Study of cetyltrialkylammonium bromide and tribromide salts in the solid phase

Some cetyltrialkylammonium tribromide salts have been studied in the solid phase using Raman spectroscopy and energy dispersive X-ray diffraction. The first technique detected the presence of asymmetric tribromides while the latter technique revealed the degree of asymmetry of the tribromide units (difference in the bond lengths between external and central Br atoms), by application of a subtraction method. In this method the spectra of the corresponding monobromide salts were recorded and differences between tribromide and monobromide curves isolated the tribromide contributions. The existence of asymmetrical and symmetrical tribromide ions has been established and the degree of asymmetry was then correlated to the steric hindrance and electronegativity of the ammonium substituents. The values of the Br-Br bond distances have been deduced; the most asymmetric tribromide has Br-Br distances at 2.38 and 2.66 Angstrom, while the symmetric tribromide has both Br-Br bond lengths equal to 2.52 Angstrom. A linear geometry is confirmed for the tribromide ions.