2026
J. Phys. Chem. 1980, 84, 2626-2630
(10) R. Shimada and L. Goodman, J . Chem. Phys., 43, 2027 (1965). (11) I.Bernal, Nature(London),200 (1913); 1318 (1963). (12) F. F. Bentley, L. D. Smithson, and A. L. Rozek, "I.R. Spectra and Characteristics Frequencies, 700-300 cm-'", Interscience, New York, p 71. (13) S. S. Mitra and H. J. Bernstein, Can. J . Chem., 37, 554 (1959). (14) D. Kearns and W. Case, J . Am. Chem. SOC.,88, 5087 (1966).
(15) R. S. Becker, "Theory and Interpretation of Fluorescence and Phosphorescence", Wiley-Interscience, New Yak, 1969, pp 157-158, 162. (16) A. Becket, A. D. Osborne, and G. Porter, Trans. Faraday Soc., 60, 873 (1964). (17) D. S. Roy, K. Bhattacharya, S. C. Bera, and M. Chowdhury, Chem. Phys. Lett., 69, 134 (1980).
UV-Visible and Infrared Spectra of Volatile Uranyl Complexes in the Gas Phase A. Ekstrom," H. J. Hurst, C. H. Randall, and H. Loeh Chemical Technology Division, Australian Atomic Energy Commission Research Establishment, Lucas Heights, Sutherland, NSW, Australia (Received: March 3, 1980)
The gas-phase UV-visible and infrared spectra of bis(1,1,1,5,5,5-hex~uoropentane-2,4-dionato)dioxo~ani~(~) (U02(HFA),)were found to exhibit a marked pressure and temperature dependence which appears consistent with the monomer-dimer equilibrium previously established for this compound. The visible spectra obtained for both the monomeric and dimeric species are unusual because the vibrational structure characteristic of the spectra of many uranyl compounds is completely absent. The IR spectra, which were in part obtained with a CW N20/C02laser spectrometer, showed three bands attributable to the asymmetric stretch (v3) of the uranyl group. A study of the pressure and temperature dependence of the intensity of these bands indicated that two could be assigned to the dimeric form and the third to the monomeric form. These observations permit the assignment of a probable structure to the dimeric form of U02(HFA)2.Some preliminary observations on the spectra of the trimethylphosphate derivative (UO,(HFA),TMP) of U02(HFA)2are also described.
Introduction Although U02(HFA)2has a trimeric structure in the solid state,l the compound is relatively volatile and exists in the gaseous state as a mixture of the monomeric and dimeric form^.^^^ The formation of U02(HFA)2,presumably in the monomeric form, during the laser irradiation of uranyl complexes such as U02(HFA)2THF(THF = tetrahydrofuran) has recently been reporteda4s5This paper describes some of the spectroscopic properties of this compound, which suggest a plausible structure for the dimeric form. Experimental Section U02(HFA)2and U02(HFA)2TMP6were prepared and UOzhandled as described p r e v i ~ u s l y . ~ 180-labeled *~ (HFA)2TMPwas prepared by reacting H2180(Yeda Pty Ltd) with UF6 to form U1802F2which was carefully dried and then reacted with the stoichiometric quantities of H(HFA) and TMP dissolved in hexane to yield U02(HFA),TMP in approximately 40% yield. Although slow and not very efficient, this method is very economical in its consumption of H2l80but was completely ineffective for the preparation of l80-1abeledU02(HFA)2.The reasons for this are not clear, but possibly the long reaction times led to the hydrolysis of the complex. Vapor pressures were obtained by interpolation of published vapor pressuretemperature results.2 The UV-visible spectra were obtained by using a spectrophotometer designed for the purpose and comprising a Jarrell-Ash Model 45-548 light source with interchangeable tungsten and deuterium light sources and a Jarrell-Ash Model 82-410 0.25-m monochromator. The tuned output radiation (band-pass 0.24 nm) was split into sample (l)and reference (I,) beams whose intensities were measured with two photomultipliers (EM1 62568). Their outputs were processed by Teledyne Philbrick operational amplifiers and logarithmic elements to give the absorbance (log ( I o / l ) )which was displayed, as a function of wavelength, on a chart recorder. 0022-3654/80/2084-2626$0 1.OO/O
