J. PhyS. Chem. 1981, 85, 1274-1276
1274
where Tl,d_d and Tl,s-rare the spin-lattice relaxation times
due to dipole-dipole and spin-rotational interaction, respectively. An important distinction between the two relaxation times is that Tl,8-r decreases with temperature contrary to T1,d-d. When the contribution of the spinrotational interaction is of the same order as that of the dipole-dipole one, the T1-temperature curve is convex. For benzene derivative^'^ the spin-rotational interaction plays an important role in their relaxation and the T1temperature curve is convex, because they are symmetric molecules with high *-electron density. The Tl-temperature relation of styrene in the Ll region is convex and it is argued that the molecules move with the character of the benzene derivative. In the L2 region, however, the styrene monomers lose the characteristic molecular motion of the benzene derivatives. As shown in Figure 3, the T1-temperature relation of ethylbenzene is not convex. The reason is that the intramolecular motion of the ethyl group enlarges the contribution of the dipole-dipole interaction in ethylbenzene. In the L2 region of styrene, it appears that the contribution of the spin-rotational interaction to Tl vanishes. This result may be considered to mean that L2 is the state where rotational freedom of molecular motion is suppressed and a few styrene molecules are associated with each other as a cluster. The cluster moves like one large molecule. With increasing pressure, the clusters become larger and Tl becomes smaller. The liquid state where the rotating freedom of molecular motion is suppressed is associated with the nematic phase of liquid crystals. In this point, the novel liquid phase of styrene can be regarded as a mesomorphic state similar to liquid crystals. It is clear that the planar structure of the styrene including the vinyl group takes part in the formation of the ordered phase, because ethylbenzene has no anisotropic liquid phase corresponding to the L2 state. Two or more molecules of styrene at high pressure cohere due to the anisotropic electric dispersion force of the double bond of vinyl group.
(13) Farrar, T.C.;Becker, E. D. “Pulse and Fourier Transform N M R ; Academic Press: New York, 1971.
(14) Green, D. K.; Powles, J. K. Proc. Phys. SOC.1965,85,87. Powles, J. G.; Figgins, R. Mol. Phys. 1966, 10, 155.
7.51
BO 120 160 180
solid
0 -30
, -b-,I
-20
-10
0
I
1
I
10
20
30
Temp. I OC I
Flgure 6. T,-temperature Isobar relation of styrene. The results shown by the open circles (0)are from T,-pressure measurements and the closed circle (0)shows results measured at atmospheric pressure. The point @ denotes a triple point.
For styrene and ethylbenzene, the proton spin-lattice relaxation process consists of two parts. These are a dipole-dipole interaction between protons, and a proton spin-rotational intera~tion,‘~ and T1 is expressed by the following equation: (TI)-’ = (Tl,d-d)-’
+ (Tl,s-J-’
Gas-Phase Infrared Spectra of Bis( 1,I,I,5,5,5-hexafluoropentane-2,4-dionato)[ ‘80]dioxouranium(VI) A. Ekstrom,’ H. J. Hurst, C. H. Randall, and H. J. Loeh Chemical Technology Dlvlslon, Australian Atomic Energy Commission Research Establlshment, Sutherland, New South Wales, Australla (Received: November 13, 1980)
The gas-phase infrared spectra of the monomeric and dimeric forms of the title compound (UOz(HFA)z),labeled with l80in the uranyl group, have been determined. The monomeric form showed three absorption bands attributable to the v3 frequency of the 16/16,16/18,and 18/18 UO$+ groups, while the dimeric form showed seven resolvable bands. These results appear consistent with an asymmetricdimer structure in which one uranyl oxygen atom acts as a neutral ligand to the other uranyl moiety. Introduction U02(HF& has been shown to have a unique trimeric structure in the solid statel and to exist in the gaseous phase as a mixture of the monomeric and a dimeric form.2 (1) Taylor, J. C.;Ekstrom, A.; Randall, C. H. Inorg. Chern. 1978,17, 3285. 0022-3654/81/2085-1274$01,25/0
Recent gas-phase infrared studies3p4showed that the monom& form had a single a s m e t r i c stretching frequency at 967 cm-’ while the dimeric form had two bands, at 963 (2) Ekstrom, A.; Randall, C. H. J. Phys. Chem. 1978,82,2180. (3) Cox, D.M.; Maas, E. T. Chern. Phys. Lett. 1980, 71, 330. (4) Ekatrom, A,; Hurst, H. J.; Randall, C. H.; Loeh, H.J. J. Phys. Chem. 1980,84, 2626.
