased up to ca. 2 × 10²³ spins per molar monomer unit. ESR spectroscopy suggested an antiferro-magnetic interaction between unpaired electrons.     

Differential correlation effects in chemisorption cluster model calculations: an FCI study

The interaction of atomic hydrogen with Cu₅ and Ag₅ cluster models simulating the Cu(100) and Ag(100) surfaces has been studied at the full configuration interaction (FCI) level in order to establish the transferability of differential correlation arising from the valence shell. It is shown that pseudopotentials that deal explicitly with one electron or eleven electrons lead to differential correlation effects which agree to within 1–2 mhartree when a localization procedure is used to separate d-shell MOs from the valence ones.     

Combined calculation method of weak exchange interactions in biradicals

A combined calculation method of weak exchange interactions in short biradicals was developed. The combined method includes two stages. The quantum chemical calculations of the high level of the biradical structure and molecular orbitals and binding energies of unpaired electrons are performed at the first stage. Information obtained at the first stage is used further to calculate the exchange interaction between unpaired electrons within the direct exchange model using the asymptotic method. This allows one to estimate the exchange interaction and to determine the dependence of this interaction on the distance between the paramagnetic centers and on their relative orientation. The method developed was used to calculate the exchange interaction in the short nitroxyl biradical containing no conjugate rings between the paramagnetic NO groups. The geometry and electronic structure of the biradical were calculated within the unrestricted DFT method (B3LYP/cc-pvdz) using the ORCA program package.     

Magnetic properties of Cu(II) bischelate complexes with 3-imidazoline nitroxides. 2. Anab initio analysis of exchange interaction mechanisms

The mechanism responsible for the emergence of ferromagnetic exchange interactions in bischelate complexes of Cu²⁺ with enaminoketone derivatives of 3-imidazoline nitroxide CuL₂ is studied by ab initio quantum chemical methods. The parameters J_cu-L and J_L-L’ of exchange interactions between the unpaired electrons of the paramagnetic centers (Cu²⁺ ion and N-O groups of nitroxyl ligands L and L’) of these complexes were calculated in terms of the full 3x3 configuration interaction between the singlet states constructed in a basis set of molecular orbitals of unpaired electrons. It is shown that for variations of the structure of the coordination polyhedron around the Cu²⁺ ion from square planar to tetrahedral the exchange interactions between the unpaired electrons of the paramagnetic centers is ferromagnetic J_Cu-L >J_L -L’>0, which agrees with the data of magnetic measurements. The principal mechanism of exchange interactions in CuL₂ complexes is the delocalization mechanism that is due to a minor transfer of spin density from the 3d-orbitals of Cu²⁺ to the Σ-orbitals of the N-O groups of L and L’ ligands. Translated fromZhurnal Strukturnoi Khimii, Vol. 38, No. 5, pp. 850–856, September-October, 1997.     

Fragment interaction analysis of structural problems in the framework ofAb initio SCF-MO computations

We describe here a procedure which allows to extend the application of the quantitative orbital interaction analysis at theab initio SCF-MO level to any kind of orbital interaction. In this new procedure a localization scheme is applied to the fragment MOs obtained with the procedure suggested by Wolfe et al. (JACS99, 1296 (1977)): in this way also the resulting σ-type fragment localized MOs have correct orbital occupancies and can be used for a PMO analysis. In addition the availability of quantitative expressions for the fragment localized MOs allows also to compute the total energy of the system in the absence of the orbital interactions under examination. For illustrative purposes the procedure is applied to the analysis of the factors which determine the preferential stability oftrans overcis diimide.     

Rational design of high-spin biradicaloids in the isobenzofulvene and isobenzoheptafulvene series

