An electron diffraction study of the molecular structure of hexamethyldisiloxane

An electron diffraction determination of the molecular geometry of hexamethyldisiloxane has removed much of the uncertainty concerning this structure. The length of the SiO bond and the SiOSi bond angle were determined to be 1.631 ± 0.003 Å and 148 ± 3°, respectively. The experimental data are consistent with a staggered conformation (C_2v symmetry) while a model with twist angles around the SiO bonds of about 30° cannot be excluded. The molecule is probably performing large amplitude intramolecular motion.     

The molybdenum-molybdenum triple bond—XVII. Syntheses,spectroscopic and structural characterizations of Mo4F4(O—t-Bu)8 and Mo2F2(O—t-Bu)4(PMe3)2 (MoMo)

Hydrocarbon solutions of Mo₂(O—t-Bu)₆ and PF₃ (2 equiv) yield Mo₄F₄(O—t-Bu)₈, I, and PF₂(O—t-Bu). Compound I contains a bisphenoid of molybdenum atoms with two short MoMo distances, 2.26 Å, and four long MoMo distances, 3.75 Å, corresponding to localized MoMo triple bonding and non-bonding distances, respectively. The tetranuclear compound may be viewed as a dimer, [Mo₂(μ₂-F)₂(O-t-Bu)₄]₂, and addition of PMe₃ to hydrocarbon solutions of I yields Mo₂F₂(O—t-Bu)₄(PMe₃)₂, II, which contains an unbridged MoMo triple bond of distance 2.27 Å. Each molybdenum atom is coordinated to two oxygen atoms, one fluorine atom and the phosphorus atom of the PMe₃ ligand in a roughly square planar manner. The overall central Mo₂O₄F₂P₂ skeleton has C₂ symmetry and NMR studies (¹H, ¹⁹F and ³¹P) are consistent with the maintenance of this type of structure in solution. Infrared and electronic absorption spectral data are reported. These are the first compounds containing fluorine ligands attached to the (MoMo)⁶⁺ unit.     

Gas phase structure and conformational properties of trifluoroacetic anhydride, CF3C(O)OC(O)CF3

The geometric structure of trifluoroacetic anhydride, CF₃C(O)OC(O)CF₃, has been studied by gas electron diffraction (GED) and quantum chemical calculations (MP2 and B3LYP with 6-31G^* basis sets). The GED analysis results in a single conformer with synperiplanar orientation of the two CO bonds. This analysis, however, cannot discriminate between a planar equilibrium structure (C_2v symmetry) with large amplitude torsional motions around the OC bonds and a nonplanar equilibrium structure (C₂ symmetry) with a low barrier at the planar arrangement. An effective dihedral angle φ(COCO=18(4)° is obtained. Both quantum chemical methods predict a nonplanar equilibrium structure of C₂ symmetry and φ(COCO)=16.5° and 13.9°, respectively.     

Kristall- und molekülstruktur von hexaphenyldistannylselenid (C6H5)3SnSeSn(C6H5)3

The crystal and molecular structure of hexaphenylditin selenide (C₆H₅)₃SnSeSn(G₆H₅)₃ was determined by X-ray diffraction data and was refined to R  0.055. The compound is monoclinic, space group P2₁, with a  9.950(4), b  18.650(7), c  18.066(6) Å, β  106.81(4)°, Z  4. The two molecules in the asymmetric unit differ slightly in their conformations, both having approximate C₂ symmetry. Bond lengths and angles are: SnSe 2.526 (2.521(3) ? 2.538(3)) Å; SnC 2.138 (2.107(16)?2.168(19)) Å; SnSeSn 103.4(1)°, 105.2(1)°. There are only slight angular distortions at the SnSeC₃ tetrahedra (SeSnC angles: 104.3(5)?114.8(4)°). The bond data indicate essentially single bonds around the Sn atoms.     

On the transferability of conformation in FC(O)S-containing compounds: Conformation and properties of methylfluorocarbonyl disulphide FC(O)SSCH3

Methylfluorocarbonyl disulphide, FC(O)SSCH₃, was prepared for the first time by reaction of FC(O)SCl with CH₃SH at room temperature. Infrared data for the vapour and matrices (Ar, Ne and N₂) as well as Raman, UV, mass and ¹⁹F, ¹³C and ¹H NMR spectra have been obtained and interpreted.From these data, the most stable conformer was deduced to have the gauche conformation with respect to the FC(O) and CH₃ groups with the syn conformation between the CO and SS bonds having C₁ molecular symmetry. This conformer is in equilibrium with another, possibly the corresponding anti, referring to the CO and SS bonds.The main structure found for FC(O)S-containing compounds seems to be the syn conformation.     

