Bond Lenghts Research Articles

Les composes de coordination des halogenures d'aluminium: mise au point sur leurs proprietes structurales et les methodes d'analyses

Les composes de coordination des halogenures d'aluminium: mise au point sur leurs proprietes structurales et les methodes d'analyses

Synthesis and crystal structure of the dimer bis{[bis(trimethylsilylmethyl)arsino]diphenylgallane}

The dimer [(Me 3SiCH 2) 2AsGaPh 2] 2, obtained from the reaction of (Me 3SiCH 2) 2AsH with Ph 3Ga, has been characterized by partial elemental analysis, NMR spectroscopy, cryoscopic molecular weight determination, and complete single-crystal X-ray analysis. Crystals of the dimer are triclinic, space group P 1 , with a 10.541(1), b 11.939(1), c 10.539(1) Å, α 99.72(1), β 83.00(1), γ 114.55(1)°, U 1187.1 Å 3 and Z = 1. The bond lenghts (GaAs 2.518(1) and GaAs′ 2.530(1) Å) and bond angles (AsGaAs′ 85.08(2) and GaAsGa′ 94.92(2)°) in the centrosymmetric, and consequently planar, four-membered ring indicate a significant degree of strain.

The X-ray structural characterization and solution dynamics of bis[(μ-2,6-dimethylphenylisocyanide)- (cyclopentadienyl)(2,6-dimethylphenylisocyanide)iron(I)

The X-ray structure of trans-[{Fe(η 5-C 5H 5)(CNC 6H 3Me 2-2,6) 2} 2] ( 1a) and the solution dynamics of both cis- and trans-[{Fe(η 5-C 5H 5)(CNC 6H 3Me 2-2,6) 2} 2] have been studied. The trans-isomer of 1a crystallizes in the space group P2 1/ n with a 14.588(4), b 8.811(2) and c 14.847(4) Å, β 92.08(2)°. The molecule lies across a crystallographic centre of inversion with a trans arrangement of cyclopentadienyl ligands and a strictly planar bridging Fe 2C 2 ring. The FeFe bond lenght is 2.518(1) Å and the bridging isocyanide ligands are symmetrically bonded to iron with a mean FeC(bridging) bond lenght of 1.928(3) Å. Solution 1H NMR spectra of 1a show the presence of both cis- and trans-isomers, and a 500 MHz 1H NMR study at low temperature reveals two distinct exchange processes; the lower energy one results in the coalescence of the signals for the inequivalent methyl groups on the bridging isocyanide ligands of the cis-isomer, whereas the higher energy process brings about coalescence of the methyl signals for the bridging and terminal isocyanide ligands of the trans-isomer. At elevated temperatures all the methyl signals coalesce.

A theoretical study of carbon chemisorption on nickel surfaces

The effective medium theory is used to calculate chemisorption energies, bond lenghts and vibrational frequencies for carbon chemisorbed on Ni surfaces. The formation of graphite-like structures on Ni(111) is discussed.

A Molecular orbital study of the rotation about the CC bond in styrene

Abstract The geometry and energy of styrene have been calculated using the 6-31G basis set as a function of the C β C 2 C 1 C 2 dihedral angle-Φ = 0°(cis), 15°, 30°, 60° and 90° — assuming that the vinyl and phenyl groups remain planar, but otherwise with full geometry optimization. Similar calculations have been carried out for 1,3-butadiene and 3-methylene-1,4-pentadiene (MPD) where rotation about 180° generates a different and not the same conformer. The torsional potential energy curve for styrene has a very flat minimum Φ = 0, i.e. the cis structure is the most stable, whereas butadiene and MPD have minima in the region Φ = 37° to 40°, indicative of more stable gauche structures. For styrene the barrier height Φ = 90° is 131.1 KJ mol −1 . These results provide strong support for the potential function obtained by Hollas and Ridley from single level vibronic fluorescence and other spectroscopic data. The distortion of the benzene ring brought about the vinyl group substitution is discussed, also the variation of the C/C and H/C bond lenghts with Φ and the change in charge on the vinyl group and the polarity of the various bonds in the conversion of the cis into the 90° gauche conformer. The stabilization energy for styrene relative to that for benzene has been evaluated according to various criteria, and, in addition, the energy associated with the distortion of the ring.

