Nonbonding Electrons Research Articles

Ligand-Accelerated Catalysis of the Ullmann Condensation: Application to Hole Conducting Triarylamines

Ligand-Accelerated Catalysis of the Ullmann Condensation: Application to Hole Conducting Triarylamines

Electronic Structure of the Electron-Poor Dinuclear Organometallic Compounds [(CpM)(CpM‘)]μ-Cot (M, M‘ = V, Cr, Fe, Co)

A systematic theoretical investigation of a variety of synfacial homo- and heterodinuclear organometallic compounds [(CpM)(CpM‘)]μ-Cot (Cot = cyclooctatetraene; Cp = cyclopentadienyl) is presented. These compounds show, depending on the metals M and M‘, a number of remarkable and even surprising properties, concerning the metal−metal distances, the magnetic behavior and the spin density distribution. Based on the results of CASSCF calculations, a so-called twinnocene model has been developed, which considers the dinuclear systems as being composed of two separated metallocene-like subunits. The formally nonbonding metal 3d electrons of the subunits interact or do not interact, respectively, in a very specific and characteristic way, to form the dimetal moieties. The model describes the electronic structure, the bonding properties and the wide range of the experimental findings for the whole class of compounds. The strategy followed in the course of the study can be generally recommended for the investigat...

The β Effect of Silicon and Related Manifestations of σ Conjugation

Stabilization of carbocations through donation of π electrons from adjacent carbon orbitals results in well-known species such as the allyl and benzyl cations. Adjacent nonbonding (n) electrons on heteroatoms also can provide considerable stabilization, as in oxonium ions such as CH2dOCH3 and iminium ions such as CH2dN(CH3)2. Less well known is stabilization of carbocations by σ orbitals. In its most common example this interaction provides stabilization of tertiary over secondary over primary carbocations. According to the valence bond representation of this interaction, a filled σ orbital donates electrons to an empty π orbital, as in 1 and 2 (R ) H).

Electron spectroscopy of resonance-enhanced vibrational excitations of gaseous and solid tetrahydrofuran

We report intramolecular vibrational excitation of tetrahydrofuran (THF) in both the gas and solid phase induced by 1–30 eV electrons. High-resolution electron-energy-loss spectra reveal selective vibrational enhancement of THF for different impact energies, indicating the presence of at least three resonances (i.e., temporary anion states), located near 4, 7.5, and 10 eV, as well as many possibly overlapping higher-lying resonances. We also report electron-energy-loss spectra for different THF film thicknesses. Besides fundamental vibrational excitation, we observe several energy loss peaks due to overtones and combination vibrational modes. The present vibrational losses are assigned in compliance with published optical measurements. The similarities between the solid THF and gaseous cyclopentane spectra suggest that the resonances in both molecules could have a common origin and hence need not be attributed to the presence of the oxygen nonbonding electrons in THF.

Sulfur and iron surface states on fractured pyrite surfaces

Pyrite has a poor {001} cleavage. Unlike most other minerals with a rocksalt-type structure, pyrite typically fractures conchoidally, demonstrating that parting surfaces are not constrained to the {001} crystallographic plane. Cleavage along {001} require rupture of only Fe-S bonds, but pyrite consists of both Fe-S and S-S bonds. Analysis of bond energies indicates that S-S bonds are the weaker bonds and they are likely to be ruptured when pyrite is fractured. With each ruptured S-S bond, two mononuclear species (formally S 1- ) are produced, one bound to one fracture surface and the second to the opposite fracture surface. This monomer is reduced to S 2- (monosulfide) during relaxation through oxidation of surface Fe 2+ ions to Fe 3+ . These surface relaxation processes explain the surface states observed in S(2p) and Fe(2p 3/2 ) X-ray photoelectron spectra (XPS) of pyrite. The S(2p) XPS spectrum is interpreted to include bulk disulfide contributions at 162.6 eV and two surface state contributions at 162.0 and 161.3 eV. The monosulfide (S 2- ) emission is near 161.3 eV, as observed in S(2p) spectra of pyrrhotite, and the 162 eV peak is interpreted to result from the surface-most sulfur atom of surface disulfide ions. The Fe(2p 3/2 ) XPS spectrum includes three contributions, a bulk Fe 2+ emission near 707 eV and emissions from two Fe surface states. One surface state is interpreted to be Fe 2+ surface ions. Their coordination is changed from octahedral before fracture to square pyramidal after fracture. The consequent stabilization of the antibonding Fe d 2 z orbital yields unpaired electrons in the valence band resulting in multiplet peak structure in the Fe(2p 3/2 ) spectrum. Similarly, each surface Fe 3+ ion, having contributed a non-bonding 3d electron to the valence band (bonding orbital), contains unpaired 3d electrons, resulting in multiplet splitting of its Fe(2p 3/2 ) signal. The high-energy tail observed in the Fe(2p 3/2 ) spectrum of pyrite is the product of emissions from both surface states with Fe 2+ multiplet peaks centered near 708 eV and the surface Fe 3+ multiplets spanning the binding energies from 708.75 to about 712 eV.