Electron diffraction study on the molecular structure of methyltrimethoxysilane

The electron diffraction data for methyltrimethoxysilane are consistent with a C₃ symmetry model, the predominant forms of which have rotational angle(s) between 100 and 155° around the SiO bond (the anti conformation of the CSiOC chain would respond to 0°). There is probably large amplitude motion around the SiO bonds. The following bond lengths and bond angles were determined: r_a(CH) 1.093 ± 0.005, r_a(SiC) 1.842 ± 0.013, r_a(SiO) 1.632 ± 0.004, r_a(OC) 1.425 ± 0.004 », ∠CSiO 109.6 ± 0.5°. and ∠SiOC 123.6 ± 0.5°.     

Glycinium trifluoroacetate

In the title compound, C₂H₆NO₂⁺·C₂F₃O₂^?, the main N—C—COOH skeleton of the glycinium cation is almost perfectly planar. The tri­fluoro­acetate anion has a staggered conformation with typical bond distances and angles. The CF₃ group is slightly disordered. The structure is stabilized by an extensive network of strong O—H?O hydrogen bonds and weaker N—H?O bonds.     

Die struktur des dreikernigen hexamethylbenzolniob-komplexes [(Me6C6)3Nb3Cl6]+ Cl- · 3 CHCl3

Well developed crystals of [(Me₆C₆)₃Nb₃Cl₆]⁺ Cl^? · 3 CHCl₃ can be obtained from a solution of [(Me₆C₆)₃Nb₃Cl₆] Cl in CHCl₃ (monoclinic, P2₁/c, a 11.850(3), b 15.906(6), c 28.529(8) Å, β 98.14(3)°, Z  4). An X-ray structure determination shows the structure of the complex cation to be highly symmetric (non-crystallographic D_3h symmetry) and to agree within narrow limits with the known structure of the corresponding 2+ cation. Important distances are: NbNb 3.347(4) and NbCl 2.504(2) Å. The C₆ rings of the hexamethylbenzene rings are not planar. The average folding angle of the C₆ groups is 156.6°. In the crystal the Cl^? anion is bonded by weak H-bridges to three CHCl₃ molecules.     

Sarcosinium tri­fluoro­acetate

The title compound, C₃H₈NO₂⁺·C₂F₃O₂^?, crystallizes in space group C2/c. The main N—C—COOH skeleton of the protonated sarcosine mol­ecule is almost perfectly planar. The tri­fluoro­acetate anion has a staggered conformation and typical bond distances and angles. The CF₃ group is probably slightly disordered. The structure is stabilized by an extensive network of strong O—H?O hydrogen bonds and weaker N—H?O bonds.     

Dynamic Jahn–Teller effect and average structure of dicyclopentadienylcobalt, (C5H5)2Co,studied by gas phase electron diffraction

The molecular structure of (C₅H₅)₂Co has been determined by gas phase electron diffraction. The best agreement between calculated and experimental intensity curves is obtained with a model with eclipsed C₅H₅ rings (symmetry D_5h), but a model with staggered rings (symmetry D_5d) cannot be ruled out. The mean CoC and CC bond distances are 2.119(3) Å and 1.429(2) Å respectively. The average angle between the CH bonds and the C₅ ring is 2.1(0.8)°. The value obtained for the CC vibrational amplitude, l(CC) = 0.055(1) Å, is significantly larger than the amplitude calculated from a molecular force field and the corresponding amplitudes in (C₅H₅)₂Fe and (C₅H₅)₂Ni determined by electron diffraction, and confirms the presence of a dynamic Jahn—Teller effect of the magnitude calculated from ESR data. The average structure is compared with those of the metallocenes of the other first row transition elements.     

Molecular structures of (SiCl3)2CH2 and (SiCl3)2CCl2 as studied by electron diffraction

An electron diffraction analysis of the molecular structures of 1,1,1,3,3,3-hexachloro-1,3-disilapropane and octachloro-1,3-disilapropane has been carried out. Deviations from the staggered conformation are indicated. The data may be approximated by models with C₂ symmetry and a small tilt of the SiCl₃ groups. The main bond lengths (r_g) and bond angles obtained for (SiCl₃)₂ CH₂ are: SiCl, 202.7(4); SiC, 186.6(6); CH, 109.8(24) pm, ClSiCl, 107.9(1); SiCSi, 118.3(7)°; and for (SiCl₃)₂CCl₂: SiCl, 202.0(4); SiC, 190.2(9); CCl, 179.6(9) pm; ClSiCl, 109.5(1); SiCSi, 120.6(9); ClCCl, 110.9(16); SiCCl, 106.3(3)°.     

