The vibrational spectrum of B4Cl4

Abstract Infrared and Raman spectra of solid B 4 Cl 4 have been measured from 35 to 3500 cm −1 . Raman depolarization measurements were made on a CCl 4 , solution, and two i.r. bands were obtained for the vapor. The i.r. spectrum is surprisingly sparse; only one band was found below 925 cm −1 . The spectra can be understood very well on the assumption of a T d 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 D 2 d , and the Raman spectra show splittings compatible with this. The eight fundamentals for T d symmetry have been assigned as follows (solid state frequencies): a 1 : 1302, 358; e : 692, 117; f 1 : [343] (inferred); f 2 : 978, 514, 110 cm −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 a 1 modes are almost completely mixed. The most interesting force constants are k r (B Cl) = 4.08 and k R (B B) = 2.3 ± 0.1 mdyn/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.