Intramolecular sulphur(II)—oxygen interaction in sulphides and disulphides with 2-methoxycarbonylphenyl and 2-nitrophenyl groups: an X-ray study

The molecular structures of methyl 2-(methylthio)benzoate, dimethyl 2,2′-dithiodibenzoate, dimethyl 2,2′-thiodibenzoate, methyl 2-(2-nitrophenylthio)benzoate, 2,2′-dinitrodiphenyl sulphide and methyl 2-(2-nitrophenylthio)phenylacetate exhibiting S(II)⋯O(carbonyl) or S(II)⋯O—(nitro) close contact have been investigated by X-ray diffreaction. Planar (ArSMe), equatorial (Ar 2S 2) and skew (Ar 2S) conformations are explained by steric and conjugative effects and sulphur(II)—oxygen interaction. Linear and non-linear X—S⋯O close contacts resulting from favourable “bond direction” and unfavouralbe “lone-pair direction” approaches (2.619–2.722 and 2.900–3.402 Å, respectively) are discussed in detail. Possible aryl ring positions, the geometry of rings with S⋯O contact and the nature of counter-atom (X = C, S) are considered as decisive factors. The results are consistent with different neighbouring group effects found earlier for o-CO 2Me, o-CH 2CO 2Me and o-NO 2 groups. Data support the mutual dependence of S—S and S⋯O distances in compounds with a linear S—S⋯O arrangement. Other bond lenghts, e.g. CO, NO and S(II)—C ar are not affected significantly by sulphur—oxygen interaction.

The molecular structure of bis(bis(trimethylsilyl)amido) magnesium, Mg{N(Si(CH 3) 3) 2} 2, as determined by gas-phase electron diffraction: a sterically hindered magnesium amide

Gaseous bis(bis(trimethylsilyl)amido) magnesium, Mg{N(SiMe 3) 2} 2, has been studied by electron diffraction at a nozzle temperature of ca 400 K. The diffraction data are consistent with a model including only monomers. Important structural features of the equilibrium conformation of the monomer are the linear NMgN arrangement and the staggered Si 2NMgNSi 2 backbone. The ligands undergo large-amplitude vibration about the NMgN axis, and the barrier to internal rotation is 3(1) kcal mol −1. Principal bond lenghts ( r a) and valence angles are: MgN 1.91(3) Å, SiN 1.703(6) Å, SiC 1.886(5) Å; ∠SiNSi 132(2)°, ∠MgNSi 114.0(1.2)°, ∠NSiC 116.4(1.2)° and ∠SiCH 111.3(1.5)°. The SiMe, groups are twisted 14(4)° away from the reference position typified by one SiC bond of each such group eclipsing the adjacent MgN bond, while the methyl groups themselves are twisted 9(9)° away from their staggered reference positions. These torsional distortions lower the overall symmetry of the molecule from D 1d to S 4.

Crystal and molecular structure of [η 5-C 5(CH 3) 5]Co(O 2C 6H 4), a cobalt( o-catecholate) complex

[η 5-C 5(CH 3) 5]Co(O 2C 6H 4) crystallizes in the orthorhombic space group Pnma with a 12.942(4), b 12.902(4), c 8.543(3) Å, V 1426(1) Å 3, and Z = 4. Least-squares refinement of 1688 independent observed reflections, F( obs) ⩾ 2.5σ( F obs), gives R F 3.79 and R wF 3.72%. The cyclopentadienyl ring contains two short (1.412(3) Å) and three longer (〈av〉 1.430(4) Å) CC bond lenghts, consistent with a slight preference for diolefin bonding. The O 2C 6H 4 fragment is best described as a catecholate with a CO bond distance of 1.338(3), and a CoO distance of 1.837(2) Å.

