Infrared and Raman spectra and structures of matrix isolated thallous halide dimers: Tl2F2 and Tl2Cl2

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Abstract
Infrared and Raman spectra are presented for monomeric and dimeric thallous halide species (TlF, Tl2F2, TlCl, and Tl2Cl2) isolated in argon matrices. From these results it is concluded that both the Tl2F2 and Tl2Cl2 molecules have planar rhombic structures rather than linear configurations as previously proposed on the basis of mass spectroscopic, infrared, and photoelectron studies. Entropy calculations and other arguments are presented to resolve this discrepancy. Our structural conclusion for Tl2F2 is in good accord with a very recent electron diffraction investigation of Tl2F2 and the structural parameters and vibrational amplitudes obtained in that work are incorporated with our frequencies in a normal coordinate analysis. Force fields are obtained for both Tl2F2 and Tl2Cl2 and a very large amplitude (31°) is calculated for the out of plane bending coordinate of Tl2F2. Assuming that the bonding in this compound is largely ionic, an average instantaneous vibrational dipole moment of 3.7 D is deduced. This very large value in a ’’nonpolar’’ molecule is in good agreement with a novel electron scattering experiment on Tl2F2 molecular beams in which vibrational dipoles in the range 2–8 D were recently reported.