SiO and GeO bonded interactions as inferred from the bond critical point properties of electron density distributions

The topological properties of the electron den- sity distributions for more than 20 hydroxyacid, geometry optimized molecules with SiO and GeO bonds with 3-, 4-, 6- and 8-coordinate Si and Ge cations were calculated. Electronegativities calculated with the bond critical point (bcp) properties of the distributions indicate, for a given coordination number, that the electronegativity of Ge (1.85) is slightly larger than that of Si (1.80) with the electronegativities of both atoms increasing with de- creasing bond length. With an increase in the electron density, the curvatures and the Laplacian of the electron density at the critical point of each bond increase with de- creasing bond length. The covalent character of the bonds are assessed, using bond critical point properties and elec- tronegativity values calculated from the electron density distributions. A mapping of the (3, u3) critical points of the valence shell concentrations of the oxide anions for bridging SiOSi and GeOGe dimers reveals a location and disposition of localized nonbonding electron pairs that is consistent with the bridging angles observed for sil- icates and germanates. The bcp properties of electron den- sity distributions of the SiO bonds calculated for represen- tative molecular models of the coesite structure agree with average values obtained in X-ray diffraction studies of coesite and danburite to within5%.

Synthesis and Characterization of Organotellurium(IV) Chlorosulphates

A variety of diorganotellurium(IV) chlorosulphate R2Te(SO3Cl)2 [R=C6H5, 4-CH3C6H4, 4-CH3OC6H4 4C2H5OC6H4, (4-C2H5OC6H4) C6H5] have been prepared by the metathetical reaction of organotellurium diacetates with HSO3Cl. The UV spectra in MeCN exhibit bands characteristic of excitations from a nonbonding to an antibonding σ molecular orbitals [η(5px)-σ*(4a1,3b2)]. The proton resonances observed in 1H NMR spectra of these compounds in d6-DMSO are considerably deshielded from that observed for the corresponding diacetates indicating the drainage of the non-bonding electrons at tellurium towards SO3Cl− group.

AM1 studies of photoelectron spectra

Five ionization potentials for the sp,ac and sp,sp conformers of (acetylthiomethyl)trifluorosilane MeC(O)SCH~SiF3 and six ionization potentials for the same conformers of (beazoylthiomethyl)trifluorosilane PhC(O)CH2SiF3 were calculated by the semiempirical AM1 method. The resulting values are in good agreement with the data of photoelectron spectroscopy only for the sp,ac conformers. The structure of the preferred conformer of (acetylthiomethyl)trifluorosilane was confirmed by measurement of its dipole moment in the gas phase. The influence of the long-range inductive effect (field effect) on the energy of nonbonding electrons of the carbonyl oxygen atom in the series of acetic acid derivatives was observed.

Physicochemical, Molecular-Orbital and Electronic Properties of Acephate and Methamidophos