Estimates for systematic empirical corrections of consistent 4–21G ab initio geometries and their correlations to total energy group increments

The structural parameters of the completely relaxed 4–21G ab initio geometries of more than 30 basic organic compounds are compared to experimental results. Some ranges for systematic empirical corrections, which relate 4–21G bond distances to experimental parameters, are associated with total energy increments. In general, for the currently feasible comparisons, the following corrections can be given which relate calculated distances to experimental r_g parameters and calculated angles to r_s-structures For CC single bond distances, deviations between calculated and observed parameters (r_g) are in the ranges of ?0.006(2) to ?0.010(2) Å for normal or unstrained hydrocarbons; ?0.011(3) to ?0.016(3) Å for cyclobutane type compounds; and +0.001(5) to +0.004(4) Å for CH₃ conjugated with CO. For CO single bonds the ranges are ?0.006(9) to +0.002(3) Å for CO conjugated with CO; and ?0.019(3) to ?0.027(9) Å for aliphatic and ether compounds. A very large and exceptional discrepancy exists for the highly strained ethylene oxide, r_s — r_e = ?0.049(5) Å and in CH₃OCH₃ and C₂H₅OCH₃ the r_s — r_e differences are ?0.029(5), ?0.040(10) and ?0.025(10) Å. Some of these discrepancies may also be due to deficiencies of the microwave substitution method caused by atomic coordinates close to inertial planes. For CN bonds, two types of NCH₃ corrections are from +0.005(6) to ?0.006(6) and from ?0.009(2) to ?0.014(6) Å; and the range for NCO is +0.012(3) to +0.028(4) Å. For isolated CC double bonds the range is + 0.025(2) to +0.028(2) Å. For conjugated CC double bonds the correction is less positive (+0.014(1) Å for benzene). For CO double bonds the corrections are ?0.004(3) to +0.003(3) Å. For bond angles of type HCH, CCH, CCC, CCO, CCO, OCO, NCO and CCC the corrections are of the order of magnitude about 1–2° (or better). Angles centered at heteroatoms are less accurate than that, when hydrogen atoms are involved. Differences in HOC and NHC angles were found in a range of ?2.3(5)° to ?6.2(4)°.     

Evidence for the distortion of C2H4 and C2H2 chemisorbed on W(100)

The adsorption of C₂H₄ on W(100) has been studied by ultraviolet photoelectron spectroscopy with hν = 21.22 eV. The spectrum measured after in initial saturation exposure at 80 K exhibits structure which correlates well with energy levels recently calculated by Demuth and Eastman (DE) for sp³ rehybridized C₂H₄. Dehydrogenation of the adsorbate, either by subsequent heating to 295 K or direct adsorption at 295 K, yields a spectrum which correlates with DE's calculation for sp² rehybridized C₂H₂. These results suggest that C₂H₄ and C₂H₂ may be distorted from their planar and linear structures respectively and that the CC bonds on these molecules are stretched by adsorption on W(100). Qualitative arguments suggest that the bonding site for both melecules is directly over a W atom and that the Dewar—Chatt model for πd bonding in organometallic compounds is applicable.     

Isolierung und struktur des dreikernigen hexamethylbenzolzirkonium-komplexes [(Me6C6)3Zr3Cl6]2+ [(Al2Cl7)−]2

Crystals of the trinuclear complex [(Me₆C₆)₃Zr₃Cl₆][Al₂Cl₇]₂ have been obtained from the reaction of ZrCl₄, hexamethylbenzene, AlCl₃, and Al in benzene. They are monoclinic, space group C2/2, with Z  4 and lattice parameters a 14.167(3), b 27.779(7), c 15.721(3) Å and β 94.27(4)°. The Zr atoms form a regular triangle. Each pair of Zr atoms is bridged by two Cl atoms. The fifth coordination site of each Zr atom is occupied by a h⁶-Me₆C₆ group. The cation is almost isostructural with the known trinuclear cation [(Me₆C₆)₃Nb₃Cl₆]²⁺. Important distances are: ZrZr 3.35, ZrCl 2.56, and Zrcenter of C₆ ring 2.17 Å. One of the two independent [Al₂Cl₇]^? anions occurs in a staggered conformation and one occurs in an eclipsed conformation.     