0 1981 American Chemical Society
The Journal of Physical Chemistty, Voi. 85, No. 9, 198 1
Gas-Phase Infrared Spectra of U02(HFA)2-’8U
1275
TABLE I: Summary of Frequencies Calculated for the Isotopically Substituted Asymmetric Dimer of UO,(HFA),a
I 800
930
I 910
32C
,
I
I
93C
QIC
“50
I
I
06‘
cw-’
Flgure 1. Infrared spectra of U1802(HFA)2obtained under various conditions: (upper curve) spectra obtained with N20 laser, 2-m cell, sample temperature of 168 OC,and sample pressure of 45 Pa; (lower curve) as above, but sample pressure of 18 Pa; (broken line) spectra obtalned with PE-225, 1.2-m cell, 168 O C , and 80-Pa sample pressure.
and 935 cm-’. Two alternate structures for the dimeric form have been suggested. In one? the two uranyl groups form a symmetric structure in which one uranyl oxygen atom from each uranyl group is bonded to the adjacent uranium atom. In the other: only one uranyl oxygen atom is bonded to the other uranium atom, giving an asymmetric structure. In this paper, we report spectra of UO2(HFAI2 labeled with lSO in the uranyl group, which provide additional evidence in support of the latter structure. Experimental Section ls0-1abeled U02(HFA)2 was prepared by reacting U1802F2with the stoichiometric quantity of H(HFA) in distilled ether to yield the U1802(HFA)2-ether adduct which was converted to pure U1802(HFA)2as described el~ewhere.~This method had been reported by us to be unsuccessfu1,4but, in the present experiment, the U1802F2 used was obtained by reacting UF6 with a large excess (X4) of H280, and the U1802F2so obtained was very carefully dehydrated by heating under vacuum. It is suspected that this modification of the previous technique minimizes the formation of a U02F2-HF adduct which does not react with H(HFA) to give the desired product. The handling of the hygroscopic U1802(HFA)zand the apparatus used for the determination of the infrared spectra have been described p r e v i ~ u s l y . ~ ~ ~ Results and Discussion Typical spectra of U1802(HFA)2in the range 970-880 cm-l obtained at various pressures and temperatures with a N20/C02CW laser and a 2-m cell and with a PerkinElmer 225 spectrophotometer and a 1.2-m cell are shown in Figure 1. A careful examination of the effects of pressure and temperature on the relative intensities of the observed bands suggested that the 967-, 948-, and 919-cm-’ bands could be attributed to the monomeric form of the molecule, while the 963-, 945-, 935-, 931-, 915-, 897-, and 886-cm-’ bands were associated with the dimeric form. Very weak bands were found at -880 cm-’ (partly obscured by the 886 band) and a t 848 cm-l. The resulting spectra are thus relatively complex, but the following assignments may be suggested. Monomer Bands. The three monomer bands are obviously assigned to the 16/16,16/18, and 18/18 forms of the molecule. The isotope shifts observed for the 16/18 (19 cm-’) and the 18/18 (48 cm-’) species relative to the 16/16 form are, as expected, very similar to those observed (5) Ekstrom, A.; Randall, C. H.; Loeh, H. J.; Taylor, J. C.; Szego, L. Znorg. Nucl. Chem. Lett. 1977, 14, 301.
