Perching complexes

The binding of ammonium and alkyl ammonium cations by [18] crown-6 and C-backbone substituted derivatives proceeds via three —H - 0 hydrogen bonds to give a perching complex, although... 

The equilibrium constant, K, is related to the rates for the complexation (7q) and decomplexation ( i) reactions by the equation K = kxlk i (Section 1.4). The forward and reverse complexation rate constants for three types of complex are given in Table 3.9. In general, it is found that the rates of both the forward and reverse reactions decrease in the order perching > nesting > capsular. 

Figure 3.64 Perching geometry in alkyl ammonium ion complexes of Me3[18]ane-03N3 held together by charge-assisted hydrogen bonds.

In each case consider whether the guest complexation will be of the capsular, nesting or perching type, or some other arrangement. Use this information to guess qualitatively the rate of guest complexation and decomplexation in each case. 

Following the preparation of 4.79, a number of other cyclic mercury crown compounds have been synthesised, which do exhibit halide complexation behaviour. Compound 4.80, forms a 1 1 polymer with bromide in the solid state in which the Hr anions perch above the Hg3 plane. The Hg—Br distances of 3.07-3.39A are considerably longer than normal Hg—Br covalent bonds (about 2.54A).61 The compound also binds SCN- with similarly long bonds as shown in Figure 4.34.60 The analogous chloride complex has a 3 2 stoichiometry suggesting a triple-decker sandwich of type [4.80 Cl 4.80 Cl 4.80]2. ... 

Computational methods have been applied to study the conformations of free and metal-complexed oxathiacrown ethers 1-4 shown in Figure 6. The results were compared to variable temperature NMR and i spin-lattice relaxation time measurements <2001JP2988>. Theoretical studies included simulated 111 NMR spectra using PERCH and molecular modeling with PM3 semi-empirical quantum-chemical calculations. The NMR and the computational data both show that Ag+ coordinates equally well to S and O atoms, Bi3+ and Sb3+ prefer O atoms, and that Ptz+ and Pdz+ prefer exo-cyclic coordination only to the S atoms in this maleonitrile macrocycle. 

Argillic horizons often correlate with the maximum depth of effective evapotranspiration, commonly 1-2 m. Water loss initiates the precipitation of secondary clays and oxides from solutes. Such precipitation may also be related to the loss of dissolved organic species and the de-complexation of soluble aluminum. The resulting low permeabilities further retard the downward percolation of pore water, commonly creating transient perched water tables directly above the hardpans. Periodic drying in this zone focuses addition secondary mineral precipitation in the vicinity of the hardpan, which then leads to a lower permeability and more clay formation. 

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