The relative energies of singlet biradicaloid and of triplet and singlet biradical electronic states for a series of benzannelated isobenzofulvenes and isobenzoheptafulvenes were calculated at the (u-)B3LYP/6-31G(d), full π-space CASSCF-CASPT2 (≤14 π-e(-)s), and full π-space RASSCF-RASPT2 (≤24 π-e(-)s) levels of theory. Both absolute and relative CASPT2 energies were reproduced quite well by the RASPT2 approach, which can be extended to much larger active spaces. RASPT2 (and DFT) calculations find that increasing benzannelation leads to triplet ground states in both hydrocarbon series, in violation of the classical principle of maximum bonding. This confirmed the expectations that the combined effects of resonance energy and aromaticity could compensate for the extra formal π-bond of the biradicaloid singlet, and that the strong exchange coupling inherent to the embedded trimethylenemethane (TMM) would manifest itself in the biradicaloids. The relative energy of the biradicaloid singlet rises rapidly upon benzannelation, as π-bonding between the high-energy delocalized GVB orbitals decreases. The underlying π-orbital topology is revealed when this weak π-bonding is artificially eliminated by a 1:1 mixing of the nondegenerate HOMO and LUMO to produce an overcorrelated valence bond (OCVB) orbital pair. For members of both biradicaloid series, the OCVB pairs are nondisjoint, revealing a limiting triplet preference with increasing benzannelation. Within the two-electron, two-orbital approximation, the effects of π-bonding in the singlet biradicaloids and orbital localization away from the acene π-system in the triplet biradicals can be analyzed as perturbations of the singlet OCVB biradicals. The application of a VB-based spin coupling scheme is discussed, in which the unpaired electrons of these species can be considered both ferromagnetically and antiferromagnetically coupled, with the strength of the latter strongly dependent on the acene subunit.     

A study of the molecular orbital localization into an extended Hartree—Fock approach: Application to the BeH2 ground state

In this article, we present a study of the localization and properties of the molecular orbitals (MOs) of polyatomic systems by using a comprehensive version of the G1 model. In this version, the wave function is written as a DODS product of univocally determined spin orbitals (MOs), “projected” on the singlet ground state. A procedure for determining the MOs is given and applied to the BeH₂ ground state. Equivalent split shell and localized MOs are found. The Be orbitals are seen to exhibit sp hybridization and the localized valence MOs are found to produce − 13.7 kcal/mol localization energy. Multistructural calculations are carried out and show that the present approach is able to describe localized and well-oriented bonds whenever the molecule under study presents only a single well-defined nonresonant chemical structure. © 1996 John Wiley & Sons, Inc.     

Transition‐Metal Chemistry of Alkaline‐Earth Elements: The Trisbenzene Complexes M(Bz)3 (M=Sr,Ba)

We report the synthesis and spectroscopic identification of the trisbenzene complexes of strontium and barium M(Bz)₃ (M=Sr, Ba) in low‐temperature Ne matrix. Both complexes are characterized by a D₃ symmetric structure involving three equivalent η⁶‐bound benzene ligands and a closed‐shell singlet electronic ground state. The analysis of the electronic structure shows that the complexes exhibit metal–ligand bonds that are typical for transition metal compounds. The chemical bonds can be explained in terms of weak donation from the π MOs of benzene ligands into the vacant (n?1)d AOs of M and strong backdonation from the occupied (n?1)d AO of M into vacant π* MOs of benzene ligands. The metals in these 20‐electron complexes have 18 effective valence electrons, and, thus, fulfill the 18‐electron rule if only the metal–ligand bonding electrons are counted. The results suggest that the heavier alkaline earth atoms exhibit the full bonding scenario of transition metals.     

Transition‐Metal Chemistry of Alkaline‐Earth Elements: The Trisbenzene Complexes M(Bz)3 (M=Sr,Ba)

We report the synthesis and spectroscopic identification of the trisbenzene complexes of strontium and barium M(Bz)₃ (M=Sr, Ba) in low‐temperature Ne matrix. Both complexes are characterized by a D₃ symmetric structure involving three equivalent η⁶‐bound benzene ligands and a closed‐shell singlet electronic ground state. The analysis of the electronic structure shows that the complexes exhibit metal–ligand bonds that are typical for transition metal compounds. The chemical bonds can be explained in terms of weak donation from the π MOs of benzene ligands into the vacant (n?1)d AOs of M and strong backdonation from the occupied (n?1)d AO of M into vacant π* MOs of benzene ligands. The metals in these 20‐electron complexes have 18 effective valence electrons, and, thus, fulfill the 18‐electron rule if only the metal–ligand bonding electrons are counted. The results suggest that the heavier alkaline earth atoms exhibit the full bonding scenario of transition metals.     