O(1) O(2) O(3) O(4) vasym urn 16 16 963 (963)b 16 18 946 (945) 843 (848)b 18 18 915 (915) 16 16 935 (935) 884 (880) 931 (931) 840noC 16 18 900(897) 863noC 18 16 836noC 18 18 8 8 6 ( 8 8 6 ) Values in parentheses a See i for number of 0 atoms. are the observed frequencies. CThesebands were not observed but are expected to be very weak.
previously for the tetrahydrofuran6 trimethyl phosphate4 adducts of U1802(HFA)2. Dimer Bands. The previous study4 of the infrared spectra of isotopically normal dimeric UOz(HFA)zindicated the dimer structure i. Random le0substitution of 0)
8
12)
i
the uranyl oxygen atoms in this molecule will give rise to 12 possible species, and a complete calculation of the vibrational frequencies is not possible without additional information such as bond distances and Raman spectra. However, as already i n d i ~ a t e dthe , ~ problem may be simplified by treating the two uranyl groups separately, group 1 being a simple linear uranyl group to which is attached a neutral ligand and which is thus essentially identical with other UOZ(HFA)z-Lewisbase adducts. Group 2 can then be regarded as a linear four-atom molecule W-Z-U-X, where Z and X are 0 atoms 3 and 4, and W represents the uranyl group 1. The v3 16/16 frequency of group 1occurs at 963 cm-’ (ref 3 and 4), and the v3 frequencies of the 16/18 and 18/18 forms can be calculated’ as 945 and 915 cm-l, while the position of the v1 band of the 16/18 form is calculated as 843 cm-l (Table I). These bands can be readily located in the observed spectra. The v3 frequency of the isotopically normal group 2 was previously found4 a t 935 cm-l. A fit of the calculated frequencies of the isotopically labeled species to the observed bands (Table I) was obtained by treating these species as linear four-atom molecules8with force constants K1 = 6.79, K z = 6.49, and K 3 = 1.21 m dyn Awl. Initial estimates of the values of K1and K z were obtained by the application of Jones’ ruleg to the uranyl bond distances found’ in the crystal structure of trimeric UOz(HFA)zand then further refined until reasonable agreement was obtained between the observed and calculated frequencies. Two other possible dimer structures which can be con~ i d e r e dare ~ ? the ~ symmetric uranyl oxygen-bridged molecule ii and the ligand-bridged form iii in which several forms of bridging may occur.( A simple analysis of the vibrational modes of these molecules readily shows that the isotopically normal molecules will have two asymmetric stretching frequencies similar to those observed only if (6) Cox, D. M.; Hall, R. B.; Horsley, J. A,; Kramer, G. M.; Rabinovitz, P.; Kaldor, A. Science 1979,205, 390. (7) Steinfeld, J. I. “Molecules and Radiation”;MIT Press: Cambridge MA, 1978; p 181. (8)Herzberg, G. “Infrared and Raman Spectra of Polyatomic Molecules”; Van Nostrand New York, 1945; p 185. (9) Jones, L. H.Spectrochim. Acta 1969,12,409.
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The Journal of Physical Chemistry, Vol. 85, No. 9, 1981
0-u
ii
u-u
II
I I I 0I 0
0
iii
there is significant interaction between uranium atoms. This interaction, which is equivalent to introducing cross terms into the potential-energy matrix of two coupled oscillators, gives rise to 20 vibrational frequencies for structure ii, which has 10 possible isotopic forms. Similarly, structure iii, which has seven isotopic forms, should give 14 such frequencies. The number of vibrational frequencies observable will be reduced by overlap between bands for both structures,
Additions and Corrections
but it appears from model calculations that the observed spectra of either of the above two molecules should be much more complex than they are found to be. It is therefore concluded that on the available evidence the observed spectra are best interpreted in terms of dimer structure i. While this assignment cannot be conclusive in the absence of further data such as Raman spectra, the formation of this molecule from the trimeric solid is readily achieved by the removal of one U02(HFA)2molecule from the latter and does not require the extensive molecular rearrangement implied by structures ii and iii. Furthermore, it is not certain that structure ii in particular is sterically possible.
Acknowledgment. We thank Mr. A. B. Waugh for the preparation of the U1*02F2used for the preparation of U1802(HFA)2and Mr. R. B. Adams for his assistance.
ADDITIONS AND CORRECTIONS 1980, Volume 84
S. H. Bauer and Nancy S. True: Kinetics of the Syn Anti Isomerization of Methyl Nitrite. Page 2507. The above article was attributed to Bauer and True. As I did not closely participate in the preparation of the final version of this manuscript, I request removal of my name.-N. S. True, University of California, Davis.
