Cage effect structure

The simple difhision model of the cage effect again can be improved by taking effects of the local solvent structure, i.e. hydrodynamic repulsion, into account in the same way as discussed above for bimolecular reactions. The consequence is that the potential of mean force tends to favour escape at larger distances > 1,5R) more than it enliances caging at small distances, leading to larger overall photodissociation quantum yields [H6, 117]. 

The hydrophobic effect. Water molecules around a non-polar solute form a cage-like structure, which ices the entropy. When two non-polar groups associate, water molecules are liberated, increasing the entropy. 

Unusual heterocyclic systems can be obtained by photodimerizations and for five-membered heterocycles with two or more heteroatoms such dimerizations need be effected on their ring-fused derivatives. Cyclobutanes are usually obtained as in the photodimerization of the s-triazolo[4,3-a]pyridine (540) to the head-to-head dimer (541). These thermally labile photodimers were formed by dimerization of the 5,6-double bond in one molecule with the 7,8-double bond in another (77T1247). Irradiation of the bis( 1,2,4-triazolo[4,3-a]pyridyl)ethane (542) at 300 nm gave the CK0ifused cyclobutane dimer (543). At 254 nm the cage-like structure (544) was formed (77T1253). 

Formation of cross products differed in CTABr from that in H20 or C6H6 Cage effects in CTAC1 depended upon substrate structure and magnetic field... 

Although it is probable that only four bonds are formed in many complexes, the cage-like structures effectively prevent the formation of complexes other than those with 1 1 stoichiometry (Chapter 3). This feature is of considerable analytical importance. 

A more detailed analysis of the cage effect has been presented by Northrup and Hynes [103]. As before, the diffusion equation was used to describe the relative motion of radicals, but the effect of solvent structures on the radicals when in close proximity to each other was incorporated via the potential of mean force (see Chap. 2, Sect. 6.6 and Chap. 8, Sect. 2.6) and hydrodynamic respulsion (see Chap. 8, Sect. 2.5 and Chap. 9. Sect. 3). The solvent structure is due to short-range liquid structure. Solvent... 

Structural effects have also been observed in Ar HCl clusters with n = 1 and 2 (Garcia-Vela et al. 1994). In this case, the effect of cluster size on the cage effect depends on the excitation energy of HI and on the specific region of the potential surface that it accesses. 

Organic quaternary ammonium ions are effective in forming silicate anions with cage-like structures. 

The silicate anion with a double five-membered ring structure is mainly formed as a crystalline solid from the tetra-n-butylammonium (N+(n-C Ho) ) silicate solutions whose N/Si ratios range from 0.78 to 1.0 (20,21). Pyridinium ions are also effective in forming silicate anions with cage-like structures (27). 

Effect of Addition of Sodium Ions to Tetramethylammonium Silicate Aqueous Solution. In zeolite synthesis, alkali metal cations are combined with organic quaternary ammonium ions to produce zeolites with different structures from the one produced with only the organic quaternary ammonium ion (2) It is then expected that other types of silicate species are formed in the silicate solutions when organic quaternary ammonium ions and alkali metal cations coexist. In such silicate aqueous solutions, however, alkali metal cations only act to suppress the ability of the organic quaternary ammonium ions to form selectively silicate species with cage-like structures (13,14,28,29). 

It is also possible that stretched (high pH) versus coiled (low pH) polymer chains may exhibit different photophysics. This was commented by Chou and Jellinek in their early report on PMAA photochemistry, and a detailed TREPR study of pH-dependent cage effect manipulation using initiator-labeled PMAA was reported by Maliakal et al. " It has been reported by Mittal and coworkers that for small molecule analogs of PAA and PMAA, the quantum yield for Norrish I cleavage drops to zero for the completely deprotonated structure. For PMAA, we also observe a pH dependence in the TREPR signal intensities (data not shown). The data in Fig. 14.7 are the hrst experimental evidence for such an effect in PAA radicals. 

Table 2. Diameters of guest molecules (A) that form clathrate hydrate structure 1 and 11, and the ratios relative to the effective cage sizes for smaller and larger cages. Effective cage size is defined as the cage size -2.9A . A cage occupied by a guest molecule is marked with an asterisk.

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