Methamidophos (Me) and its N-acetylated derivative, acephate (Ac), are water soluble insecticides that have similar insecticidal potency, but different mammalian toxicity. Me is a potent inhibitor, while Ac is a poor inhibitor of mammalian AChE (mAChE). At physiological pH, both insecticides exhibit similar accumulation in RBC, while Ac exhibits greater binding to plasma proteins than Me. These differential effects of Ac and Me are attributed to the differences in their physicochemical, molecular-orbital and electronic properties. Ac and Me are freely soluble in aqueous solution, moderately soluble in ethyl-acetate (EtAct) and insoluble in n-hexane. The solubility of these insecticides in aqueous solution and the partitioning of these insecticides from aqueous solution into EtAct are independent of the pH of the aqueous solution. At pH 8, Me did not react with o-phthalaldehyde (a NH 2 selective dye), but γ-amino-butyric acid (pK a 10) did. Thus, despite the presence of an amino group, Ac and Me do not exhibit pH dependent solubility in aqueous and in organic solvents. Ac has two O atoms with non-bonding electrons (PO δ − and CO δ −) where PO and CO point in opposite directions. Me has only one O atom with non-bonding electrons (PO δ −). However, because of charge translocation, the CO group of Ac exists as CO − and the PNH 3 + group of Me exists as PNH 2 + at a pH lower than their pK a. The PN bond of Me, but not of Ac, is hydrolyzed at pH 2. Thus, the presence of an electron rich domain stabilizes Ac's PN bond. The CH 3SP bond of both insecticides is similarly hydrolyzed at pH 11. This indicates that the two compounds are considerably similar except that Ac has an additional electron rich domain. At physiological pH, therefore, the functional differences between these insecticides may be due to the differences in their electronic structure. We propose that, similar to a previous model for cationic inhibitors of AChE [13], the PO δ − group of Me forms hydrogen bonds within the oxyanion-hole causing the leaving group (SCH 3) to orient towards the “gorge” opening. This orientation allows the P atom of Me to interact with Ser 200, resulting in the phosphorylation of the enzyme. For acephate, either PO or CO, but not both, interact within the oxyanion-hole. This destabilizes the binding of Ac to the active center, resulting in reduced AChE phosphorylation.

Evidence concerning the relative nucleophilicities of non-bonding and π-bonding electrons in furan from the rotational spectrum of furan···ClF

The rotational spectra of the three isotopomers C4H4O···35ClF, C4H4O···37ClF and C4D4O···35ClF of a complex formed between furan and chlorine monofluoride have been observed by means of a pulsed-nozzle, Fourier-transform microwave spectrometer fitted with a fast-mixing nozzle to preclude any chemical reaction among the component gases. Transitions allowed by all three components µa, µb and µc of the electric dipole moment were observed, those associated with µc exhibiting a small doubling (ca. 20 kHz). Spectral analysis yielded rotational constants, centrifugal distortion constants and (for the isotopomers C4H4O···35ClF and C4H4O···37ClF) all five independent components of the Cl nuclear quadrupole coupling tensor χαβ (α, β = a, b, c). Diagonalisation of the tensor χαβ led to the values of the components χii (i = x, y, z) in the ClF principal axis system and the direction cosines θαz (α = a, b, c) relating the ClF axis (z) to the principal inertial axis system. The geometry obtained by fitting the principal moments of inertia of the three observed isotopomers is consistent with the θαz values and has the ClF subunit approximately perpendicular to one of the C(2)–C(3) bonds of furan. The end δ+Cl of ClF thus appears to interact with the π electron density between these two atoms. The distance of the Cl atom from the point of intersection of the extrapolated ClF axis with the ring is r(···Cl) = 2.726 A. The doubling of the c-type transitions is interpreted in terms of a motion in which the interaction switches from C(2)–C(3) to C(2′)–C(3′). The contrast between the angular geometries of furan···HCl and furan···ClF is discussed in the context of the relative nucleophilicities of the n-pair and the aromatic π-electrons of furan.

Use of Mössbauer Spectroscopy for the Characterization of Order/Disorder Phenomena in Tin(II) Containing Ionic Conductors and for Making Predictions Regarding Possible Electron Contribution to the Conduction

ABSTRACTMössbauer spectroscopy has been seldom used for the characterization of ionic conductors. However, since the introduction of divalent tin in MF2 fluorites (M = Sr, Pb and Ba) to form MSnF4, PbSn4F10 or the Pb1−xSnxF2 solid solution, all of which have structures closely related to the fluorite type, results in an enhancement of the fluoride ion conductivity by up to three orders of magnitude, and since 119 Sn is the second best Mössbauer nuclide, it seems that the Mössbauer technique could provide useful information about how tin(II) modifies the fluorite structure and leads to such a tremendous enhancement of the fluoride ion mobility. The MF2/SnF2 systems contain some number of materials that show order/disorder phenomena (between M and Sn, and also between different fluorine atoms) which make it difficult to understand them from diffraction data only. Mössbauer spectroscopy has been invaluable in helping understand the local structure at tin. By probing the valence electronic structure of tin, we can also make predictions on the possible long range mobility of the tin(II) non-bonded electron pair, which would make the material an electronic conductor or a mixed conductor.

