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Abstract: Cu K-edge X-ray absorption spectroscopy (XAS) has been used for the in situ structural characterization of Cu monolayers prepared on both bare Au(111) and butanethiol self-assembled monolayer (BT-SAM)-modified Au(111) electrodes by underpotential deposition (UPD) from a sulfate supporting electrolyte. in conjunction with electrochemical measurements, the XAS studies enable assignment of the evolution in interfacial Structure of both systems as a function of the applied electrochemical potential. The measurements of Cu adlayer formation on unmodified Au(111) from sulfate supporting electrolyte, ail extensively studied and model UPD system, allow a reevaluation of XAS as an alternative or complementary technique to more established methods for the structural investigation of the electrified interface, such as X-ray diffraction (XRD) and surface X-ray scattering (SXS). The experiments reported in this paper indicate that recent technological advances enable XAS studies of UPD monolayers to be conducted on a time scale comparable to that of XRD and SXS. Meanwhile, the XAS data are consistent with the accepted model for the evolution in structure of the Cu adlayer under increasingly reducing conditions: following the formation of a commensurate (root 3 x root 3)R30 degrees (theta(sc) = 0.67) adlayer in which the Cu adatoms Occupy the 3-fold hollow sites, additional UPD results in I Cu-(1 x 1) monolayer structure. Theoretical modeling of the XAS data also allows a Structural assignment for the coadsorbed anion overlayer and illustrates that the sulfate ions adopt a (root 3 x root 3)R30 degrees (theta(sc) = 0.33) arrangement on the Surface of both Cu adlayers. The XAS characterization of Cu UPD oil BT-SAM-modified Au electrodes indicates that the Cu adatoms reside in face-centered cubic 3-fold hollow sites of an unreconstructed Au(111) surface. This is significant because it Suggests that formation of the Cu UPD adlayer lifts, at least in part, the organothiol-SAM-induced reconstruction of the Au substrate surface. The local environment of the Cu adatoms presents the possibility that adlayer regions with a Cu-(1 x 1) structure form on the Au surface, which Support a coadsorbed (root 3 x root 3)R30 degrees (theta(sc) 0.33) overlayer of BT sulfur atoms residing in the 3-fold hollow sites of the UPD monolayer.

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Abstract: The prototypical thiourea inclusion compound, cyclohexane/thiourea, in which cyclohexane guest molecules are located within tunnels in a hydrogen-bonded thiourea host structure, is known to exist in three phases (denoted I, II, and III). The stable phase at ambient temperature (phase I) undergoes a second-order transition to phase II at ca. 148 K, and phase II undergoes a first-order transition to phase III at ca. 127 K. The structural properties of phases I-III have been determined in the present work from synchrotron X-ray powder diffraction data, employing the Rietveld refinement technique. The structural properties determined for phase I are in agreement with those reported previously from single-crystal X-ray diffraction data. The structures of the two low-temperature phases (phases II and III) have not been reported previously, and we emphasize the advantages of using X-ray powder diffraction techniques in these cases due to the occurrence of crystal twinning on entering these phases. In phase II, the observed distortion of the thiourea tunnel is consistent with an increase of orientational ordering of the guest molecules in comparison with phase I, although a unique determination of the disordered guest substructure could not be established. On entering phase III, however, the Rietveld refinement indicates a further distortion of the thiourea tunnel structure, which is associated with ordering of each guest molecule in a single discrete orientation. Importantly, structural aspects of the guest substructure in this phase (particularly concerning the tilt angle of the C3 axis of the guest molecule relative to the tunnel axis) are in excellent agreement with results on the orientational characteristics of the guest molecules in phase III deduced from a previous single-crystal 2H NMR study.