The absorption cell (either 0.5 or 1.0 m long) consisted of a glass tube (5 cm diameter) and quartz windows. The cell was enclosed with a heating jacket and fitted with heatable stainless steel valves which led to a high-vacuum system for the evacuation of the absorption cell and with an independently heatable sample reservoir. The whole system was enclosed in a light-tight box, and spectra were obtained as the difference between the absorbance readings of the empty and full cell. IR spectra over an extended frequency range were obtained by using a PE225 recording spectrophotometer and the multipass cell (Figure l),which was constructed for this instrument and aligned with a He/Ne laser. The sample beam traversed the cell 12 times, giving an effective path length of 1.4 m. The empty cell transmitted approximately 40% of the incident radiation. IR spectra in the range 900-980 cm-' were also obtained by using a laser spectrophotometer which consisted of a line tuneable CW C02/N20laser having a discharge length of 3 m. The chopped laser beam was reflected at shallow angles by partially reflecting 10-m concave and plane mirrors and divided into reference and sample beams by a beam splitter. The sample beam passed through the absorption cell, and the transmitted beam intensity (I)was measured with a pyroelectric detector (Molectron, P3), A similar detector simultaneously recorded the intensity (lo) of the reference beam. The outputs of the two detectors were passed into two phase-sensitive amplifiers (Keithley 840 Autoloc), and the outputs of these converted to the form log ( I o / I )by using analogue ratioing modules and logarithmic units (Teledyne Philbrick). The values of log ( I o / l )and a wavelength marker (Io)were recorded on a twin pen chart recorder. Spectra were obtained from the difference of the values of log (Io/l)with the absorption cell filled with a sample at the appropriate temperature and pressure and the empty cell at the same temperature. The absorption cell consisted of a glass tube (2 m X 5 cm diameter) fitted with salt windows at the Brewster angle and attached to the cell with Vacseal. The cell was 0 1980 American Chemical Society
The Journal of Physical Chemistty, Vol. 84, No. 20, 1980 2627
UV-Visible and IR Spectra of Uranyl Complexes
PLAN VIEW
I
'LIGHT PATH
U
U
Figure 1. Diagram of 1.4-m path length cell for PE225 recording spectrophotometer: (A) aluminum block; (B) adjustable base plate; (C) stainless steel bellows valves; (F) metal-glass joints; (G) glass cell; (H) heaters; (I) asbestos insulation; (P) glass plate supporting mirror systems; (S) adjustable stainless steel rod controlling distance between mirrors M; (T) external mirrors; (W) sodlum chloride window attached to cell with Vacseal.
0
enclosed by am oven (2.5 m X 35 cm X 35 cm) fitted with heaters, an air circulating fan, and suitably positioned salt windows for the entrant and exit beams to reduce the thermal gradient in the oven. The apparatus was fully instrumented with automatic temperature control. Stainless steel valves, fitted internally, connected the cell to a high-vacuum system and to an external, and independently heatable, sample reservoir. This arrangement permitted the independent variation of both the sample pressure and the absorption cell temperature.
Results and Discussion UV-Visible Spectra. A comparison of the spectra obtained for U02(HFA)2and U02(HF'A)2TMP(Figure 2), under approximatelysimilar conditions of temperature and pressure, illustrates the striking difference observed in the visible spectra of these compounds. While the spectrum obtained for lJ02(HFA)2TMPshows the vibronic structure typical of the uranyl ion in a variety of environments? the spectrum of U02(HFA)2shows a featureless band extending to 5/50 nm. The spectrum obtained for U02(HFA)2TMPiin the gas phase was unaffected by variations in the sample pressure and temperature and was virtually identical with that obtained when the compound was dissolved in nonpolar solvents such as hexane. In contrast, the gas-phase1 spectrum of U02(HFA), was found to depend significantly on sample pressure and temperature, the general eiffect at constant temperature being an increase in the effective absorption cross section8 with increasing sample pressure at any wavelength. This effect appeared consistent with previously observed2dimerization of U02(HFA)2. To obtain tlhe absorption spectra oE the monomeric and dimeric forms, the absorption cross section of U02(HFA)2 samples was measured in the range 500-230 nm at five sample pressures in the range 6.2-73.6 Pa at a constant temperature of 180 'C. At any wavelength, the observed cross section ((E)is given by eq 1, where EM and EDare E = EM($)+ ED(l-- X ) (1) the absorption cross sections of the monomeric and dimeric species at that, wavelength and x is the fraction monomer present at the particular sample pressure which could be calculated from the known2equilibrium constants for the
nm
Flgure 3. Resalved visible spectra of monomeric (broken line) and dimeric (solid line) UO,(HFA), at 180 OC.
