Rotational isomerism of the tetraethylammonium cation in solution and in the solid state

Abstract

The tetraethylammonium ion Et4N+ is shown to present rotational isomerism in aqueous or organic solutions between an all-trans conformation (tt.tt or Greek cross) and a trans–gauche one (tg.tg or Nordic cross). The Raman spectra of the two rotamers are distinct and their temperature dependence indicates that (tt.tt) is more stable than (tg.tg) by about 4.1 kJ mol−1, i.e. by a value typical of two trans–gauche rotations. In some investigated solid salts at room temperature, the cation adopts one of these two conformations depending on the crystal structure and local packing effects induced by the anion. However, some other derivatives with bulky anions such as (Et4 N)2 CdBr4 and Et4 N(CF3 SO2)2 N exhibit much more complicated conformational situations for the cation as a function of temperature. Copyright © 2000 John Wiley & Sons, Ltd.