Nonbonding Electrons Research Articles

Sn- K and Pb- L3 EXAFS, X-Ray Diffraction, and 119Sn Mössbauer Spectroscopic Studies of Ordered β-PbSnF 4 and Disordered Pb 1- xSn xF 2 ( x = 0.3, 0.4) Solid Solutions and PbSn 4F 10: High Performance Fluoride Ion Conductors

Fully and partly disordered superionic conductors in the PbF 2/SnF 2 system have been studied using X-ray powder diffraction, 119Sn Mössbauer spectroscopy, and Pb- L3 and Sn- K EXAFS (extended X-ray absorption fine structure). X-ray diffraction is seriously limited because of the large degree of disorder at the metal site. The solid solution Pb 1- x Sn xF 2 ( x = 0-0.30) is fully disordered as is the stoichiometric compound PbSn 4F 10 ( x = 0.80). On the other hand, some degree of ordering occurs in Pb 0.6Sn 0.4F 2 ( x = 0.40) and β-PbSnF 4 ( x = 0.50). 119 Sn Mössbauer spectra show that the tin(II) lone pair is stereoactive in these materials. Therefore, the tin environment is highly distorted and its nonbonded electron pair is not a charge carrier in the conduction mechanism. The Sn- K and Pb- L 3 EXAFS spectra indicate average first shell distances of R Sn-F = 2.12(1) Å and R Pb-F = 2.55(1) Å. There is a clear second shell Pb-Pb signal in the Pb- L 3 EXAFS but no higher shell signal in the Sn- K EXAFS, indicating a higher degree of disorder and a lower rigidity of next-neighbor fluorine shells around Sn than around Pb. The results are discussed in relation to previous studies of ordered MSnF 4 fluoride ionic conductors and to proposed mechanisms for ionic conduction in these superionic materials.

Ab initio atomic and electronic structure calculations of selenium systems in different dimensions

Ab initio calculations are performed to investigate the atomic and electronic structures of selenium for different dimensions. In particular, the role played by non-bonding electrons in the formation of the condensed phase is clarified. The systems studied are zero-dimensional (0D) dimers and trimers, one-dimensional (1D) helical chains, two-dimensional (2D) plates consisting of trimers, and three-dimensional (3D) trigonal selenium. Stable configurations and electronic structures are calculated for these systems. From the results for the 1D chains and the 3D system, it follows that the interchain interactions are induced and that the intrachain bonds are slightly weakened by interactions between the non-bonding electron pairs. The interactions between trimers in the 2D system is much smaller than the interaction between chains in the 3D system. This difference between the 2D system and the 3D system is due to the characteristic distributions of non-bonding electrons on the 0D trimer and the 1D chain.

The Ziegler—Natta olefin insertion reaction into a metal—alkyl bond for second-row transition metal atoms

The Ziegler—Natta olefin insertion reaction into a metal—alkyl bond has been studied for the second-row transition metal atoms, without additional ligands and with two hydride ligands added. Correlation effects of all valence electrons are included in the calculations using reasonably large basis sets. The results are compared to the corresponding insertion into a metal—hydride bond which has been studied previously. For most metals the barrier height for the olefin insertion into the metal—alkyl bond is about 20 kcal/mol higher than for insertion into the metal—hydride bond. The origin of this difference is the directional character of the methyl bond. The hydride can continuously change the direction of its bond from the metal to the olefin, but for the methyl group a large part of the metal—methyl bond has to be broken before the methyl group can start to bind towards the olefin. For the reaction mechanism, the d character of the bond at the transition state and the importance of repulsion between non-bonding electrons on the metal and the electrons on the olefin is emphasized.

Electronic structure of silicon-oxygen high polymers

The electronic structure of silicon-oxygen high polymers (siloxane and siloxene) have been calculated by the first-principle local-density-functional method. Oxygen's lone-pair (OLP) states play an important role in the electronic structure of these polymers, in accordance with oxygen's spatial position. In siloxane, where oxygen atoms position in the Si skeleton backbone, non-bonding ( n) electrons in the OLP states cut Si's σ-electron delocalization, and widen the band gap. On the contrary, when oxygen atoms are located outside of the Si skeleton (siloxene), the characteristic σ- n mixing occuring at the band-edge states has a potential to reduce the band gap.

