Cations mutual arrangements

In the sandwich complex with Ba2+ cations, the mutual arrangements of 2-styrylbenzothiazole molecules 9 are favourable for the occurring of the reaction of the photocycloaddition. Upon irradiation with blue light, 9 undergoes stereoselective [2+2]-photocycloaddition (PCA) to afford isomeric cyclobutane derivatives + (78%) and E+Z (22%) isomers with relatively high combined quantum yield (0.13) (Scheme 11) [32],... 

Fig. 5. Structure of the phosphaalkyne [(dme)jLi] [IMi -S] 30 % probability measurement at -100 3 °C the reproduction of cation and anion reflects their mutual arrangement in the solid.

Fig. 8.5 Singlet probability of the surviving radical pairs as a function of the number of pairs (e + Solv ) using an external field of 0.33 T. A mutual dififusion coefficient (D ) of 0.5 ps was used for S / c-RH+. Cations were arranged on aline 20 A apart, with anions Gaussian distributed from their respective cations with a standard deviation of 80 A. Hyperfine coupling constant was averaged using Eq. (8.23)...

To achieve the state of lowest energy, the cations and anions will tend to maximize attractions and minimize repulsions. Attractions are maximized when each cation surrounds itself with as many anions as possible, with the proviso that neither the cations nor the anions touch. To illustrate, consider the four anions surrounding cations of increasing radii as shown in Fig. 3.2. The atomic arrangement in Fig. 3.2a is not stable because of the obvious anion-anion repulsions. Figure 3.2c, however, is stabilized by the mutual attraction of the cation and the anions. When the anions are just touching (Fig. Z.lb), the configuration is termed critically stable and is used to calculate the critical radii at which one structure becomes unstable with respect to another (Worked Example 3.1). 

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