Elizabeth M. Page

Hydrogen bonding in the gas-phase: the molecular structures of 2-hydroxybenzamide (C7H7NO2) and 2-methoxybenzamide (C8H9NO2), obtained by gas-phase electron diffraction and theoretical calculations.

The structures of 2-hydroxybenzamide (C7H7NO2) and 2-methoxybenzamide (C8H9NO2) have been determined in the gas-phase by electron diffraction using results from quantum chemical calculations to inform restraints used on the structural parameters. Theoretical methods (HF and MP2/6-311+G(d,p)) predict four stable conformers for both 2-hydroxybenzamide and 2-methoxybenzamide. For both compounds, evidence for intramolecular hydrogen bonding is presented. In 2-hydroxybenzamide, the observed hydrogen bonded fragment is between the hydroxyl and carbonyl groups, while in 2-methoxybenzamide, the hydrogen bonded fragment is between one of the hydrogen atoms of the amide group and the methoxy oxygen atom.

Determination of the molecular structure of (CH3)3SiCHCH2 by gas-phase electron diffraction and ab initio methods

Abstract The molecular structure of (CH 3 ) 3 SiCHCH 2 has been determined by a combination of gas phase electron diffraction and ab initio molecular orbital calculations. The molecule was found to exist in a syn conformation in which the dihedral CCSiC angle is 0°. The SiC methyl and SiC vinyl bond lengths were refined as an average value and the difference between them fixed at the difference found from ab initio calculations. Other ab initio results were also used as constraints in the refinements. The main molecular parameters were found to have the following values; r g ( C  C ) = 1.359(10) A , r g ( SiC methyl ) = 1.877(3) A , r g ( SiC vinyl ) = 1.867(3) A , ∠(C methyl SiC vinyl ) = 112.6(0.8)°, ∠(SiCC) = 124.6(1.8)°. Reasons for the preferred syn conformation in this and related compounds are discussed. The structure offers no evidence for any special interaction between the Si atom and the π-orbitals of the CC bond.

THE MOLECULAR STRUCTURE OF CYCLOPENTADIENYL VANADIUM TETRACARBONYL DETERMINED BY GAS-PHASE ELECTRON DIFFRACTION

The molecular structure of cyclopentadienyl vanadium tetracarbonyl, C5H5V(CO)4, has been determined by gas-phase electron diffraction at 95°C assuming local C4v symmetry for the V(CO)4 moiety and C5v symmetry for the C5H5V group. The relative positions of these two groups could not be determined unequivocally. Models with an eclipsed (torsion angle, τ(CVXC) = 0° where X is the centre of the cyclopentadienyl ring) or staggered configuration (τ = 9°) fit the experimental data equally as well as a model which allows for free rotation of the C5H5 ring with respect to the V(CO)4 group. The CH bonds were allowed to bend out of the plane of ∠VCO was allowed to be non-linear. For both the staggered and the eclipsed models ∠XCH = 171(19)° (CH bent away from the metal) and ∠VCO = 177(11)° (CO bent slightly towards the ring). The remaining parameter values are almost identical for both models and for the staggered model are: rg (VC(ring)) = 2.281(13) A, rg(VC(CO)) = 1.963(7) A, rg (CC = 1.405(6) A, rg(CO) = 1.135(4) A, ∠(OCVCO) = 75.4(2.0)°, R = 11.4%.

An evaluation of the use of a commercial scanner to obtain experimental data produced by gas-phase electron diffraction and recorded on photographic plates

Abstract A commercial scanner (Agfa Arcus II) has been used to retrieve electron-diffraction data from photographic plates. The data thus obtained from five different molecules has been analysed and the results compared with the original published data. Excellent agreement was observed between bond distances and amplitudes obtained from refinements on data collected from this scanner, a similar scanner and a micro-densitometer. It is planned to use the Agfa Arcus II scanner for future measurement of electron-diffraction intensity data from photographic plates.