Phase and conductivity studies on the systems MIIX2–MVIO3(MII= Sn or Pb; MVI= Mo or W; X = Cl or Br)

The molten systems MIIX2–MVIO3(MII= Sn or Pb; MVI= Mo or W; X = Cl or Br) appear to be homogeneous liquids with up to 40 mol% of MVIO3. These liquids are shown by phase studies and the X-ray diffraction data of the cooled melts to be sols. The frozen products are intensely coloured and show semiconducting properties with evidence for two conduction mechanisms. The Mössbauer spectroscopic data for Sn-containing systems show that there is an increasing loss of electron density at the Sn atoms with increasing oxide content. This work provides further evidence for the ability of ns2 species such as SnII to transfer some of their non-bonding electron pair density to either solid-state or cluster bands.

The carbides Gd3Mn2C6 and Tb3Mn2C6

AbstractCrystals of the title compounds were obtained by arc‐melting cold‐pressed pellets of the elemental components, followed by annealing the reaction products in an argon atmosphere slightly below the melting point. The crystal structures of these isotypic, hexagonal carbides (P63/m, Z=2) were determined from single‐crystal X‐ray data; Gd3Mn2C6: a=815.0(2) pm, c=504.93(9) pm, R=0.012 for 526 structure factors and 18 variable parameters; Tb3Mn2C6: a=810.5(2) pm, c=500.5(2) pm, R=0.025 (225 F′s, 18 variables). The carbon atoms form pairs with C—C bond distances corresponding to double bonds. The three‐dimensional, polyanionic managanese carbon network contains relatively large trigonal‐bipyramidal voids formed by three lanthanoid and two manganese atoms. The rationalization of chemical bonding on the basis of the 18‐electron rule suggests that these voids are filled by nonbonding electrons of the adjacent manganese atoms.

The effect of solvation on molecular Rydberg states: Dioxane clustered with nonpolar solvents

One color 2+1 mass resolved excitation spectroscopy is employed to obtain molecular Rydberg 3s←n transition spectra of 1,4-dioxane clustered in a molecular beam with nine nonpolar solvents. The solvents are Ar, Kr, CH4, CD4, CF4, SiH4, Si(CH3)4, ethane, n-propane, cyclohexane-h12, and cyclohexane-d12. Spectral results are interpreted in terms of cluster size, isotope effects, and model calculations. A Lennard-Jones–Coulomb 6-12-1 potential is used to model the intermolecular interactions and predict minimum energy cluster geometries, cluster binding energies, and intermolecular force constants which are used to calculate van der Waals vibrational frequencies. The results show that for simple solvents (i.e., Ar, CH4) the calculations offer a simple interpretation of the observed spectra in terms of multiple cluster geometries with distinct transition energies; however, as the solvent becomes more complex, the cluster spectra also become more complex, and the number of calculated minimum energy cluster geometries increases. Complex spectra are interpreted as a distribution of cluster geometries with similar transition energies. For all of the clusters, the electronic origins are blue shifted with respect to the bare 1,4-dioxane origin. This observation is consistent with a model in which the excited state intermolecular potential becomes more repulsive due to the increased radial distribution of a nonbonding electron upon excitation into the 3s Rydberg state.

Conformational stability and molecular structure of dimethyl (methylthio) phosphine from electron diffraction studies and ab initio calculations

The molecular structure of dimethyl(methylthio)phosphine, (CH 3) 2PSCH 3, has been determined from a combined gas-phase electron diffraction study at 20°C and ab initio calculations utilizing both the 3–21G * and 6–31G * basis sets. Two distinct conformers, trans and cis or near-cis (the cis conformer has the thiomethyl group eclipsing the non-bonded electron pair on the phosphorous atom and the trans conformer has this methyl group staggered with respect to the two methyl groups attached to the phosphorous atom) were identified with the isomeric composition of 78±5% of the cis form. The differences between the geometrical parameters were fixed at the values obtained from the ab initio calculations. The determined heavy atom structural parameters for the cis conformer are r(PS)=2.111±0.002, r(PC)=1.862±0.002, r(SC)=1.769±0.006, ∠ PSC=102.3±2.6, ∠ CPC=96.7±0.8 and ∠ CPS=96.8±1.5 where the distances are in Å and the angles in degrees. The parameters which differ for the trans conformer are r(PS) =2.103±0.002, ǒ PSC=108.5±2.5 and ∠ CPS=99.9±1.5. The rms amplitudes of vibration with their associated uncertainties have been determined for some of the distances. In addition, the geometry has been calculated by ab initio Hartree-Fock gradient calculations with geometry optimization at the 3-21G * and 6-31G * levels, and the computed structural parameters are in reasonable agreement with those obtained from the electron diffraction data. Features of the structure are compared with those of similar molecules.