Molecular Structure of Tetrahydroxycalix[4]arene [-(C<Subscript>6</Subscript>H<Subscript>3</Subscript>OH)-CH<Subscript>2</Subscript>-]<Subscript>4</Subscript> in the Gas Phase

Gas-phase electron diffraction and HF/6-31G*, HF/6-31G**, and B3LYP/6-31G* ab initio calculations were used to find that in the gas phase at 242°C the calix[4]arene [-(C₆H₃OH)-CH₂-]₄ molecule possesses a C₄ conformation. Geometric parameters of the molecule were determined, and the energies of C-H?O hydrogen bonds (7.3 kcal mol^?1) were estimated by the AM1 method.     

Studies of layered uranium (VI) compounds. VI. Ionic conductivities and thermal stabilities of MUO2PO4 · nH2O,where M = H,Li,Na,K,NH4 or 12Ca, and where n is between 0 and 4

We have measured the ionic conductivities of pressed pellets of the layered compounds MUO₂PO₄ · nH₂O, and correlated the results with TGA data. The conductivities (in ohm^?1 m^?1), at temperatures increasing with decreasing water content over the range 20 to 200°C, were approximately as follows: Li⁺4H₂O, 10^?4; Li⁺, Na⁺, K⁺, and NH₄⁺3H₂O, 10^?4, 10^?2, 10^?4, and 10^?4; H⁺, Li⁺, and Na⁺1.5H₂O, 10^?2, 10^?4, and 10^?4; Na⁺1H₂O, 10^?5; H⁺, K⁺, and NH₄⁺0.5H₂O, all 10^?5; and H⁺, Li⁺, Na⁺, K⁺, NH⁺₄, and $\text{1}\text{2}\text{Ca}{}^{\mathrm{2+}}\text{}\text{OH}{}_2\text{O}$, 10^?5, 10^?5, 10^?4, 10^?5, 10^?5, and 10^?6. A ring mechanism is proposed to account for the high conductivity found in NaUO₂PO₄ · 3.1H₂O. The accurate TGA data showed that most of the hydrates had water vacancies of the Schottky type, and should be represented as MUO₂PO₄(A ? x)H₂O, where x can be between 0 and 0.3.     

An ab initio study of the geometry and energy of the four planar conformers of glyoxylic acid and the glyoxylate ion

The structures and energies of the four planar conformers of glyoxylic acid and the glyoxylate ion have been studied ab initio using the unscaled 4—31G basis set with full geometry optimization. Changes in the CO, OH and CO bond lengths in the conversion of the cc conformer into the ct and tt conformers, and into the tc conformer, are consistent with the formation of four-membered and five-membered hydrogen-bonded ring structures, respectively. Changes in the distances between the nearest non-bonded atoms around each C atom reveal that the internal geometry of the CHO and COOH groups is significantly affected by cis—trans isomerization with respect to the OCCO backbone, and that the geometry of the CHO group is affected by proton dissociation from the COOH group. Furthermore, the movement of the component atoms in each functional group, characterized as clockwise or anticlockwise about the C atom, results in some cases in a rotation of the functional group as a whole. Whereas experiment shows the tc conformer to be more stable than the tt by 1.2 ± 0.5 kcal mol^?1, the calculations find the tt conformer to be the most stable, separated in energy from the ct, tc and cc conformers by 0.4, 1.4 and 10.7 kcal mol^?1, respectively. Augmentation of the 4—31G basis set in several forms, and use of (9,5/4) and (9,5/4,1) basis sets, only served to decrease slightly the tt/tc energy difference, not change the sign. The calculated proton affinity of the glyoxylate ion with respect to the tt conformer is 342.7 kcal mol^?1, compared to 357.7 kcal mol^?1 for the formate ion. The expectation energy differences Δ V_nn, Δ V_ee and Δ V_en for the cis—trans isomerization of the ct and cc conformers are opposite in sign to those for the glyoxal reaction, and in magnitude they all far exceed the ΔE_T values, which shows that hydroxyl group substitution has a much greater influence than a comparison of only the ΔE_T values would suggest.     