ChemInform Abstract: CIS-1-CHLORO-3-FLUOROPROPENE: GAS PHASE MOLECULAR STRUCTURE AND CONFORMATION AS DETERMINED BY ELECTRON DIFFRACTION

Abstract Gaseous cis -1-chloro-3-fluoropropene has been investigated at 20°C by electron diffraction. Only one conformer was observed, but small amounts ( v (φ) = V 0 (1 + (φ/φ 0 ) 4 −2(φ/φ 0 ) 2 ), φ = 180 — τ, with φ 0 = 37(10)° and V 0 = 1.0(1.0) kJ mol −1 . The results for bond lenghts ( r a ) and bond angles (∠ α ) are as follows: r (CC) = 1.347(14) A, r (CC) = 1.493(14) A, r (CCl) = 1.737(5) A, r (CF) = 1.388(9) A, ∠CCC = 125.4(8)°, ∠CCCl = 122.7(6)°, ∠CCF = 111.2(8)°. The quoted error limits are 2 σ, where σ includes uncertainties in the voltage and nozzle-to-plate distances and corrections for correlation in the experimental data.

EXAFS study of mixed crystals of the A IIB III2X VI4 family

In this work we studied the nature of the near-neighbour environment in ternary and pseudoternary defect chalcopyrites of the AB 2X 4 family by using the extended X-ray absorption fine structure (EXAFS) technique. The EXAFS was measured at the K-edge of Zn, Ga and Se elements in ZnGa 2 (S xSe 1-x) 4 and Ga and Se edges in CdGa 2 (S xSe 1-x) 4 mixed crystals.NN distance were determined for all the compositions by a linear fitting of differences of phases and by a fitting procedure in k-space. We found bond lenghts nearly constant and close to the respective distances in the end member crystals over all the composition range, whereas the unit cell parameter a(x) changes accordingly to the virtual crystal approximation (Végard's law).

Cis-1-chloro-3-fluoropropene: gas phase molecular structure and conformation as determined by electron diffraction

Gaseous cis-1-chloro-3-fluoropropene has been investigated at 20°C by electron diffraction. Only one conformer was observed, but small amounts (<10%) of a second form could not be excluded. The average CCCF torsional angle (τ) was found to be τ = 149(4), where τ = 0° corresponds to the fluorine atom eclipsing the carboncarbon double bond. The potential function for the torsion about the CC bond could be approximated by v(φ) = V 0 (1 + (φ/φ 0) 4−2(φ/φ 0) 2), φ = 180 — τ, with φ 0 = 37(10)° and V 0 = 1.0(1.0) kJ mol −1. The results for bond lenghts ( r a) and bond angles (∠ α) are as follows: r(CC) = 1.347(14) Å, r(CC) = 1.493(14) Å, r(CCl) = 1.737(5) Å, r(CF) = 1.388(9) Å, ∠CCC = 125.4(8)°, ∠CCCl = 122.7(6)°, ∠CCF = 111.2(8)°. The quoted error limits are 2 σ, where σ includes uncertainties in the voltage and nozzle-to-plate distances and corrections for correlation in the experimental data.

The preferred conformation(s) of trimethyl phosphate as derived from NMR spectra of partially oriented molecules and potential energy calculations

The 1H and 31P NMR spectra of oriented trimethyl phosphate have been investigated. Potential energy calculations on the system indicate that the gauche (with the rotation angle φ around the CP bond(s) ⋍ 60°) and the trans (with φ ⋍ 155°) are the preferred conformers with the gauche having lower energy than the trans. Assuming reasonable bond lenghts and bond angles and similar order parameters of the threefold symmetry axis for the two preferred conformers, it is shown from NMR studies that the gauche conformer is predominant. Solvent/temperature dependence of the conformer ratio is evident from the NMR spectra. The 1H and the 31P chemical shift anisotropies are determined from the spectra in mixed liquid crystals of opposite diamagnetic anisotropies.

