Knowledge Bank

Infrared and Raman spectra of solid $B4Cl4$ have been measured from 35 to 3500 $cm-1$. Raman depolarization measurements were made on a $CCl4$ solution, and two infrared bands were obtained for the vapor. The infrared spectra are surprisingly sparse; only one band was found below $925cm-1$. The spectra can be understood very well on the assumption of a $Td$ structure in which the boron atoms form a tetrahedral cage, with the $B-Cl$ bonds directed away from the center of the tetrahedron. In the crystal the symmetry is known to be distorted very slightly to $D2d$, and the Raman spectrum shows splittings compatible with this. the eight fundamentals for $Td$ symmetry have been assigned as follows (solid state frequencies): $a1$: 1302, 358; e: 692, 117; $f1$: 343 (Inferred); $f2$: $978,514,110cm-1$. They are believed to be reliable, except that 110 and 117 may be interchanged. A normal coordinate calculation has been made. It shows that the two $a1$ modes are almost completely mixed. The most interesting force constants are $kr(B-C1)=4.08$ and $kR(B-B)=2.3\pm 0.2$ millidynes/A. The former is In the normal range, but the latter is quite low by analogy to some related compounds. This reflects the electron-deficient nature of the molecule.