Gaseous electron-diffraction investigations : I. Development of high-temperature nozzles : II. Molecular structure and barrier to internal rotation of diboron tetrafluoride and diboron tetrabromide : III. Molecular structure and composition of 2,3-butanedione and oxalyl chloride at high temperature Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/g732dd73m

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  • The design and construction of two new sample injection systems or nozzles, for use with the Oregon State electron diffraction apparatus are discussed. One of these nozzles was intended for use with volatile samples the vapors of which were to be heated or cooled over the range -196 to 540°C; it is currently in regular use. The other nozzle, equipped with a sample volatilizing oven, was intended for use with relatively involatile materials, that is, materials having vapor pressures of only a few torr in the range 150 to 1200°C. Only a prototype of this nozzle was constructed. The structures and particularly the torsional properties of the molecules diboron tetrafluoride, diboron tetrabromide, biacetyl (2,3- butanedione), and oxalyl chloride (ethanedioyl dichloride) have been investigated, For B₂F₄ nozzle-tip temperatures of -50, 22, and 150°C were used. In contrast to B₂Cl₄ which is staggered in the gas phase, the B₂F₄ molecule has a coplanar equilibrium conformation (symmetry D₂h). The effect of temperature change on the amplitude of torsional motion is commensurate with a description of this motion as slightly hindered internal rotation. The average value for the rotational barrier V₀ is 0.418 (156) kcal/mol based on an assumed hindering potential of the form 2V = V₀(1-cos 2o). An estimate of the fundamental torsional frequency gives 20(4) cm⁻¹. For B₂Br₄ nozzle-tip temperatures of 23, 90, 150 and 305°C were used. The molecule is like B₂C1₄ in that it has a staggered equilibrium conformation (symmetry D₂d). The average of the rotational barrier for the four temperatures based on the assumed potential function for hindered rotation V = V₀(1 - cos 2o) is V₀ = 3.07(.33) kcal/mol. higher than that for B₂F₄ or B₂C1₄. The estimated value for the torsional frequency is 18(4) cm⁻¹. An earlier electron diffraction investigation of biacetyl at a nozzle-tip temperature of 228°C revealed only a trans conformer, in contrast to certain similar conjugated systems which have substantial amounts of gauche rotameric forms as well. New experiments at 525°C have been carried out with the same result: there is no evidence for the presence of any but the trans conformer. The structure and conformational properties of oxalyl chloride were studied by reanalyzing the data from experiments at 0, 80, and 190°C obtained from an older investigation together with new data taken from experiments at 405 and 525°C in terms of a more sophisticated model, The experiment at 525°C resulted in essentially complete decomposition into, apparently, phosgene and carbon monoxide. In agreement with the former study, all evidence still indicates the gas to be a mixture of trans and gauche conformers. Making the approximation that the difference between the trans (torsion angle o) = 0⁰) and gauche conformers lies only in the torsion angle about the C-C bond, the average gauche torsion angle over all four temperatures is og = 93.5(8.2)⁰. The conformational analysis was based on an assumed rotational potential function of the form 2V = ³∑i=₁Vi(1 - cos io). The average values of the three potential constants were found to be V₁ = 1.94(31), V₂ = -0.53(.24), and V₃ = 0.70(12) kcal/mol. Equilibrium constants were evaluated from the temperature dependence of the composition with the energy difference ΔE° = Eg° - Et° = 1.72(σ=0.29) kcal/mol and entropy difference ΔS° = Sg° - St° = 1.8(σ=0.8) cal/deg begin subsequently obtained.
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