P, Si, Ge, B) and a monovacant Dawson derivative anion P2W1706i10-were adopted as the ligand for cerium(III). The formation of the complexes [CeL]lm-3)-and [CeL2]<2m-3>-was confirmed by means of absorption spectroscopy, where Lm~denotes an unsaturated heteropolytungstate anion of P, Si, or Ge, while clear evidence for the complexation of BW11O399-was not obtained. Electrochemical behaviors of heteropolytungstocerates(III) were studied by cyclic and differential pulse voltammetry in aqueous solution. The redox potential of the cerium(IV/III) couple was considerably lowered (below +1 V vs Ag/AgCl) upon coordination of heteropolytungstate in aqueous solution at pH 4.5. The magnitude of the potential shift for [CeL]lm"3>" increased in the order PWnOgg7-, P2W1706i10-, GeWnOag8-, and SiWuOag8-. A similar trend was observed for [CeL2]12m-3>-with larger potential shift than those of [CeL]lm-3>-. The shift of the redox potential for cerium(IV/III) is caused primarily by the large negative charge of the unsaturated heteropolytungstate.
Atomic hydrogen has been stably encapsulated in both solution and crystal at room temperature. Upon gamma-ray irradiation of [(CH(3))(3)Si](8)Si(8)O(20), which is the trimethylsilylated derivative of the silicate anion with a double four-ring (D4R) cage, electron spin resonance (ESR) spectra revealed that a single hydrogen atom is encapsulated in the center of the D4R cage and is stable for periods of many months. Attack by chemically reactive species such as oxygen was prevented by the D4R cage, but the ESR signal of the hydrogen atom was sensitive to the magnetic interaction caused by the presence of the O(2) molecule near the cage.
B3LYP level optimizations were performed on the structures of the octasilsesquioxane (Si 8 O 12 H 8 , HT 8 ) double four-ring (D4R) cage and single hydrogen atom-trapped HT 8 (H@HT 8 ). Moreover, the transition state in the detrapping process of the hydrogen atom from the D4R cage was examined. The basis sets used were 6-31G** for HT 8 and (3 1 1/1*1*/1*) for the trapped hydrogen atom. Both HT 8 and H@HT 8 were structure-optimized with O h molecular symmetry and the resulting cage conformations were similar. The trapped H atom was located at the center of the D4R cage. The weak interaction between the D4R cage and the trapped H atom in H@HT 8 was determined by examining the singly occupied molecular orbital (SOMO) [8a 1g ] of H@HT 8 . The SOMO was constructed from an antibonding interaction between the lowest unoccupied molecular orbital (LUMO) [8a 1g ] of HT 8 and the 1s orbital of the trapped H atom. For the transition state, the structure was optimized with C 4V molecular symmetry. As a result, the position of the Si 8 cube framework was unchanged, and four O atoms in a silicon single four-ring were displaced, thereby opening one of the oxygen windows of the D4R cage. The detrapping H atom was located near the center of the oxygen window and the MO illustrations showed a change in shape from spherical to ellipsoid. Consequently, it is clear that the detrapping process is not due to the formation of chemical bonding. The calculated activation and reaction energies of this detrapping process were +98.6 and -26.1 kJ/mol, respectively. In addition, single-point calculations at the MP2 level were done for each optimized structure, and the obtained activation and reaction energies were +128.7 and -9.3 kJ/mol, respectively. Both calculated activation energies were comparable to Sto ¨sser's experimental data (+109.6 ( 3.1 kJ/mol) for H•:Si 8 O 12 (OSi(CH 3 ) 3 ) 8 (Q 8 M 8 ). Furthermore, additional explanations are given on the IR vibrational frequencies of HT 8 and H@HT 8 and the hyperfine coupling constant for caged atomic hydrogen by ESR.
We investigated the encapsulation behavior of hydrogen and deuterium atoms in double four-ring (D4R) cages of octasilsesquioxane ([(CH3)3SiO]8Si8O12, Q8M8) under variable experimental conditions when dissolved in n-hexane and n-hexane-d
14. Encapsulated atomic H and D were detected simultaneously upon γ-ray irradiation in room temperature ESR spectra for samples of Q8M8 suspended in n-hexane-d
14. Although the encapsulation of H, generated from D4R cage substituents, decreased with increasing n-hexane-d
14, encapsulation of D atoms increased remarkably with increasing n-hexane-d
14.
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