chemical thermodynamics of neptunium and plutonium - U.S. ...

136 9. Neptunium group 17 (halogen) compounds and complexesThe excellent agreement between the second- and third-law analyses for the databy Chudinov and Choporov gives confidence to this study, which in addition measuredthe absolute values of the pressures, as distinct from the relative measurements byKleinschmidt et al. It may be noted that were the NpF 4 (g) molecule taken to havelower symmetry than tetrahedral (see below), the entropies and Gibbs energy functions,−[G ◦ m − H m ◦ (298.15 K)]/ T, would be increased, thus worsening the second- and thirdlawdisagreement from the study by Kleinschmidt et al. However, in view of theirappreciably lower second-law enthalpy of sublimation (which should be independentof cross-sections), and the uncertainties in the electronic contributions to the calculatedthermal properties of the vapour, the uncertainty in the selected value for the enthalpyof sublimation sub H(NpF 4 , 298.15 K) = (313.0 ± 15.0) kJ·mol −1has been increased substantially.This is identical to the assessed values for the enthalpy of sublimation of UF 4 (cr)(313 kJ·mol −1 ,[92GRE/FUG]), although this value should probably be reduced by2-3 kJ·mol −1 to correct for revised thermal functions for the gaseous actinide tetrafluorides(cf. the discussion in the next section). The derived enthalpy of formation ofNpF 4 (g) is thus f H ◦ m (NpF 4, g, 298.15 K) = −(1561 ± 22) kJ·mol −1This value (and the enthalpy of sublimation) are slightly different from those suggestedby [2000RAN/FUG], since a small previously undetected error in a regression equationin [70CHU/CHO] has been corrected.9.1.2.5.bStandard entropy and heat capacityKonings et al. [96KON/BOO] have recently studied the infrared spectrum of UF 4 (g)between 1300 and 1370 K. Based on this, and a re-analysis of the previously determinedgas electron diffraction data, they have demonstrated that the UF 4 (g) moleculealmost certainly has tetrahedral symmetry and that the entropy of sublimation calculatedfrom the vapour pressure measurements (see [92GRE/FUG]) is in substantialagreement with this model when the newly determined smaller stretching vibrationfrequency is used to calculate the thermal functions of UF 4 (g). These molecular parametersfor UF 4 (g) have thus been adopted for the NpF 4 (g) molecule, except for a smalldecrease in the M-F distance from 2.059 × 10 −10 mforUF 4 (g) to 2.05 × 10 −10 mforNpF 4 (g). The ground-state energy level was assumed to have a statistical weight of 6,and the higher electronic levels were taken to be the same as those suggested for thegaseous species UF − 4by Glushko et al. [82GLU/GUR]. The calculated values for theentropy and heat capacity of NpF 4 (g) at 298.15 K areSm ◦ (NpF −14, g, 298.15 K) = (369.8 ± 10.0) J·K−1·molCp,m ◦ (NpF −14, g, 298.15 K) = (95.3 ± 5.0) J·K−1·mol