Sc5Re2C7, a Complex Carbide with C3-Units

Abstract The title compound was obtained by arc melting of the elemental components and subsequent annealing in a high frequency furnace. It crystallizes with the orthorhombic space group Cmmm, a = 779.26(7) pm, b = 1362.0(1) pm, c = 320.62(3) pm, V = 0.3403 nm3, Z = 2. The structure was determined from single-crystal X-ray data and refined to a residual of R = 0.021 (692 F values and 21 variables). It is closely related to the NaCl-structure. Three of the seven carbon atoms per formula unit form linear C3 -units with C-C bond lengths of 134 pm. They are coordinated to ten Sc atoms forming bicapped quadratic prisms. Additional isolated C atoms are located in octahedral voids formed by four Sc and two Re atoms. Together with the Re atoms they form two-dimensionally infinite [Re2C4 11-]n polyanions. The Re atoms have highly distorted tetrahedral carbon coordination with Re -C bond lengths of 200 and 208 pm corresponding to bond orders of about two. This coordination seems to be compatible with the 18-electron rule. The near-neighbour coordination of the Re atoms is assymmetric. It is suggested that this is due to the space requirements of nonbonding electrons. Samples of Sc5Re2C7 are semiconducting and weakly paramagnetic.

Cumulene Carbenes in Space and in the Laboratory

Astronomical searches for H2CCC and H2CCCC, based on frequencies from our laboratory identifications, have resulted in detections toward TMC-1 and IRC+10216. These new interstellar species are possibly the first of a new family of highly polar carbon chains; they are only the second and third carbenes (carbon molecules with two nonbonded electrons) known in space.

The VUV spectroscopy of deuterated hydrazine, N 2D 4

The VUV absorption spectrum of N 2D 4 has been measured in the spectral region 1050–2100 Å using synchrotron radiation. The spectrum, which is largely unstructured, gradually rises from a threshold of 2630 Å and is dominated by three broad peaks of increasing magnitude at ≈ 1940, 1685 and 1400 Å followed by a drop in the absorption at ≈ 1340 Å and then a monotonic rise to the low-wavelength experimental cutoff at 1050 Å. The spectrum can be interpreted either in terms of Rydberg excitations of the nonbonding nitrogen electrons with the broad features resulting from spectral congestion or alternatively, the broad nature of the spectrum may be due to absorption of molecular valence states involvingσ ∗←n and σ ∗←σ transitions. Mulliken proposed the existence of such valence transitions by analogy with corresponding feature in the isoelectric diatomic halogen molecules.

One-bond C–H NMR coupling constants in 1,2,4-trioxanes: a reversed perlin effect

In a reversal of the Perlin effect, the values of 1JC–H for the equatorial protons at C(3) and C(6) in 1,2,4-trioxanes are less than those for the corresponding axial protons; this is ascribed to homoanomeric interactions between equatorially directed nonbonding electron pairs on oxygen and the equatorial C(3)–H or C(6)–H σ* orbitals.

Electronic structure of some I–III–VI2 chalcopyrite semiconductors studied by synchrotron radiation

Photoemission spectra were measured for the I–III–IV2 chalcopyrite semiconductors CuInSe2, CuInS2, CuInTe2, and CuGaS2 with various photon energies (32 to 140, and 1253 eV). The partial density-of-states (DOS) of Cu 3d orbital is selectively observed at the photon energies beyond 100 eV. The DOS of Cu 3d spreads from the top of valence band (VB) to 5 or 6 eV below. Three dominant peaks are labeled as A, B, and C, the structures of which are interpreted as antibonding, nonbonding, and bonding states. Reflection spectra were also measured for CuInSe2 from 2 to 100 eV at room temperature and 98.5 K. The In 4d core reflection is observed at 17.5, 18.9, 19.8, and 21.2 eV at 98.5 K; very similar characteristics are also observed at 5.28, 6.21, 7.35, and 8.67 eV. These contributions are due to the Cu 3d nonbonding electrons. Photoemission and reflection experiments thus confirm conclusively the existence of nonbonding Cu 3d states in I–III–VI2 chalcopyrites. This result proves the theoretical model that attributes the upper VB to the antibonding state, in contrast with the bonding state of sp3-hybridized semiconductors.

