Revised estimates of the appropriate polarizabilities lead to the conclusion that dispersion forces are not the major factor responsible for the anomalous dissociation energy of Fz in the halogen series (F2< Clz >Br2 >Iz). Approximate calculations of the generalized Coulomb term in a simple valence-bond model, using pure p-bonds and no hybridization, suggest that the greater part of the anomaly lies in this term.
The infrared spectra for the di-and hexa-hydrates of uranyl nitrate, for anhydrous uranyl acetate, and for sodi~im zinc uranyl acetate hexahydrate have been obtained using the potassiun~ bromide pressed disk technique. Where possible, samples were prepared by freezedrying aqueous solutions of the appropriate compound and potassiu~n bromide. The uranyl nitrate hydrates gave spectra with pealcs characteristic of both nitrate and nitrato groups. For ~~r a n y l acetate the type of spectra obtained depended upon the ratio of ~lrarlyl acetate to potassium bromide in the freeze-dried mixture. I t is post~llated that, for relatively large ratios of uranyl acetate to potassium bromide, the carbonyl oxygen atoms of the acetate groups are chelated to the ~~r a n i u n~ atom. The spectrum of sodium zinc ~~r a n y l acetate hesahydrate is quite similar to that reported by Jones for sodi~un ~~r a n y l acetate. Using the 033 cm-' peal;, the Beer-La~nbert law \\.as obeyed for samples prepared by freeze-drying solutions containing uranyl nitrate and potassium bromide.
IN'TRODUCTIONThe authors have been interested in the infrared spectra of uranyl compo~incls and in the possible applicatioil to a quantitative determination of uraniunl by infrared spectrophotol~~etry. Various investigations have been made of the infrared spectra of uranium compoui~cls (1, 2, 3, 4, 5 , 6, 7). The repeated occurrence in the infrared spectra of ~~i -~~l y l compounds of a strong absorption band a t 10.87 p (920 cnl-I) led to the suggestion by Leconlpte and 'reymann (2) that this band coulcl be used for the identification of uranyl ions. For watel--soluble substances the potassium bromide clisl; technique (8, 9), combinetl with the freeze-drying of aqueous solutions containing both the substance whose spectrum is being investigated and potassium bromide, gives good quantitative results (10).
EXPERIIvIENTAL -1 pparatus and ReagentsThe spectra were recorded with a Perkin-Elmer model 21 double-beam infrared spectrophotometer equipped with a sodium chloride prism.The sample chamber of the freeze-drying apparatus mas a 250-mm vacuuin desiccator from which the ground-glass sleeve had been removed. The desiccator was connected to a I-liter trap by means of a 55/40 standard taper ground-glass joint and 40-mm I.D. glass tubing. A length of rubber tubing and a 18/7 ball and soclcet joint were used to connect the trap to a high vacuum system. A freezing mixture of dry ice -acetone was used t o cool the trap.The die used to prepare the potassium bromide dislts was of the type described b17 Bauer, Epp, and Lemieux (11). The disk holder is removable and fits into an adapter which can be inserted into the standard sample slide 011 the spectrophotometer. A 20-ton hydraulic press was used to apply pressure to the die.Reagent grade chemicals were used. Sodiu~n zinc uranyl acetate was prepared froin zinc uranyl acetate and sodium chloride solutions using the method ol Barber and IColthoff (12).
Sample PreparationThe freeze-dried i-riixtures were prepared by nl...
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