Infrared spectra of matrix-isolated uranium oxide species. II. Spectral interpretation and structure of UO3

Abstract

The midinfrared spectra of matrix-isolated UO3 and its oxygen-18 isotopes have been successfully interpreted in terms of a C2ν T-shaped structure. Values of the two bond-stretching and two bond-interaction force constants determined by a normal coordinate analysis using 13 observed frequencies reproduce all peaks in the observed spectrum to within ± 0.6 cm−1. The following assignments for U16O3 species trapped in argon matrices have been obtained: ν1(A1) = 843.50, ν2(A1) = 745.65, ν4(B1) = 852.60 cm−1. The structure and intensity distribution are discussed in terms of current bonding theory for uranyl compounds.

Infrared Absorption Study of Uranium Trioxide Phases

“…If the U0 3 molecule is approximated as uranyl monoxide (U0 2+ -O 2 "), then the linear structure of the uranyl ion, would result in T-shape for U0 3 . This was confirmed by GABELNICK et al [13] who stated that the linearity of the uranyl ion can be inferred from the absence of the symmetric stretching frequency Vy at 843.5 cm" 1 for the υ 16 θΓ and υ 18 θΓ isotopes in their spectra.…”

“…The shortest U-O bond distance was found to be 1.796 Â. WAIT [11][12][13][14] 750cm" 1 Fund., F\ u , U-0 stretch. [8].…”

“…These results were explained to indicate that U0 3 has a C 3v or lower symmetry (perhaps C 2v ). Finally, GABEL-NICK et al [13,33] proved that U0 3 has a C 2 ", T-shaped equilibrium geometry, which is much less common for MX 3 -type molecules than the C 3v or£> 3A (for d 3h -only one vibrational mode of a planar species is infrared active). The C 2v , T-shape is consistent with the bonding in other uranium-oxygen species.…”

“…The infrared spectra of U0 3 phases have been studied by several investigators [2][3][4][5][6][7][8][9][10][11][12][13][14][15], Although the spectra were similar, these workers differed in their interpretation and assignment of the observed bands. In particular HOEKSTRA and SIEGEL [2] observed two absorption bands in the uranyl region which are indicative of symmetric (i^s = 860 cm" 1 ) and asymmetric (v" = 930 cm" 1 ) vibrations.…”

“…OHWADA [8] noticed a strong band at 750 cm" 1 which is reasonably assigned to the higher frequency fundamental (F lu ) M-0 stretch which is triply degenerate. GABELNICK et al [13] showed that U0 3 has a C 2v , T-shaped equilibrium geometry. This structure which is much less common for MX 3 -type molecules than structures of C 3v or D^ symmetry, is consistent, if the U0 3 molecule is approximated as uranyl monoxide, (U0 2 ) 2+ (0) J ".…”