Crystal structure of benzyltriphenylphosphonium pentachlorosulphido-tungstate(VI) and a study of the vibrational spectra of salts containing [WCl5Y]–(Y = O or S) ions

The salt [PPh3(CH2Ph)][WCl5S] is formed when WCl4S is treated with [PPh3(CH2Ph)]Cl (1 : 1 molar ratio) in Ch2Cl2 solution. Crystals are monoclinic, space group P21/c, with a= 10.572(9), b= 18.413(13), c= 14.250(11)A, and β= 104.20(8)°. The salt consists of discrete [PPh3(CH2Ph)]+ and [WCl5S]– ions with the W–S bond length being 2.132(13)A and the W–Cl lengths spanning the range 2.246(17)–2.461(12)A. The i.r. and Raman spectra of a series of salts containing the [WCl5Y]– anion (Y = O or S) have been recorded and are reported, together with an assignment of the a1 stretching modes.

The molecular structure of hexamethyldigermane determined by gas-phase electron diffraction with theoretical calculations for (CH3)3M-M(CH3)3 where M = C, Si, and Ge.

Gas-phase electron diffraction (GED) data together with results from ab initio molecular orbital calculations (HF and MP2/6-311+G(d,p)) have been used to determine the structure of hexamethyldigermane ((CH(3))(3)Ge-Ge(CH(3))(3)). The equilibrium symmetry is D(3d), but the molecule has a very low-frequency, large-amplitude, torsional mode (phiCGeGeC) that lowers the thermal average symmetry. The effect of this large-amplitude mode on the interatomic distances was described by a dynamic model which consisted of a set of pseudoconformers spaced at even intervals. The amount of each pseudoconformer was obtained from the ab initio calculations (HF/6-311+G(d,p)). The results for the principal distances (r(a)) and angles (angle(h1)) obtained from the combined GED/ab initio (with estimated 1sigma uncertainties) are r(Ge-Ge) = 2.417(2) A, r(Ge-C) = 1.956(1) A, r(C-H) = 1.097(5) A, angleGeGeC = 110.5(2) degrees, and angleGeCH = 108.8(6) degrees. Theoretical calculations were performed for the related molecules ((CH(3))(3)Si-Si(CH(3))(3) and (CH(3))(3)C-C(CH(3))(3)).

The molecular structure of diethyl cadmium determined by gas-phase electron diffraction

Abstract The molecular structure of diethyl cadmium (C2H5)2Cd, has been studied by gas-phase electron diffraction. The molecule was found to consist of a central linear CCdC fragment terminated at each end by a methyl group From consideration of the data it seems that there is a substantial degree of rotation about the CdC bonds Results from a model in which free rotation about the CdC bonds was assumed yielded the following values for the molecular parameters: rg(CdC) = 2.133(6) A, rg(CC) = 1.537(7) A, ∠CdCC = 115.81(1.1)°.

Oxygen, sulphur, and selenium abstraction from WCl4Y (Y = O, S, or Se) by triphenylphosphine. Crystal and molecular structure of tetrachlorobis(triphenylphosphine)tungsten(IV)

Excess of triphenylphosphine has been allowed to react with the series of compounds WCl4Y (Y = O, S, or Se). The products from each reaction have been analysed and subjected to spectroscopic studies which have shown that in each reaction abstraction of the chalcogen atom took place thus reducing tungsten(VI) to tungsten(IV) and yielding WCl4·2PPh3 and P(Y)Ph3. With Y = O a second product, WCl4O·P(O)Ph3·PPh3, was also isolated. The complex WCl4·2PPh3 crystallises in the monoclinic space group P21/n, with a= 9.605(8), b= 21.320(13), c= 9.313(8)A, β= 117.5(1)°, and Z= 2. The WCl4·2PPh3 molecules are centrosymmetric with two equivalent W–Cl distances [2.320(5) and 2.344(6)A] and a long W–P distance [2.629(6)A].

Infrared and electronic spectra of matrix isolated chalcogenide halides of tungsten(VI), WYX4(Y = S or Se; X = F, Cl, or Br)

Characteristic i.r. fundamentals have been observed for the six tungsten thin- and seleno-tetrahalides WSF4, WSeF4, WSCl4, WSeCl4, WSBr4, and WSeBr4, isolated as monomers in nitrogen matrices. The electronic spectra of these species have similarly been recorded, and the principal features assigned on the basis of vibrational fine-structure. The results are compared with data previously obtained for the tungsten oxotetrahalides.