Synthesis, spectroscopic investigation and molecular structure of pentacarbonyl-5-t-butyl-5-aza-2,8-dithia-1-stannobicyclo[3.3.0 1,5]octanechromium(0)

The compound pentacarbonyl-5-t-butyl-5-aza-2,8-dithia-1-stannobicyclo-[3.3.0 1,5]octanechromium(0) was prepared in good yield by the reaction of 5-t-butyl-5-aza-2,8-dithia-1-stannobicyclo[3.3.0 1,5]octane with Cr(CO) 6 in THF under UV-irradiation. The crystal and molecular structure was determined from three-dimensional X-ray data. The crystals are monoclinic, space group P2 1/ c. The unit cell, with dimensions a 14.963(9), b 10.026(5), c 13.565(5)Å, β 114.68°(5), contains 4 molecules. The structure was solved by the Patterson-method. The full-matrix refinement with the 2734 independent reflexions gave a final R value of 0.034. The eight-membered ring adopts an asymmetric boat-boat conformation with a tin-nitrogen bond lenght of 2.40 Å; the tin-chromium bond length is 2.62 Å.

Substituted heptaphosphanortricyclenes: Derivatives and homologues of P 7(SiMe 3) 3

Homologues and derivatives of P 7(SiMe 3) 3 can be synthesized from Li 3P 7·3solv or Na 3P 7, or by cleavage of the PSiMe 3 bond with RX. The reaction of Li 3P 7·3DME with the halogen-containing compounds Ph 3SiCl (Ph  C 6H 5), H 3SiI, Me 3SnBr (Me  CH 3), i-C 3H 7Br, CpFe(CO) 2Br yields P 7(SiPh 3) 3, P 7(SiPh 3) 3, P 7(SnMe 3) 3, P 7(i-C 3H 7) 3 and P 7[Fe(CO) 2Cp] 3. Na 3P 7 reacts with Me 3MCl (M  Si, Ge, Sn) yielding P 7(MMe 3) 3. The reaction of Li 3P 7·3DME with PMe 2Cl leads to P 2Me 4, but P 7(PMe 2) 3 is not formed. Cleavage of the PSi bond in P 7(SiMe 3) 3 by Me 3SnBr or Me 3SnCl gives the compounds P 7(SiMe 3) 3- n (SnMe 3) n with ( n = 1, 2 or 3) depending on the molar ratio. The reaction with HI yields mixtures of H 3- n P 7(SiMe 3) n ,while I 2 converts P 7(SiMe 3) 3 into P 2I 4, PI 3 and Me 3SiI. Crystals of the Ge and Sn compounds are less sensitive towards oxidation and hydrolysis than P 7(SiMe 3) 3. The compounds have been identified by 31P NMR and mass spectra. An X-ray structure analysis has shown P 7(MMe 3) 3 (M = Si, Ge, Sn, Pb) to be isotypical (space group P2 1 (No. 4). The compounds crystallize as pure enantiomers. Bond lenghts and angles vary with their position in the P 7 cage (PP = 222.2,218.8, and 218.0 pm) and are almost unaffected by the substitution. The PM bond lengths are 228.8 (Si); 235.5 (Ge); 253.9 (Sn); 261.7 pm (Pb), showing a small lengthening with respect to calculated values. The cone angle of the bridging P atom decreases with increasing size of M. The P 7 cage vibrations are almost unchanged by the subsitution, whereas ν(PM) and ν(MC 3) change in the usual manner.

Approximate Relativistic calculations within the one-center approximation for the series CH 4 TO PbH 4

Using one-component relativistic theory combined with an Xαβ local exchange, we have carried out calculations for the series CH 4 to PbH 4 within the spherically symmetric one-center expansion approximation. Bond lenghts, breathing force constant, ionization potentials and spin-orbit splitting of the filled valence p shell thus obtained, are found to be in excellent agreement with Dirac-Hartre-Fock results.

