In supramolecular chemistry [1] synthetically designed organic constituents interact noncovalently, in a directed and specific way to form host-guest complexes of higher complexity. The ability to tailor the molecular interplay with respect of chemical design, specificity, and molecular switching opens up the development of new molecular materials for artificial molecular recognition, molecular organization, and selfassembly. We have used mechanical single-molecule force spectroscopy to investigate the binding of individual resorc-[4]arene-ligand host-guest complexes. By using diluted samples of the host and guest molecules that are modified with a long linker which is attached to an atomic force microscope (AFM) tip, we were able to prevent multiple binding and to observe single host-guest unbinding events in a supramolecular system for the first time. The molecular binding forces, their dependence on external loading rates, the rate of dissociation, and the molecular cavity length directly relate to the molecular properties of the supramolecular species and are consistent with an activated decay of a metastable bound state, a finding already established for biological receptor-ligand complexes. This result allows new insights into the mechanisms, kinetics, and thermodynamics of intermolecular association in chemistry and biology, and opens new possibilities in the investigation, design, and development of synthetic receptor systems.Calixarenes are model receptor systems providing synthetic receptor cavities for the inclusion of small cationic guests, such as alkali-metal or ammonium ions. [2][3][4][5] Organic cations, such as ammonium ions, play a significant role in molecular recognition processes in nature (e.g. in protein side chains). Calix[n]arenes, generally, are a class of macrocyclic compounds formed by the base-catalyzed condensation of nphenol derivatives and formaldehyde. [2,3] The resorc [4]arenes [6,7] considered herein are calixarenes formed from four

A new modification of boron, resulting from the pyrolytic decomposition of BI z on a heated surface (800 °C. to 1100 °C.), has been found to have a simple rhombohedral structure, with a 0 = 5.057 /~ and c~ = 58.06 °, space group R3m, Z ----12. The atomic parameters have been obtained from a single crystal using the precession method. The structure may be described as composed of units of nearly regular icosahedra in a slightly deformed cubic close packing. This requires the use of three-center or delta bonds, wherein two electrons are shared by three boron atoms at the vertices of an equilateral triangle.

The crystal structure of CaF%O 4 has been determined from single-crystal studies. The crystal system is orthorhombic, the space group is Pnam--D~, and the unit cell dimensions are a 0 --9-230, b 0 ----10.705, c o ----3-024 A. The coordination of oxygens around Fe is that of a distorted octahedron; around Ca there occurs a coordination of nine oxygens in a configuration first noted in CaTi20 4 and present also in CaV20 a, with which CaF%O 4 is isomorphous.

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is less and not greater when unjustified assumptions are made.The difference between the standard deviation 0.035 J~ and that obtained by Carpenter, 0.017 A, must be attributed mainly to the different method of calculation used. The accuracy of the determinations is approximately equal; in the present work more structure amplitudes (87 compared with 62) were used and more independent values for each observation were obtained; on the other hand the value of R is larger, being 0-142 compared with Carpenter's 0.110 for the same planes. Choice of weighting factor can provide only a partial explanation for the discrepancy; the ratio between the standard deviation obtained by the approximate Fourier method and by the method of least squares with Carpenter's weighting factor is 1:0.92. It seems likely that the larger value of the standard deviation gives a true picture of the accuracy obtainable in this structure analysis; this view is confirmed by Carpenter's statement that movement of the nitrogen atom by 0.05/~ produced a barely perceptible change in the agreement index.