Chain dynamics of bilayer n-decylammonium chloride studied by deuteron NMR spectroscopy

N-decylammonium chloride is a polymorphic solid which exhibits a variety of layered phases. The molecular dynamics and phase transitions in these phases have been studied using²H NMR of selectively deuterated n-decylammonium in the chain positions 1 and 6 and in the ammonium group. Measurements were performed over the temperature range of the interdigitated phase I, the non-interdigitated monoclinic phase , the tetragonal phase , and the monotropic phase , obtained upon cooling the sample from the -phase. In phase I, below 321 K, the hydrocarbon chain is rigid except for ND₃ rotation about its own symmetry axis. In phase , between 321 K and 325 K, a fast motion of the hydrocarbon chains is assigned to an exchange between two pairwise nonequivalent positions. The chains are slightly tilted in this phase. In phase above 325 K the chains rotate freely around their long axes experiencing additional transgauche conformational dynamics. In the non-interdigitated phase the spectra can be interpreted by assuming a reorientation between two variously occupied potential wells about the axis determined by the C-N bond, which is believed to be parallel to the normal bilayer axis.