Exchange interactions in tetranuclear copper(II) complexes

A good description of the exchange effect from the experimental point of view is only possible using relatively simple systems. The two spin case ( e.g. Cu(II)) is the simplest one, nevertheless it also poses a number of problems. If more than two paramagnetic centres are coupled, then the problems become even more complicated. On the other hand when one has more than two centres, the chances of transferring the linear case to a three-dimensional case are increased. The latter case is far more important for the problems in question. In copper(II) the number of centres are equal to 4 and thus one of the possible molecular shapes is the tetranuclear cuban-like type. The main part of the contribution is concentrated around our present knowledge of the exchange effect in this type of tetranuclear Cu(II) complex. The exchange pathway runs along the diamagnetic bridge ligands. When one considers the lowest symmetry case C 1, 6 different exchange integrals for describing the exchange effect are required. The simplest description still needs two independent integrals. In principle, the tetranuclear cuban type structures can be divided in to two different types. Type I forms a tetrameric unit, consisting of two weakly coupled Cu 2O 2 dimers. The coupling occurs perpendicular to the plane of the ring. Type II can be derived from a boat like eight membered Cu 4O 4 ring. In the case of the former type of structure there is an overall antiferromagnetic effect and in the latter type one finds an overall ferromagnetism. There is a continuous series of examples verified between the two types in the groups of aminoalcohol or iminoalcohol ligands. The greatest point of interest at present is the question of which geometrical factors (bond lenghts, bond angels) determined the strength and the sign of the exchange effect. These relationships for the tetrameric cuban type clusters known up to the present time are presented. Moreover, the strength and the sign of the exchange integrals depends also on the type of ligands near the paramagnetic centre, e.g. saturated or unsaturated groups. This relates to the fact that the energy level of each paramagnetic center depends upon its chemical surrounding. In this contribution we present examples which can explain the relation between the changes in the chemical structure and the strength of the exchange effect. A good example is the change of the molecular structure and also the exchange integrals under the influence of different solvents. This relates to the fact that the different interaction energies between the exchange coupled molecule and the solvent cause different modifications with different magnetic behavior. The aims of this contribution is to explain these small effects.

Structural distortions in alkylcobaloximes containing triphenylphosphine as axial ligand. The X-ray structures of trans-bis(dimethylglyoximoto)(bromomethyl)(triphenylphosphine)cobalt(III) and trans-bis(dimethylglyoximato)(cyanomethyl)(triphenylphosphine)cobalt(III)

The crystal structure of two cobaloximes, [Co(CH 2Br)(Hdmg) 2(PPh 3)] ( 1) and [Co(CH 2CN)(Hdmg) 2(PPh 3)] ( 2) (H 2dmg = dimethylglyoxime), are reported and discussed. Compound ( 1) crystallizes in the space group P2 1 / c with cell parameters a = 10.221(7), b = 15.23(1), c = 19.27(1) Å β = 108.0(1)°, Z = 4; ( 2) crystallizes in the space group P2 2 / c with cell parameters a = 8.653(4), b = 16.55(1), c = 19.99(1) Å, β = 104.7(1)°, Z = 4. Both structures have been solved by Patterson and Fourier methods and refined by least-squares methods to final R values of 0.057( 1) and 0.028( 2), using 1746( 1) and 2950( 2) independent reflections. The cobalt atom is displaced 0.065( 1) and 0.084( 2) Å above the (Hdmg) 2 4-N equatorial donors towards PPh 3, the two Hdmg planes making dihedral angles of 2.7( 1) and 11.1° ( 2) and bending towards the alkyl group. The PPh 3CoCH 2Br fragment of ( 1) is characterized by a PCoC angle of 175.1(4)° and CoP and CoC bond lenghts of 2.399(3) and 1.998(15) Å respectively; the corresponding figures for ( 2) are 177.7(1)°, 2.391(1) and 2.043(3) Å respectively. The CoCH 2Br angle is 125.2(9)° and the CBr distance is 1.84(1), significantly shorter (∼0.1 Å) than values reported for bromoalkyls. The CoCH 2CN angle is 116.5(3)° and the CCN distance is 1.424(5) Å, slightly but significantly shorter (∼0.04 Å) than those reported for R 3CCN compounds. The present results are compared with those reported for the analogues [Co(CH 2X)Co(Hdmg) 2(PPh 3)], (X = H, t-C 4H 9, CF 3) and discussed in terms of steric and electronic influences. Comparison with the [Co(CH 2X)Co(Hdmg) 2(py)] series is discussed, pointing out the differences provoked by substitution of PPh 3 with py on the geometry of the Co(CH 2X)(Hdmg) 2 moiety.

