Solid-state NMR analyses of the crystalline phase transitions for 1,20-eicosanediol

The structure and dynamics for the monoclinic and hexagonal crystals of 1,20-eicosanediol (HO-(CH2)(20)-OH) have been investigated mainly by solid-state C-13 and H-1 NMR spectroscopies. In the heating process of DSC curve, a small endothermic peak, which is assigned to the phase transition from one monoclinic phase (M phase) to another monoclinic phase (M' phase), is observed at 80 degrees C. Moreover, another endothermic peak attributed to the phase transition from the M' phase to the hexagonal phase (H phase) also appears at 95 degrees C. CP/MAS C-13 NMR and H-1 CRAMPS spectra reveal that two magnetically different structures exist in chain ends associated with hydrogen bonding in the M and M' phases. The evaluation of the 13C chemical shift for the CH2 resonance line by considering the gamma-gauche effect indicates that about 3% gauche conformations are included in the inner CH2 seyuenees in the ii phase. FT-IR measurements also confirm the existence of the kink defects in the H phase. It is also found by Tie and H-1 CRAMPS measurements that the two-site exchange motion for the hydroxyl groups, because of the jump rotation around the C-O bund, occurs in the H phase. To fulfill the cylindrical symmetry in the H phase, the overall random jump rotation around the chain axis may be induced for more than 80% molecular chains with the almost planar zigzag conformation. However, less than 20% chains will undergo the partial jump rotation by the introduction of kink defects probably as a result of the rather strong fixation of chain ends through intermolecular hydrogen bonding. (C) 2000 Elsevier Science B.V. All rights reserved.