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

Abstract

Solid-state 13C NMR measurements have been carried out for the first time for the pure n-alkanethiol ( n-C n H 2 n+1 SH) and α,ω-alkanedithiol (HS–(CH 2) n –SH) crystals to characterize molecular motions in each crystal phase. The comparison of 13C spin-lattice relaxation time ( T 1C) values for n-alkanethiol and α,ω-alkanedithiol crystals determined in this work and those for n-alkane or α,ω-alkanediol crystals reported in previous works reveals that the intermolecular S⋯S interaction between SH groups is stronger than the van der Waals interaction between CH 3 groups but weaker than the hydrogen bonding between OH groups as well as the covalent bonding between CH 2 carbons. For the melt-crystallized and solution-crystallized n-tetracosanethiol ( n-C 24H 49SH) samples, which have the same side chain packing structure, remarkable changes are observed in 13C NMR 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–(CH 2) 20–SH) sample forms polyethylene-like rigid long-chain crystals through the intermolecular S⋯S interaction in contrast to the case of n-alkanes in which each crystalline stem is separated by the CH 3 end groups.