Solid-state 13C NMR studies on the molecular motions for n-alkanethiol and α,ω-alkanedithiol crystals

Solid-state C-13 NMR measurements have been carried out for the first time for the pure n-alkanethiol (n-CnH2n+1SH) and alpha,omega-alkanedithiol (HS-(CH2)(n)-SH) crystals to characterize molecular motions in each crystal phase. The comparison of C-13 spin-lattice relaxation time (T-1C) values for n-alkanethiol and alpha,omega-alkanedithiol crystals determined in this work and those for n-alkane or alpha,omega-alkanediol crystals reported in previous works reveals that the intermolecular (SS)-S-... interaction between SH groups is stronger than the van der Waals interaction between CH3 groups but weaker than the hydrogen bonding between OH groups as well as the covalent bonding between CH2 carbons. For the melt-crystallized and solution-crystallized n-tetracosanethiol (n-C24H49SH) samples, which have the same side chain packing structure, remarkable changes are observed in C-13 line widths and T-1C values. These changes seem to reflect the difference in the monoclinic-to-rotator phase transition behavior, in accord with the results obtained by previous DSC and X-ray analyses. It is also found that the 1,20-eicosanedithiol (HS-(CH2)(20)-SH) sample forms polyethylene-like rigid long-chain crystals through the intermolecular (SS)-S-... interaction in contrast to the case of n-alkanes in which each crystalline stem is separated by the CH3 end groups. (C) 2002 Elsevier Science B.V. All rights reserved.