High-temperature infrared spectra of LaCl3, LaBr3, and LaI3

The gas-phase infrared spectra of LaCl3, LaBr3 and LaI3 have been measured between 1350 and 1400 K. The spectra show a gradual change in the contour of the stretching fundamentals: from a sharp band for LaI3 to a complex band for LaCl3. To support the interpretation, ab initio molecular orbital calculations have been carried out. The calculations resulted in a flat potential energy hypersurface around the global minimum. They also showed that the infrared intensity of the ν1 and ν4 fundamentals is low. Based on this information, the ν3 and ν2 fundamentals were assigned. A distinction between planar and pyramidal geometry cannot be made on the basis of the present results.