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Title: Synthetic and thermochemical studies of tungsten halides.
Authors: Taylor, P.
Award date: 1975
Presented at: University of Leicester
Abstract: Stoichiometric quantities of tungsten oxide tetrafluoride and xenon difluoride interact in the melt to give stable white crystalline compounds, which have the compositions, XeF2.WOF4 and XeF2.2WOF4, by weight. The new adducts, which have been characterised in the solid state and in solution by Raman and 19F n.m.r. spectroscopy respectively, are best formulated as covalent structures containing Xe--P--W bridges. From the 19F n.m.r. spectroscopic investigation of the dissolution of XeF2.2WOF4 in SO2CIF, the first evidence for complex equilibria involving Xe--F--W and Xe--O--W bridged species has been observed. The proposed structure of the compound, XeF2oWOF4, has been borne out by an X-ray crystallographic study. The adduct is monoclinic (space group: P2I/c) with a=5.440, b=9.969 , c=I2.I66, B=92.I?, U=659.4, z=4, and Dc=4.5g cm-3. The crystal contains discrete FXeFWOF4 units in which the xenon atom is essentially linearly coordinated between two fluorine atoms. The terminal and bridging Xe-F distances are I.98 (0.03) and 2.04 (0.03) respectively. The distance between the tungsten atom and the bridging fluorine atom is 2.I8 (0.02) . The remaining four fluorine atoms and the oxygen atom, which is trans to the bridging fluorine atom, complete a distorted octahedral environment about the tungsten atom. From measurements of their respective enthalpies of oxidative hydrolysis at 25?C, the standard enthalpies of formation of KWF6, RbWF6, CsWF6, KMoF6, RbMoF6, and CsMoF6 have been determined as -53I.8 I.4, -534.6 2.I, -540.0 I.5, -496-I 2.2, -50I.8 I.9, and -506.3 I.7 Kcal. Mol-1. respectively. Values for the electron affinities of gaseous tungsten hexafluoride and molybdenum hexafluoride have been estimated as II7 I.I and I23.9 I.4 Kcal. Mol-1. The fluoride ion affinities of crystalline tungsten pentafluoride and molybdenum pentafluoride have been derived as II7.6 I.I and 98.8 I.4 Kcal. Mol-1. respectively at 25?C. The standard enthalpies of formation of K2WBr6, Rb2Wbr6, and Cs2WBr6 have been calculated via their enthalpies of alkaline hypochlorite hydrolysis at 25C to be -254.6 2.I, -264.4 I.4, and -270-8 I.6 Kcal. Mol-1. respectively. The sum of the first and second electron affinities of gaseous tungsten hexabromide has been assessed as 85.3 Kcal. Mol-1. and the sum of the first and second bromide ion affinities of crystalline tungsten tetrabromide has been estimated to be 6.3 Kcal. Mol-1. In addition, the standard enthalpies of formation of WF5(OMe), WF4(OMe)2, WF2(OMe)4, and WF5(Net2) have been determined as -4I8 3, -405 4, -363 4, and -380 4 Kcal. Mol-1. respectively. The decomposition reactions of these ternary compounds have been discussed with respect to the thermochemical results. The introduction of fluorides into tungsten incandescent lamps has been investigated. It has been shown that lamps coated internally with metaphosphate glass eliminate the reaction between fluorine and the quartz envelope. Attempts to eliminate or modify the attack of fluorine at the cooler regions of the filament by either physical or chemical methods have been ineffective. Temperature measurements along a tungsten filament in the absence and presence of tungsten hexafluoride has confirmed the existence of a tungsten fluorine regenerative cycle. Impure dipotassium hexafluorotungstate (IV) has been prepared by the action of potassium iodide on potassium hexafluorotungstate (V) at elevated temperatures. The compound has been characterised by infrared spectroscopy and by Debye X-ray photography. Attempts to prepare tungsten dioxide difluoride have been unsuccessful.
Level: Doctoral
Qualification: Ph.D.
Rights: Copyright © the author. All rights reserved.
Appears in Collections:Theses, Dept. of Chemistry
Leicester Theses

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