A solid-state NMR study of the 2,15-dimethylhexadecane/urea inclusion compound

Solid-state 2H- and 13C NMR studies were performed for the first time on an urea inclusion compound with a doubly branched alkane guest molecule. Variable temperature 2H NMR measurements were done on two samples with 2,15-dimethylhexadecane deuterated either at two methylene groups (positions C3 and C14) or at both terminal isopropyl groups (positions C1, C1′, C2, C15, C16 and C16′). The 13C NMR data demonstrate that the inner chain segments exist in the trans-conformation. A comprehensive analysis of the experimental 2H NMR data demonstrates that the guest molecules undergo restricted overall rotation (modeled with a degenerate 2-site jump process) around the channel long axis, which is a major source for spin-lattice relaxation. Moreover, it is found that the orientation of the methyl groups is perpendicular to the long molecular axis (i.e. gauche conformation at both chain ends) which gives rise to a better molecular packing and stronger van-der-Waals interactions. Activation energies of 24.3 (±1.0) kJ/mol are derived for the overall molecular rotation, while for methyl group rotation values of 14.4 (±0.6) kJ/mol (between 210 and 250 K) and 12.0 (±1.7) kJ/mol (between 160 and 200 K) are obtained. The relatively large activation energies for the guest motions can be attributed to the considerable spatial confinements imposed by the urea host lattice. For a consistent description of the available 2H NMR data, overall molecular wobbling of the guest molecules has to be taken into account as well.