Rotational spectrum of thiophene···HCl Does thiophene act as an aromatic π-type electron donor or an n-type electron donor in hydrogen-bond formation?

The rotational spectra of the four isotopomers [32S]-thiophene···H35Cl, [32S]-thiophene···H37Cl, [32S]-thiophene···D35Cl and [34S]-thiophene···H35Cl of a complex formed between thiophene and hydrogen chloride have been observed by using a pulsed-nozzle, Fourier-transform microwave spectrometer. Rotational constants, centrifugal distortion constants and Cl-nuclear quadrupole coupling constants χaa, χbb-χcc and χab were determined. Interpretation of the spectroscopic constants led to the conclusion that the observed complex has Cs symmetry, with the Cl atom of HCl lying almost directly above the centre of mass of the thiophene ring but with the H atom of HCl pointing at the π-electron density near to the S atom. The S···H–Cl nuclei are almost collinear [ϑ=0.9(6)°] but the relatively large distance r(S···H)=2.7474(29) A indicates that the S···H interaction is weak. The angle φ between the C2 axis of thiophene and the S···H internuclear line was found to be 64.53(16)°. The distance r(*···Cl)=3.693 A from the centre of mass (*) of the thiophene ring to Cl and the angle (S*Cl)=98.9° are very similar in magnitude to the corresponding quantities in thiophene···Ar. Indeed, a comparison of thiophene···Ar, thiophene···HCl, benzene···Ar and benzene···HCl revealed a strong family relationship between the geometries of these four complexes. It is concluded that the non-bonding electron pair carried by S in thiophene is so weakly nucleophilic that when thiophene forms a hydrogen bond with HCl it does so via the aromatic π-electron system. In this respect, thiophene resembles benzene and is in stark contrast to its oxygen analogue, furan, with which HCl forms a hydrogen-bonded complex of C2v symmetry via the non-bonding electron pair on O.

Synthesis and Characterization of Organotellurium(IV) Chlorosulphates

A variety of diorganotellurium(IV) chlorosulphate R2Te(SO3Cl)2 [R=C6H5, 4-CH3C6H4, 4-CH3OC6H4 4C2H5OC6H4, (4-C2H5OC6H4) C6H5] have been prepared by the metathetical reaction of organotellurium diacetates with HSO3Cl. The UV spectra in MeCN exhibit bands characteristic of excitations from a nonbonding to an antibonding σ molecular orbitals [η(5px)-σ*(4a1,3b2)]. The proton resonances observed in 1H NMR spectra of these compounds in d6-DMSO are considerably deshielded from that observed for the corresponding diacetates indicating the drainage of the non-bonding electrons at tellurium towards SO3Cl− group.

Kinetics of Acyclic Diene Metathesis (ADMET) Polymerization. Influence of the Negative Neighboring Group Effect

The influence of the negative neighboring group has been quantified kinetically using hydrocarbon monomers containing oxygen and sulfur. Rate constants were determined for the ADMET polymerization of bis(4-pentenyl) ether (3), bis(5-hexenyl) ether (4), bis(4-pentenyl) sulfide (5), and bis(5-hexenyl) sulfide (6) using both Schrock's catalyst Mo(CHCMe2Ph)(N-2,6-C6H3-i-Pr2)(OCMe(CF3)2)2 [Mo] and Grubbs' phenyl version of the ruthenium metathesis catalyst RuCl2(CHPh)(PCy3)2 [Ru]. Both catalysts experience the effect of functional group presence. Further, a pure hydrocarbon version of the negative neighboring group effect has also been demonstrated using the monomer 1,5-hexadiene. In this case, [Mo] catalysis produces linear 1,4-polybutadiene in a reaction that exhibits a typical ADMET rate constant and activation energy. On the other hand, [Ru] catalysis leads principally to the formation of cyclic compounds rather than linear polymers. This change in mechanism for [Ru] catalysis can be attributed to multiple factors including the dissociation of phosphine ligands, thermodynamic parameters, and π complexation with the adjacent olefin in the monomer, a phenomenon similar to heteroatom nonbonded electron complexation.

