Molecular structure of monomeric uranium tetrachloride determined by gas electron diffraction at 900 K, gasphase infrared spectroscopy and quantum-chemical density-functional calculations

Abstract

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 t₂ stretching and deformation modes have been assigned at ν₃ 337.4 and ν₄ 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_e= 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.