STUDY OF CETYLTRIALKYLAMMONIUM BROMIDE AND TRIBROMIDE SALTS IN THE SOLID-PHASE

Some cetyltrialkylammonium tribromide salts have been studied in the s olid phase using Raman spectroscopy and energy dispersive X-ray diffra ction. The first technique detected the presence of asymmetric tribrom ides while the latter technique revealed the degree of asymmetry of th e tribromide units (difference in the bond lengths between external an d central Br atoms), by application of a subtraction method. In this m ethod the spectra of the corresponding monobromide salts were recorded and differences between tribromide and monobromide curves isolated th e tribromide contributions. The existence of asymmetrical and symmetri cal tribromide ions has been established and the degree of asymmetry w as then correlated to the steric hindrance and electronegativity of th e ammonium substituents. The values of the Br-Br bond distances have b een deduced; the most asymmetric tribromide has Br-Br distances at 2.3 8 and 2.66 Angstrom, while the symmetric tribromide has both Br-Br bon d lengths equal to 2.52 Angstrom. A linear geometry is confirmed for t he tribromide ions.