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 N-14 and Br-18 solid state NMR, X-ray diffraction, and theoretical calculations, The investigation represents a necessary step toward further N-14 and Br-81 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 CxH2x+1(CH3)(3)N+Br- polycrystalline samples with different n-alkyl chain lengths (x = 1, 12, 14, 16, 18) were recorded and modeled, The N-14 and Br-18 quadrupolar coupling interaction parameters (C-Q, eta(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 N-14 and Br-81 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.