Change of geometry of polyacetylene upon charge transfer

Crystal orbital calculations on cis- and trans-polyacethylene chains indicate two kinds of geometry changes on charge transfer. (1) increase (decrease) of the CC bond lenght and CCC bond angles in the trans conformer taking up (losing) electrons, and (u) ̇ a tendency towards bond-length equalization on electron uptake and loss for both conformers.

Direct observation of atomic processes: Silicon adatoms on tungsten surfaces

A wide variety of the behaviors of single silicon adatoms on tungsten surfces have been studied in the field ion microscope (FIM). Existing works are summarized, and some new results are presented. Diffusion parameters for single silicon atoms on the W{110} plane were found to be E d = 0.70±0.07 eV and D 0 = 3.1 × 10 -4 × 10 ±1.28 cm 2 s -1, where E d is the activation energy of diffusion and D 0 the diffusivity. Although chain-form atomic clusters were often formed when more than one adatom was present on a plane, the clusters were not stable during diffusion. Silicon atom transport on the W{110} plane is thus achieved dominantly by migration of single silicon atoms. The desorption field as a function of radial distance from the W{110} plane center was measured, and the data were related to the field distribution on the plane at a given applied voltage. The binding energy of silicon adatoms on the W{110} plane was estimated from the desorption field. Relative pair energies for the Si-Si interaction on the W{110} plane for several bond lenghts were obtained from the relative frequencies of observing these bonds in the FIM. The maximum binding occured at a separation √2 a, with the bond aligned in the [1 1 0] direction, the maximum antibinding occured at a distance a in the [001] direction, and a weak binding occured at 2 a along the [001] direction; a was the substrate lattice constant. Using the pair energy data, the relative binding energy in an adlayer for various adlayer structures was calculated. The [ 2 √2 √3× 4 √3 R 35.26° superstructure, which is conventionally expressed as p(2×1), had the lowest energy. Our subsequent thermal equilibration experiments with deposited silicon adatoms at about 300 K confirmed the existence of such a structure. Since further heatings at about 300 K changed only the adlayer shape and composites, but did not change the structure of the superlattice, the observed superstructure was the equilibrium adlayer structure at a low degree of coverage at about 300 K. Monte Carlo computer simulations using the measured pair energies predicted the same equilibrium superstructure. In addition, many growth defects of the two-dimensional lattice, such as antiphase domains and stacking faults, were observed. The computer simulations also showed these and other defects. On the W{112} plane, no cooperative walk of silicon adatoms similar to that of metal adatoms was found. However, the staggered and straight bond configurations were observed much more often than that expected from a random distribution. When two silicon adatoms were deposited onto the same surface channel, they were mobile although the √3 a bond separation seemed to occur more often. In addition, interactions of plane edges with silicon adatoms and some other observations are discussed.

Bond orders in tricarbonyl[η 4-cyclobutadiene]iron complexes

Valence bond structure-resonance theory is used to calculate bond orders in tricarbonyl cyclobutadieneiron complexes. CFe bond lenghts show a linear correlation with the bond orders. CFe bond lengths in benzocyclobutadiene complexes V and Vi (Table 1) which had been called “surprising”, are rotationalized. A graph theoretical method to calculate the bond orders is illustrated.