A facile trans to gauche conversion in layered silver butanethiolate

Raman, X-ray diffraction (XRD), UV-visible reflectance, and C-13 and Ag-109 CPMAS NMR studies have been carried out on three different forms of the layered solid AgS(CH2)(3)CH3: a white all-trans compound, a bright yellow gauche C(1)-C(2) conformational isomer, and a phase containing both species. The structure of these compounds comprises a slab of Ag and S atoms, alkyl chains extending in both directions, and layer stacking. Raman spectroscopy and C-13 NMR show that the all-trans species is less stable, converting cleanly to the gauche form. This transformation (E(a) approximate to 60 kJ/mol) is accompanied by a color change from white to yellow, a phenomenon previously observed upon melting of mesogenic AgSR (R = alkyl) compounds. XRD shows that the 15.56 Angstrom interlayer spacing in the trans form increases to 16.88 Angstrom in the gauche form. On the basis of Ag-109 NMR data and bond lengths from structurally-characterized Ag thiolate model compounds, S-S spacings of 4.4-4.8 Angstrom were found, which agree with those reported for two-dimensional self-assembled monolayers of organothiols (SAMs) on well-defined Ag surfaces. These findings shed light on the molecular origins of AgSR mesogenic behavior and reinforce the idea that Ag-S interactions play a dominant role in self-assembly of organothiols on Ag.