J. Phys. Chem. 1982, 86. 1921-1922
These are mixtures of the inner yalence orbitals, 3bl, 5a1, la2 of (CH3)20 and the 2e' orbital of BF3. The precise correspondence is not as clear as that for the outer valence orbitals, but a suggested distribution into two plus three MOs is given by the calculations (Table II). The last peak which is a quite sharp occurs at 19.9 eV and is the ionization from the loa' orbital which contributes a weak u bond between the boron and the three fluorine atoms with considerablyB2S character. It derives from the 2a' orbital of BF, which has been observed at 21.5 eV in a He I1 PE spectrum2* and was predicted to be particularly sharp.24 The stabilization of 1.6 eV is in accord with the general trend.
Conclusion The formation of a 1:l electron transfer complex, (CH3)20.BF3,has been studied by a combination of He I PE spectroscopy and photoionization mass spectrometry. Since this is a weak complex ita formation is promoted by the adiabatic expansion of the constituent gases through a nozzle inlet. The He I P E spectrum of the complex is extracted using a spectrum stripping procedure. The geometry has been studied using the INDO, MNDO and Gaussian 70 (STO-3G) SCF methods. The results indicate that the geometry of ref 15 is more resonable than (26) W. C. Price, A. W. Potts, and D. G. Streets, "Electron Spectroscopy", D. A. Shirley, Ed., North Holland, Amsterdam, 1972, p 187.
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that of the earlier electron diffraction work, and so this structure has provided the basis for electronic structure calculations at the 4-31G level. The Koopmans IP's are, in general, in exellent agreement with the experimental values and assist in estimating the relative shifts of the MO's of each constituent upon complexation. Thus, the Mo's of (CH3)20are stabilized by 0.7-2.8 eV; the first IP, by 2.35 eV. The MO's of BF3 are destabilized by 1.4-3.3 eV. These results are a reflection of the transfer of electronic charge from (CH&O to the empty pn orbital of BF3 the SCF calculations indicating a transfer of 0.09 electron. This is not large, indicative of the weak nature of the complex compared to, say, the (CH3)3N-BF3complex, which can be isolated as a solid and where the first IP shifts by 3.74 eV.3 We now anticipate extension of this work to even weaker complexes such as van der Waals molecules. During the concluding stages of this work we found that a molecular structure conference2' included an abstract reporting a similar study. Although specifics were limited, the results appear to be essentially the same as those described herein. Acknowledgment. We are grateful to the Natural Sciences and Engineering Research Council of Canada for their Financial support. (27) K. Nomoto, Y. Achiba, and K. Kimura, Abstract of Molecular Structure Conference, Japan, 1980, p 638.
COMMENTS Spiltting of the Luminescent Excited State of the Uranyl Ion
Sir: Brittain and Perry have reported the luminescence spectra of some uranyl compounds1 and conclude the following: "The data can best be explained by invoking a split excited state and arguments are presented which indicate that this splitting is due to a descent in symmetry experienced by the uranyl ion when it is placed in a crystal field". The splittings reported lie in the range 53-93 cm-'. In recent years Denning and co-worker~~?~ have developed a highly successful model of the electronic structure of the uranyl ion and we have used this model to interpret the luminescence spectra of a variety of uranyl compounds.4-s In this model the lowest excited state is IIg.(u6), the degeneracy of the state being preserved at sites of C3 ( z parallel to O - U a ) and higher symmetry, but even in lower symmetries the splittings of the excited states are expected (1) Brittain, H. G.; Perry, D. L. J.Phys. Chem. 1981,85, 3073. (2) Denning, R. G.; Snellgrove, T. R.; Woodwark, D. R. Mol. Phys. 197pI. 419. --._,32. --I
(3) Denning, R. G.; Snellgrove, T. R.; 1979, 37, 1089, 1109.
Woodwark,D. R. Mol. Phys.
(4) Flint, C. D.; Tanner, P. A. J . Chem. SOC.,Faraday Trans 2 1978, 74, 2210. ' (5) Flint, C. D.; Tanner, P. A. J. Chem. SOC., Faraday Trans. 2 1979, 75, 1168. (6) Flint, C. D., Tanner, P. A. Mol. Phys. 1981, 43, 933. (7) Flint, C. D.; Tanner, P. A. Mol. Phys. 1981,44, 411. (8) Flint, C. D., Tanner, P. A., J.Chem. Soc., Faraday Trans. 2 1981, 77, 1865. 0022-365418212086-192 l$Ol.25/0
to be zero in the first order of the equatorial perturbati~n.~ The position is particularly clear for RbU02(N03)3where the ion occupies a D3 sitegJOso that a lIgstate must be unsplit but Brittain and Perry report an excited state splitting of 77 cm-l. If correct this would cast serious doubts on Denning's model which was derived from a most careful spectral study of CsU02(N03), (which is structurally similar to RbU02(N03),). We argue that the analysis presented in ref 1 in incorrect and illustrate our argument by reference to our data on RbU02(N03)3. However, similar considerations apply to the other compounds reported in ref 1. The general appearance of the low-temperature luminescence spectrum of RbU02(N03)3 It consists is similar to that of other uranyl of an electronic origin accompanied by a large number of vibronic origins corresponding to lattice modes and the vibrations of the U02(N03)3entity. The uranium oxygen distance of the UO?+ grouping is appreciably longer than in the ground state so that Franck-Condon progressions in the uranium-oxygen symmetric stretching mode (vl) appear based on all these electronic and vibronic origins. However, the antisymmetric uranyl stretching mode v2 (and some of the nitrate modes) have higher wave number than v1 so some overlapping of spectral features occurs. As detailed below it was the failure to recognize this over(9) Hoard, J. L.; Stroupe U S . Atomic Energy Commission, TID-5290, Vol. 1, 323, 1958. (10) Sharma, P., Thesis, University of London, 1981.
0 1982 Amerlcan Chemical Society
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The Journal of Physlcal Chemkby, Vol. 86, No. 10, 1982
Additions and Corrections
TABLE I: Wave Numbers and Assignments of
I
,
495
,