The 119Sn Mössbauer and solid-state NMR and the crystal and molecular structure of tin(II) bisfluorosulfate, Sn(OSO2F)2

The reaction of tin(II) fluoride with fluorosulfuric acid has been shown to produce tin(II) bisfluorosulfate. Crystals of Sn(OSO2F)2 are monoclinic, space group P21/c with a = 5.195(1), b = 9.709(1), c = 13.861(1) Å, β = 110.12(1)°, Z = 4, R = 0.029, and Rw = 0.030 for 1461 unique reflections. The structure consists of a three-dimensional framework of fluorosulfate groups linked by O—Sn—O bridges with the two crystallographically independent fluorosulfates acting as tridentate bridging ligands between tin atoms. There are four short bonds (Sn—O = 2.338(3), 2.350(3), 2.398(4), and 2.427(3) Å) and two longer ones (Sn—O = 2.695(3) and 2.702(3) Å) around tin. The four short bonds have a disphenoidal, AX4E, primary geometry with the lone pair occupying an equatorial site. If the two longer interactions are also considered the geometry is distorted octahedral, AX4Y2E. Two more non-bonding contacts of 3.420(4) and 3.319(4) Å complete an approximately dodecahedral arrangement of oxygen atoms around the tin. The fluorosulfate groups deviate significantly from C3υ symmetry and have the mean dimensions, S(1)—O = 1.419(4), S(1)—F = 1.541(4), S(2)—O = 1.435(3), S(2)—F = 1.539(3) Å, O—S(1)—O = 114.3(2)°, F—S(1)—O = 104.1(3)°, O—S(2)—O = 114.2(2)°, and F—S(2)—O = 104.1(2)°, respectively. The solid-state 119Sn nmr spectrum shows only one single resonance with a shift of −1534 ppm which is substantially different from that of a solution in HSO3F. The 119Sn Mössbauer spectrum shows an unresolved quadrupole splitting as a result of a distorted environment around tin, produced by the stereochemically active non-bonding electron pair. Key words: tin(II) bisfluorosulfate, 119Sn Mössbauer, solid-state nmr, X-ray crystallography

N-heteroatomic effect on the photophysics of polyphenyl system: 2,6-diphenyl pyridine

From the detailed analysis of emission and absorption spectra of 2,6-diphenyl pyridine (DPP), m-terphenyl (MTP) and 2,6-luitidine (LTD) in different solvents, a striking change in absorption and emission characteristics of DPP compared to others could be observed. Consideration of through-space interaction of nonbonding electrons of the N atom and the π electrons of the outer rings becomes significant in explaining the dramatic change of absorption and emission character of the title compound in relation to a polyphenyl, MTP. The lowest excited singlet state of DPP is identified as 1L a, indicating a crossover of states relative to MTP.

Photochemistry of an unusual exsudatinite in Permian Basin shales

An unusual variety of exsudatinite is found in four Middle Paleozoic shales of the Permian Basin. Its strong blue to blue-green ( λ max = 450–470 nm) fluorescence highlights it prominently along sedimentary bedding partings and microfractures. Unlike most primary liptinites, fluorescence spectra of this microfracture exsudatinite are characterized by additional structure in the form of competitive peaks in the green wavelengths (480–500 nm) and a broad secondary structure in the yellow wavelengths. Subtle flow structures are common along the extent of the maceral which may reach maximum widths of approximately 10 μm and lengths of 100 μm. Time-resolved fluorescence data indicate a complex fluorescence of several nanosecond and sub-nanosecond lifetimes. Unperturbed fluorescence lifetimes (3.0–7.4 ns) are significant in Woodford and Wolfcamp shales, but are minor or absent in Atoka and Canyon Group shales. These lifetimes are interpreted to represent aromatics and heteroaromatics (3–5 rings) in relatively unperturbed fluorescence microenvironments where the radiative rates are high. In contrast, two other groups of shorter lifetimes suggest that the natural decays of molecules are influenced by photochemical interactions that are shorter than natural fluorescence decay. We speculate that intermediate lifetimes (1.3–2.6 ns) indicate charge transfer perturbations to various degrees by peripheral electron-donating functionals. These exciplexes represent the inter- and intramolecular transfer of non-bonded electrons from ground state donors to proximal and coplanar excited acceptors. The shortest lifetimes (〈0.30 ns) may be ascribed to several processes that severely perturb the fluorescence.