The crystal structure of Ba2V2O7

Ba₂V₂O₇ is triclinic with a = 13.571(3), b = 7.320(2), c = 7.306(2) Å, α = 90.09(1), β = 99.48(1), β = 99.48(1), γ = 87.32(1)°, V = 7.15.1 ų, Z = 4, and space group $\text{P}\text{1}$. The crystal structure was solved by Patterson and Fourier methods and refined by full-matrix least-squares analysis to a R_w of 0.034 (R = 0.034) using 2484 reflections measured on a Syntex $\text{P}\text{1}$ automatic four-circle diffractometer. The structure has two unique divanadate groups that are repeated by the b and c lattice translations to form sheets of divanadate groups parallel to (100). These sheets are linked by four unique Ba atoms that lie between these sheets. Ba(1) and Ba(3) are coordinated by eight oxygens arranged in a distorted biaugmented triangular prism and a distorted cubic antiprism, respectively. Ba(2) is coordinated by 10 oxygens arranged in a distorted gyroelongated square dipyramid and Ba(4) is coordinated by nine oxygens arranged in a distorted triaugmented triangular prism. These coordination numbers are substantiated by a bond strength analysis of the structure, and the variation in 〈BaO〉 distances is compatible with the assigned cation and anion coordination numbers. Both divanadate groups are in the eclipsed configuraton with 〈VO(br)〉 bond lengths of 1.821(4) and 1.824(4) Å and VO(br)V angles of 125.6(3) and 123.7(3)°, respectively. Examination of the divanadate groups in a series of structures allows certain generalizations to be made. Longer 〈VO(br)〉 bond lengths are generally associated with smaller VO(br)V angles. When VO(br)V < 140°, the divanadate group is generally in an eclipsed configuration; when VO(br)V > 140°, the divanadate group is generally in a staggered configuration. Nontetrahedral cations with large coordination numbers require more oxygens with which to bond, and hence O(br) is more likely to be three coordinate, with the divanadate group in the eclipsed configuration. In the eclipsed configuration, decrease in VO(br)V promotes bonding between O(br) and nontetrahedral cations, and hence smaller nontetrahedral cations are generally associated with smaller VO(br)V angles.     

Ab initio study of the O4 molecule

SCF-CI calculations were done on tetratomic oxygen complexes at various geometries. The results point to the existence of a metastable covalent molecule O₄ completely different from the van der Waals structure (O₂)₂ detected experimentally. At its equilibrium geometry, the O₄ molecule is a quasi-square (r(OO) ≈ 1.4 Å), slightly twisted out of plane, corresponding to the symmetry group D_2d. The activation energy of the reaction O₄(¹A_g) → 20₂(X ³Σ^?_g) is found to be ≈ 15 kcal/mole, that of the inverse reaction, ≈ 75 kcal/mole.     

Preparation and structural characterization of [(η5-C5H5)2Zr(SC6H5)]2O. A qualitative description of the bonding in oxo-bridged dicyclopentadienyl-transition metal dimers

The hydrolysis of (η⁵-C₅H₅)₂Zr(SC₆H₅)₂ was shown previously by IR spectroscopy to produce an oxo-bridged complex. The molecular structure of this material has been determined by X-ray diffraction methods and consists of two (η²-C₅H₅)₂Zr(SC₆H₅) units linked by an oxo bridge. The ZrOZr bond is nonlinear at 165.8(2)° with a Zr?Zr interatomic separation of 3.902(1)Å. The two independent SZrO bond angles of 98.7(1) and 103.3(1)° are consistent with a d° electronic structure for each zirconium atom. The relatively short ZrO distances of 1.968(3) and 1.964(3) Å support the presence of partial double-bond character arising from the donation of electron density from filled p_π-orbitals on the oxygen atom to unfilled d-orbitals on the electron deficient d⁰ metal atoms. This bonding feature requires based upon orbital symmetry arguments that the (ML)₂O molecular core in [(η⁵-C₅H₅)₂ML]₂O complexes must be nonplanar with a dihedral angle between the two LMO planes less than 90°. For [(η⁵-C₅H₅)₂Zr(SC₆H₅)]₂O, dihedral angle of 61.7° was observed. The compound crystallizes in an orthorhombic space group, Pbca, with refined lattices parameters a 16.458(4), b 20.281(5), and c 17.016(4) Å. Full-matrix least-squares refinement of 2613 diffractometry data I > σ(I) led to a final discrepancy index R(F₀²) = 0.044.