A Facile Trans to Gauche Conversion in Layered Silver Butanethiolate

Raman, X-ray diffraction (XRD), UV-visible reflectance, and ¹³C and ¹⁰⁹Ag CPMAS NMR studies have been carried out on three different forms of the layered solid AgS(CH₂)SCH₃: 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 ¹³C NMR show that the all-trans species is less stable, converting cleanly to the gauche form. This transformation (E_a ≈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 Å interlayer spacing in the trans form increases to 16.88 Å in the gauche form. On the basis of ¹⁰⁹Ag NMR data and bond lengths from structurally-characterized Ag thiolate model compounds, S-S spacings of 4.4-4.8 Å 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.