Übergangsmetallkomplexe mit Nitronylnitroxylradikalliganden

The stable free radicals, the isomers of 4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide with the 2-substituentR (R=para-,meta-,ortho-pyridyl), have been prepared and used as ligands in copper(II), palladium(II) and platinum(II) complexes. The magnetic moments and the EPR spectra of the complexes and the free radicals have been investigated. Most of the complexes show a considerable intramolecular interaction between the radicalic groups of the ligands. No intramolecular interaction was found, however, between the transition metal ions and the unpaired electrons of the ligands. But by analysis of the EPR spectra in the solid state there was found in some cases an intermolecular interaction between the metal ion [copper(II)] and the unpaired electrons of the ligands.

     

σ versus π-radical: Tuning the electronic nature of neutral carbon (I) compounds with three non-bonding electrons

The bonding and reactivity of the hypo-coordinated compounds with one, two, and four non-bonding electrons namely, carbon-centered free radical, carbenes, and carbones were well earlier established. Here, we report stability, bonding and reactivity of compounds RCL, where R is one-electron donor group (R =  CH₃ ( a ),  CHO ( b ), and  NO₂ ( c )) and L is two-electron donor ligand (L = cAAC ( 1 ), CO ( 2 ), NHC ( 3 ) and PMe₃ ( 4 )), having three non-bonding electrons. The ground states of molecules exist in a doublet with a lone pair of electrons and an unpaired electron at the central carbon atom (C1). The spin hops over from π- to σ-type orbitals is observed as the π-acceptor strength of the donor ligand increases. The replacement of the methyl group by  CHO and  NO₂ indicate that the cAAC and  CHO substituted compounds gives a σ-radical except in compound 2c . These molecules show very high proton affinity and exothermic reaction energy for the hydrogen atom addition indicating dual reactivity namely, radical and lone pair reactivity.     

Enhanced Zintl anions by carbon doping in Al-Na clusters and new magic structure Al6Na4C

This article investigated the low-energy structures of Al₆Na _mC (m = 2, 4, 6, 8) clusters and their electronic structures by using genetic algorithm combined with density functional theory and configuration interaction methods. The computations show that the C atoms prefer sitting at the center, whereas the Na atoms tend to locate at the outside of the clusters. The valence molecular orbitals (MOs) agree well with the prediction of the jellium model. The stronger attraction of the central carbon to the valence electrons will depress the potential energies locally, which makes the 2S level go obviously lower and the 2P and 1D orbitals form a sub-band. The 26 valence electrons in Al₆Na₄C form closed 1S²1P⁶2S²1D¹⁰2P⁶ shells and correspond to a new magic structure. The MOs and electron localization function show that the sodium cores are exposed at the outside of the valence electrons and form naked cations. The contraction of the valence electrons because of the carbon doping enhances the charges on the Al₆C moieties, and the Na⁺ cores on the peripheries are ionically bonded to the Zintl anions (Al₆C)^q−. The Al₆Na₄C has a tetrahedral structure with symmetry T_d, and it may be used as building blocks to synthesize Zintl solid.     

The “Inverted Bonds” Revisited: Analysis of “In Silico” Models and of [1.1.1]Propellane by Using Orbital Forces

This article dwells on the nature of “inverted bonds”, which refer to the σ interaction between two sp hybrids by their smaller lobes, and their presence in [1.1.1]propellane. Firstly, we study H₃C−C models of C−C bonds with frozen H-C-C angles reproducing the constraints of various degrees of “inversion”. Secondly, the molecular orbital (MO) properties of [1.1.1]propellane and [1.1.1]bicyclopentane are analyzed with the help of orbital forces as a criterion of bonding/antibonding character and as a basis to evaluate bond energies. Triplet and cationic states of [1.1.1]propellane species are also considered to confirm the bonding/antibonding character of MOs in the parent molecule. These approaches show an essentially non-bonding character of the σ central C−C interaction in propellane. Within the MO theory, this bonding is thus only due to π-type MOs (also called “banana” MOs or “bridge” MOs) and its total energy is evaluated to approximately 50 kcal mol⁻¹. In bicyclopentane, despite a strong σ-type repulsion, a weak bonding (15–20 kcal mol⁻¹) exists between both central C−C bonds, also due to π-type interactions, though no bond is present in the Lewis structure. Overall, the so-called “inverted” bond, as resulting from a σ overlap of the two sp hybrids by their smaller lobes, appears highly questionable.