14N and 81Br Quadrupolar Nuclei as Sensitive NMR Probes of n-Alkyltrimethylammonium Bromide Crystal Structures. An Experimental and Theoretical Study

This is the first time a comprehensive study has been carried out on n-alkyltrimethylammonium bromide salts using 14N and 81Br solid state NMR, X-ray diffraction, and theoretical calculations. The investigation represents a necessary step toward further 14N and 81Br NMR characterization of the environment of cationic and anionic groups in materials, accounting for the amphiphilic properties of cationic surfactants. The NMR spectra of five C x H2x+1(CH3)3N+Br− polycrystalline samples with different n-alkyl chain lengths (x = 1, 12, 14, 16, 18) were recorded and modeled. The 14N and 81Br quadrupolar coupling interaction parameters (C Q , η Q ) were also estimated from spectrum modeling and from computer simulation. The obtained results were discussed in depth making use of the experimental and reoptimized crystal structures. In the study, both 14N and 81Br nuclei were found to be sensitive probes for small structural variations. The parameters which influence the NMR properties the most are mobility, deviation of C−N−C bond angles from T d angles, and variations in r(N−Br) distances.