MOLECULAR-STRUCTURE OF MONOMERIC URANIUM TETRACHLORIDE DETERMINED BY GAS ELECTRON-DIFFRACTION AT 900-K, GAS-PHASE INFRARED-SPECTROSCOPY AND QUANTUM-CHEMICAL DENSITY-FUNCTIONAL CALCULATIONS

The gas electron diffraction pattern of monomeric UCl4 has been recorded with a nozzle temperature of 900 K. The data are in good agreement with a model of T(d)symmetry and a U-Cl bond distance of r(a) = 250.3(3) pm. The root-mean-square vibrational amplitudes are l(U-Cl) = 8.9(3) pm and l(Cl... Cl) = 34.3(10) pm. The gas-phase infrared absorption spectra have been recorded from 25 to 3400 cm(-1) at temperatures ranging from 700 to 900 K. and the t(2) stretching and deformation modes have been assigned at v(3) 337.4 and v(4) 71.7 cm(-1) respectively. The molecular structure of UCl4 has been optimised by density-functional calculations under D-2d symmetry. It is tetrahedral with bond distance r(a)= 251 pm. The calculations yield values for the IR-active modes which are in good agreement with the experimental counterparts. The 'best values' for the two IR-inactive frequencies were obtained by refining a diagonal symmetry force field to the four calculated (density functional) frequencies, the two observed frequencies and the vibrational amplitudes obtained by gas electron diffraction. The entropies of gaseous UCl4 calculated from these normal modes and a symmetry number of 12 corresponding to T(d)symmetry are in good agreement with the experimental counterparts.