dimerization reaction. The available data thus yielded five equations similar to eq 1 for various values of x, which could be solved by least-squares methodsgto yield values of EMand EDat any wavelength. The results obtained for the visible region are shown in Figure 3, while those obtained in the ultraviolet are summarized in Table I. The latter data show the spectra of dimeric U02(HFA)2and UO,(HFA),TMP to be similar, dimeric U02(HFA)2having an intense band at 354-364 nm, which appears as a shoulder in monomeric UO2(HFA)* The full implications of these results on the current understanding7 of the spectra of the uranyl group are difficult t o assess, but it would appear that chemical environment of the uranyl group in both the monomeric and dimeric form would be quite unusual. Infrared Spectra. The gas-phaseIR spectra determined with the 1.2-m cell and the PE225 spectrophotometer appeared typical of those obtained for other metal-HFA derivatives, and the majority of the observed bands can be readily assigned (Table 11). Only minor differences are found when the spectra of UO2(HFAI2 and U02-
2628
The Journal of Physical Chemistry, Vol. 84, No. 20, 1980
Ekstrom et al.
TABLE I: Summary of the Ultraviolet Absorption Bands of UO,(HFA), and UO,(HFA),TMP at 180 "C absorption peak wave- 10'8(absorption length, cross section), nm cmz molecule-'
comvd UO,(HFA), (monomer)
UO,(HFA), (dimer) UO,(HFA),TMP
a
355 (shIa 300 (sh) 270 364 315 285 3 54 300 27 5
8.6 15.4 21.5 27.8 31.4 33.0 19.9 24.2 23.0
sh = shoulder.
TABLE 11: Summary of Infrared Spectra of UO,(HFA), and UO,(HFA),TMP" ._
~
UO,(HFA),, cm- '
UO,(HFA),TMP, cm-'
1645 (s)"
1645 (8) 1640 (sh)
u(C=O)
1565 (w) 1535 (w) 1455 (6) 1260 (s) 1215 (s) 1165 (m) 1148(m) 1105 (m) 1065 (m)
u(C=C), 6 (C-H)
1620 (s) 1565 (w) 1535 (w) 1430 (s) 1260 (8) 1225 (s) 1185 (s) 1145(m) 1105 (m) 966 (m) 935 (m) 880 (vw) 815 (m) 745 (m) 663 (m)
assignmentb
Figure 4. Laser spectrometer scans over the range 920 to 980 cm-' of UO,(HFA),: (0)24.5 Pa, 165 O C ; (0) 22.1 Pa, 210 OC.
Y
?
i
?
u(C=O) ?
u(C-C), u(C=C) u(CF3) ?
6(C-H) TMP peak u3(u02)
954 (m)
870 (m) 838 (vw) 805 (m) 770 (w) 745 (m) 660 (m)
u3(u02)
VI(UO,)c TMP peak U~(UO,)~ 6(CF,) TMP peak u(C-CF,) ?
a s = strong, sh = shoulder, m = medium, w = weak, vw = very weak. Assignments were made following ref 10. e Observed only at high sample pressures and low sample temperatures, Le., conditions favoring dimerization. Observed only in IsO-labeled compounds (see text).
(HFA)2TMP are compared. However, while UOz(HFA)2TMP shows only a single, sharp asymmetric (vJ uranyl stretch band at 954 cm-l, U02(HFA)zshows, at this resolution, two bands, located at 935 and 965 cm-l, whose relative intensities changed significantly with variations in both sample pressure and temperature. An interesting aspect of the spectrum of U02(HFA)2was the appearance, at high sample pressure (60 Pa) and low temperatures (165 "C)
