No abstract

No abstract

I 1 J The g tensors and linewidths vary slightly for different samples. A representative EPR spectrum gives g , = 2.26, g,, = 2.02 (solid-state at 77 K). H,,,0.32x0.04x0.04mm3, hexagonal, P6,22, a = b =12.1121(5), c =17.3157(10),&, V = 2199.9(2) A3, 2 = 6, psalcd = 1.800 g~m -~, Mo,,radiation ( ; . = 0.71073 A), p = 1.22 mm-'. Data were collected on a Siemens CCD Smart system in the range 4<20< 54". 11859 measured reflections, 1625 unique, 1276 with F,40(F,) were used to refine 98 parameters to R(wR) = 0.0479(0.0786), GOF = 1.185; F Z refinement in SHELXTL 5.0. A $J scan absorption correction gave min. and max. transmission factors of 0.79 and 0 91. The highest peak in the final difference map = 0.527 e k ' . Crystallographic data (excluding structure factors) for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publlcation no. CCDC-179.110. Copies of the data can be obtained free ofcharge on application to The Director,CCDC, 12Union Road,CambridgeCB21EZ,UK(fax: 1nt.code +(1223) 336-033; e-mail: teched@chemcrys.cam.ac uk). [14] a) The tight binding band structure calculations used an extended Huckel-type Hamiltonian: R. Hoffmann, J Chem. Phys. 1963, 39, 1397. b) A modified Wolfsberg-Helmholz formula was used to evaluate the nondiagonal H,, values: J. Ammeter, H.-B. Biirgi, J. Thibeault, R. Hoffmann, J Am. Chem. SOC. 1978,100,3686. c) The exponents and parameters used were taken from R. H. Summerville, R. Hoffmann, L Am. Chem. Soc. 1976, 98, 7240. I151 M.-H. Whangbo, R. Hoffmann, J Am. Chem. Soc. 1978, 100.6093, [16] Preliminary data support a localized system as a result of strong electron correlation effects in one dimension. Application of pressure (ca. 25 kbar) is expected to restore metallic character (room temperature conductivity = 2 Scm-'):

Apertureless scanning near-field optical microscopy (SNOM) in the reflection-back-to-the-fiber configuration is possible on rough surfaces of practical importance and reaches 15 nm lateral resolution. The feedback for constant distance mode is provided by shear-force atomic force microscopy. Any artifacts in the atomic force microscopy will translate into artificial optical responses. Very sharp tapered tips (radius ca. 15 nm, apex angle`10°) are required. Only these give the strongly enhanced reflectance in shear-force distance, do not break as easily as do more blunt tips, and can follow the topography in order to provide chemical SNOM contrast without topographic errors. In the absence of significant reflectance enhancement, if blunt or abrased or broken tips are used, not SNOM but artificial 'optical contrast' is recorded. Metal-coated tips are unsuitable: they become hot, give poor resolution and are the source of numerous additional artifacts that are summarized for comparison. The SNOM artifacts are subdivided into:* topographic errors-if chemical uniform topography does give some optical response; * lack of spatial correlation-if the optical signal does not reproduce the site of the emitter or of the chemical contrast; * tip imaging-very large features and more or less negative differentials therefrom; * contrast inversion-false sign of optical contrast; * split optical contrast-signal is split into two unequal parts with the same (false) sign of the contrast; * artificial stripes-instead of chemical/emissive contrast artificial stripes, that are not always 'interference fringes' or 'edge-contrast'.The artifacts can be easily recognized in published images from different techniques and modes of SNOM. Their analysis would be facilitated if full x/y/z data were published instead of two-dimensional images. We therefore disclose interactively analyzable VRML data in the EPOC version.

No abstract

The solid-solid chemical reaction of benzopinacol (1) with p-toluenesulphonic acid monohydrate (2) to give a quantitatively proton-catalysed pinacol rearrangement with formation of triphenylacetophenone (3) in the absence of a solvent was studied preparatively and mechanistically using atomic force microscopy (AFM) measurements and known crystal structure data. The reaction rate is dramatically enhanced if the water of reaction is continuously removed. AFM reveals that no reaction occurs on (001) of 1 whereas (100) exhibits distance dependent craters and volcano-like mounds over areas extending to 1.5 m m from the contact edge of the crystals with 1 undergoing phase rebuilding while reacting. A mechanism resembling the formation of membrane potentials is seen for the first time in crystals. Thus it appears that catalytic protons migrate without their counterions from one molecular compartment to the next by proton consumption at its inside and proton liberation at its outside, which is the inside of the next compartment. The uppermost molecular layer determines reactivity or non-reactivity. On (001) of 1 the hydroxyl groups occur with their hydrogens up. Hence no AFM features are found and crystals of 2 do not become adhered to 1. However, on (100) the hydroxyl hydrogens point down (free electron pair up). Protonation is possible, chemical reaction is indicated by the formation of the AFM features and crystals of 2 adhere firmly to the surface of 1 after reaction.

No abstract

A low-temperature scanning confocal and near-field optical microscope Rev.Reflection scanning near-field optical microscopy with uncoated fiber tips: How good is the resolution really?Reflection-back-through-the-fiber scanning near-field optical microscopy ͑SNOM͒ measurements with uncoated tapered fiber tips, that remain cold, were performed under shear-force distance control for opaque and transparent rough samples. The feedback dynamics of the commercial ͑DME͒ RASTERSCOPE SNOM were tested with a 3000/500 nm gold grating at very high scan rates. Slow rates give rise to thermal broadening due to heating of the gold surface by partial light absorption. Enhancement of the light intensity at the base by reflection from steep walls on four sides is reported, the steepness contrast being less important. Doped gallium arsenide substrates give materials contrast. Extremely rough porous silicon has been probed. Organic chemical contrast is seen for the first time with SNOM on partly oxidized anthracene at large dislocations. Anthracene, anthraquinone, and their charge-transfer complex are clearly discriminated by different reflectivities. Irradiated ␣-cinnamic acid exhibits a chemically uniform surface and, thus, no SNOM contrast despite the anisotropic m high features that are observed. Photolithography on light-absorbing 2,5-dibenzylidene-cyclopentanone crystals with the tip illuminated at an Ϸ10 nm distance provides an easy, inexpensive, and rapid performance test of single tips. Very long local irradiation creates features that widen and reach heights of 300 nm.