A FACILE TRANS TO GAUCHE CONVERSION IN LAYERED SILVER BUTANETHIOLATE

Raman, X-ray diffraction (XRD), UV-visible reflectance, and C-13 and A g-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 br ight yellow gauche C(1)-C(2) conformational isomer, and a phase contai ning both species. The structure of these compounds comprises a slab o f Ag and S atoms, alkyl chains extending in both directions, and layer stacking. Raman spectroscopy and C-13 NMR show that the all-trans spe cies is less stable, converting cleanly to the gauche form. This trans formation (E(a) approximate to 60 kJ/mol) is accompanied by a color ch ange from white to yellow, a phenomenon previously observed upon melti ng of mesogenic AgSR (R = alkyl) compounds. XRD shows that the 15.56 A ngstrom interlayer spacing in the trans form increases to 16.88 Angstr om in the gauche form. On the basis of Ag-109 NMR data and bond length s from structurally-characterized Ag thiolate model compounds, S-S spa cings of 4.4-4.8 Angstrom were found, which agree with those reported for two-dimensional self-assembled monolayers of organothiols (SAMs) o n well-defined Ag surfaces. These findings shed light on the molecular origins of AgSR mesogenic behavior and reinforce the idea that Ag-S i nteractions play a dominant role in self-assembly of organothiols on A g.