THE UPS AND DOWNS OF BRIDGES

The low-temperature H NMR spectra for calix[4]- and calix[5]arenes reveal that all four or five phenolic units are identical.225 This is compatible with a cone conformation with C4v and C5v symmetry, respectively.226,227 For calix[8]arenes the same pattern is interpreted by a / -symmetrical pleated loop conformation, with a regular up-and-down of the methylene bridges. Calix[7]arenes, however, show seven singlets for the OH protons and seven (overlapping pairs of doublets) for the methylene protons. This total lack of any symmetry element (C,) makes this conformation chiral.228... 

The sensor consists of a bridge of tubes with three vertical tubes all connected by horizontal tubes at the top and the bottom. Pure carrier gas enters at the center of the right hand vertical tube and splits into two streams one passing along the lower horizontal tube and the other along the upper horizontal tube. The eluent from the column enters the center of the middle tube and the flow also splits into two streams and each meets the respective flow from the right-hand tube. The flows in the two horizontal tubes finally pass up and down the left-hand vertical tube to meet at the center and then exit to waste. Flow sensors are situated in the horizontal tubes between the right-hand vertical tube and the center vertical tube. When only carrier gas is present in the system, the horizontal flows are equal and the... 

Figure 41. Two charge-hopping mechanisms. The donor injects an electron (or hole) into the bridge which consists of discrete redox units, (a) The bridge units are nearly degenerate. Consequently, the injected electron (or hole) moves randomly and reversibly up and down the bridge, finally becoming irreversibly trapped by the acceptor, (b) The bridge units constitute an ordered redox cascade the electron or hole moves essentially irreversibly along the bridge towards the acceptor.

Figure 8.5 Hydrogen-bonded corrugated urea layer in ](n-C3H7)4N ]2C404 6(NH2)2C0-2H2O (1) at z = 0 comprising wide ribbons derived from two independent urea molecules, as well as bridging urea molecules of the third kind that point alternately up and down between ribbons. Broken lines represent hydrogen bonds. Symmetry transformations a (1 — x, 2 — y, —z), b (1 x, 1 y, z) and c (1 — x, 1 — y, —z)...

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