Ferromagnetic Spin Alignment in Three-Dimensional Hydrocarbons

A new three-dimensional (3D) hydrocarbon is proposed as a model of organic ferromagnets. This compound is the 3D-carbon-atom network composed of the benzen rings and the bridging carbon atoms with the itinerant π electrons and the nonbonding electrons. This π-electron system is studied within the Kondo-Hubbard model with the interlayer-transfer energy using a mean-field theory and periodic boundary conditions. The total energy, the energy bands, and the spin densities have been computed. The amplitude of spin density at the bridging carbon atom is most largest. As a result, the spins at the bridging carbon atoms align ferromagnetically in the 3D directions. It turns out that the ferromagnetic ground state is stabilized than the nonmagnetic one through the antiferromagnetic correlation, the ferromagnetic exchange interaction and the topological structure of this compound.

Determination of the sign of the quadrupole coupling constant of in silicate glasses by Mössbauer spectroscopy

High-field Mössbauer spectroscopy was performed on a range of tin-containing ternary and binary silicate glass systems. All the glasses unambiguously exhibited a positive value for for the valence state. Determination of the sign of indicates that the electron distribution around each Sn nucleus is similar at all the tin atoms, which is a significant finding for glasses. These results suggest that the electronic state of the is dictated by its non-bonding electron orbital, which has strong character, and which provides the dominant contribution to the electric field gradient present at the nucleus.

Photoinduced anisotropy in vitreousAs2S3:mA reflectance-difference study

Results of the study of photoinduced anisotropy in a model chalcogenide glass ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$ using reflectance-difference spectroscopy are presented. We find that anisotropy can be induced in the energy range much exceeding the energy of the photons of the exciting light and that not only defects but also main covalent bonds of the glass are reoriented by linearly polarized light. We further demonstrate that the sign of the photoinduced anisotropy, especially at higher energies, strongly depends on the photon energy of the exciting light. The observed difference is explained by photoinduced change in the bonds topology involving a conversion between bonding and nonbonding electrons. Strong differences are observed between bulk glass and thin films which are shown to be caused by interference effects.

The effect of interchain coupling on energy bands and ferromagnetic order in quasi-one-dimensional organic polymer ferromagnets

Considering the interchain interaction as an interchain electron transfer, a theoretical model is developed to describe quasi-one-dimensional organic ferromagnets. We also take into account the interaction between the itinerant electrons and the distortion of the lattice, the Hubbard electron - electron repulsion, and the antiferromagnetic correlation between the electrons and the nonbonding electrons at the site radicals. By employing the mean-field approximation, four energy bands which exhibit a three-dimensional anisotropic aspect are obtained. When the interchain coupling increases, the energy gap decreases. In the case of small dimerization order parameter, due to the interchain coupling, the band gap may disappear and the conjugated electrons in the main chain show ferromagnetic order in the ground state. The interchain coupling destabilizes the ferromagnetic ground state of the system.

Diels-Alder reactions of N-silyloxy 1-azadienes

Novel 1-(t-butyldimethylsilyloxy)-1-aza-1,3-butadienes 1 and 2 are prepared by the reaction of O-( t-butyldimethylsilyloxy)hydroxylamine with methyl vinyl ketone and methacrolein, respectively. Azadienes 1 and 2 through sharing of oxygen nonbonding electrons are activated and thus their Diels-Alder reactions with a number of halobenzoquinones, napthoquinones and N-phenylmaleimide regiospecifically give low to good yields of various pyridine heterocycles.

Photochemical Desorption of 4-Vinylbenzoic Acid Adsorbed on Silver Colloid Surfaces

Temporal changes in the surface-enhanced Raman scattering (SERS) of 4-vinylbenzoic acid (4vba) adsorbed on silver colloid surfaces were studied using a method in which the solution of the adsorbate-covered colloid solution flowed through a capillary at differing rates. In this way the photochemical effect of the illuminating laser was studied quantitatively. 4vba appears to exist on the colloid surface in two forms: one that photodesorbs under the influence of the laser and one that does not. An analysis of the SERS spectra suggests that the former adopts a surface geometry in which the benzene ring and carboxylate group are adsorbed flat on surface but where the vinyl group is not bonded directly to the surface. In the latter, all three groups are adsorbed on surface simultaneously: the benzene and vinyl groups are π-bonded, while the carboxylate group is adsorbed through a nonbonding electron pair of one of its oxygens. The photodesorption rate constant was found to be proportional to laser power. Its...