Raman Spectra and Factor Group Analyses of Crystalline Boron Trihalides

Abstract

The Raman spectra of the crystalline boron trihalides have been recorded at ·77°K in the region of the fundamentals. The spectra of boron trichloride, boron tribromide, and boron triiodide have been interpreted in terms of the factor group (C6h) known to pertain for the trichloride; the trifluoride is shown to have a different crystal structure. Halogen isotopic structure to the a1′ and a2″ bands of the trichloride as well as to the a1′ band of the tribromide has been resolved, the separation between the isotopic components of these bands amounting to 4.2, 2.2, and 1.2 cm−1, respectively. The Raman spectra of the trichloride, tribromide, and triiodide as liquids or solutions have also been recorded. The asymmetric stretching mode (e′ symmetry) is the most sensitive fundamental to change of state, being depressed from its liquid phase values by 11–22 cm−1 for each halide; the other three fundamentals only suffer very small shifts on change of state. The infrared spectrum of solid boron triiodide has been measured in the 1000–70 cm−1 region, and one fundamental has been reassigned; the appropriate force constant has been recalculated.