Coordination Numbers Larger Than Six Among Electronegative Elements

Abstract A review of recent structure determination of non-metal anions containing simple ligands, mostly halogenes, is given, where the total number of ligands exceeds 6. It will be shown that these anions exist in the form of regular polyhedra, the pentagonal bipyramid and the square antiprism. A non-bonding electron pair (or two) will influence such geometries in a complex way: the general principle that such pairs are stereochemically active needs to be carefully revised in compounds of large coordination numbers.

The ?-bromocyclohexanone ring: The relationship between its conformation and the ultraviolet and infrared absorption of the keto group

A series of diterpenoids whose C-ring was an α-bromocyclohexanone showed a discontinuity in the relationship between the BrCCO torsion angle and the Br…O contact distance at about 50°, below which the Br…O distance remained effectively constant at about 3.0 A. This effect is caused by the non-compressibility of the van der Waals radii of the Br and O atoms. Analysis of the parameters for all compounds in the Cambridge Data File containing an α-bromocyclohexanone ring confirmed this phenomenon. Once the Br…O distance approaches 3.0 A, the BrCC and CCO angles are forced open to accommodate the Br…O compression strain as the torsion angle is further reduced. An approximate value of the Br…O distance, d in A, can be estimated for any torsion angle, o, by the empirical equation d2 = R – S cos ϕ + T cos2 ϕ where R = 11.73, S = 3.62 and T = 0.75. There is a linear relationship between the Br…O separation and both the infrared vibration frequency v and the ultraviolet absorption wavelength δmax of the CO group: the greater the distance, the lower is the frequency v and longer the wavelength δmax. Thus measurement of the ultraviolet and infrared spectra of a compound containing an α-bromocyclohexanone system can yield information about the Br…O distance and hence the conformation of the six-membered ring. There are also systematic trends in the effect on the spectroscopic properties of the keto group that accompany changing the halogen from fluorine through to iodine. When the halogen is axial, its electronegativity has a maximum influence on the CO vibration frequency by a ‘through-bond’ inductive process. When the halogen is equatorial, it exerts a steric compression on the non-bonded electrons in the 2p orbital of the oxygen atom. This enhances hybridization to the sp2 state and this simultaneously strengthens the CO bond and enlarges the n π* energy gap. This ‘through-space’ influence is simply a process of steric compression, dependent on the size of the halogen atom. It accounts well for all of the systematic trends in both infrared and ultraviolet frequencies of the CO group when the halogen is equatorial and eclipses the CO bond.

On the bonding in B 2H 6 and the lone pairs in H 2O: The use of localized molecular orbitals in spin-coupled calculations

The implementation of a recently proposed procedure for generating localized SCF molecular orbitals makes it possible to carry out spin-coupled calculations on the bridging region of B 2H 6 and on the non-bonding electrons in H 2O. The description that emerges for B 2H 6 is fairly similar to the conventional view of three-centre two-electron “banana bonds”, but the orbital picture for the non-bonding electrons in H 2O turns out to be significantly different from the classical valence bond idea of doubly-occupied lone pairs.

Intensity of the Secondary Transition of Some 1,3-Benzodioxole Derivatives

Abstract 1,3-benzodioxole derivatives display a charge transfer (C.T.) transition n0 → π∗ φ, involving the non-bonding electrons of the oxygen atoms (n0 stands for the pπ orbitals of the oxygen atoms) and a π∗ φ orbital. It has been studied in a preceding paper for derivatives with a strongly perturbating subtituent: X, in para to one of the oxygen atoms (figure 1).1 These derivatives will be named underneath Bzd-X (for example Bzd-CO2H). In the present work, we want to study the intensity of the secondary transition which obeys to rules very different from those of the C.T. transition.