Tetrapods

AlSalman,A.,Tortschanoff, A.,Mohamed, M. B., Tonti, D., van Mourik, F. and Chergui, M. (2007) Temperature effects on the spectral properties of colloidal CdSe nanodots, nanorods, and tetrapods. Appl. Phys. Lett., 90, 093104. 

Figure 8. A collage of TEM images of Pt shapes (a) cubes, (b) symmetrical planar tripod, (c) cw-bipod, (d) tra s -bipod, (e) unsymmetrical tripod, (f) tetrapod, (g) monopod, (h) V-shaped bipod, and (i) tripod. Scale bar is 50 nm.

Figure 9. ED patterns of (a) cw-bipod and (b) planar tetrapod and their corresponding TEM images (inset). Scale bar is 50 nm.

Dulka J. (1993). Sex pheromone systems in goldfish comparable to vomeronasal systems in Tetrapods Brain Behav Evol. 42, 265-280. 

Parsons T. (1967). Evolution of the nasal structures in the lower Tetrapods. Am Zool 7, 397-413. 

Triply bridging carbonates between three zinc centers have been identified in nine different X-ray structures deposited in the CSD 458,461,465-467 For example, a binuclear ft-OH zinc complex with a tetradentate /V-donor ligand absorbs atmospheric carbon dioxide to a triply bridged carbonate.468 Examples are also known where the metal atoms are in varying coordination environments. The complex cation [Zn3(bipyridine)6(/U3-C03)(H20)2]4+ contains one penta- and two hexacoordinate zinc centers.469 A tetrapodal compartmental ligand forms a tetrameric complex with zinc that contains the carbonate bridging between three of the four zinc centers.470... 

Manna L, Milliron DJ, Meisel A, Scher EC, Alivisatos AP (2003) Controlled growth of tetrapod-branched inorganic nanocrystals. Nat Mater 2 382-385... 

Marin 0., Smeets W. J., Gonzalez A. (1998). Evolution of the basal ganglia in tetrapods a new perspective based on recent studies in amphibians. Trends Neurosci. 21(11), 487-94. 

Manna, L., Scher, E.C., and Alivisatos, A.P. (2000) Synthesis of soluble and processable rod-, arrow-, teardrop-, and tetrapod-shaped CdSe nanocrystals. Journal of the American Chemical Society, 122 (51), 12700-12706. 

Tetrapodal Pentadentate Nitrogen Ligands Aspects of Complex Structure and Reactivity Andreas Grohmann... 

Stecca, B., Southwood, C. M., Gragerov, A. et al. The evolution of lipophilin genes from invertebrates to tetrapods DM-20 cannot replace proteolipid protein in CNS myelin. /. Neurosci. 20,4002-4010, 2000. 

Parallel diffuse reflectance UV (DRUV) and EPR spectroscopic investigations (51,52,54) have provided evidence that the nature of the oxo intermediates formed on contact with H202 depends on the intrinsic local structure and environment of the Ti ions. The tetrapodal structures seem to generate oxo species the concentrations of which correlate with selectivity in the epoxidation of alkenes. 

Fig. 2. Schematic representations of the two different tetrapodal environments Model A, characterized by 3 Ti-O-Si angles of 140° and 1 at 160° Model B, characterized by 2 Ti-O-Si angles of 140° and 2 at 160° [Reproduced from Gleeson et al (47) by permission of the PCCP Owner Societies].

Table I illustrates the utility of DRUV-visible data in determining the surface structures involving Ti. Samples of TS-1 were prepared by three different methods or treatments. Samples 1 and 2 were prepared by conventional hydrothermal synthesis and sample 3 by synthesis in a fluoride medium. TS-2 was synthesized as reported (7). At least five bands could be discerned by deconvolution (Fig. 3), at 205, 228, 258, 290, and 330 nm. Band 1 at 205 nm is assigned to tetrahedral, tetrapodal Ti present in TS-1, TS-2, and Ti-beta. Band 5 at 330 nm is assigned to an...

The presence of two types of titanium sites in TS-1 (tetra- and tripodal) was also suggested by the cyclic voltametry experiments of Bodoardo et al. (158). The tripodal Ti(OSi)3(OH) showed a redox couple at 0 V and the tetrapodal Ti(OSi)4... 

The various spectroscopic techniques had revealed that Ti4+ ions in TS-1, Ti-beta and, Ti-MCM-41 are 4-coordinate in the dehydrated state. Tetrapodal Ti(OSi)4 and tripodal Ti(OH)(OSi)3 are the main Ti species. Upon exposure to H20, NH3, H202, or TBHP, they increase their coordination number to 5 or 6. On samples in which the Ti4+ has been grafted onto the silica (referred to as Ti f MCM-41), a dipodal Ti species (Ti(OH)2(OSi)2) may also be present. As a result of interaction with the oxidant ROOH (R = H, alkyl), the formation of 7)1- and p2-peroxo (Ti-O-O-), hydroperoxo (Ti-OOH), and superoxo (Ti02 ) species has been observed experimentally (Section III). A linear correlation between the concentration of the p2-hydroperoxo species and the catalytic activity for propene epoxidation has also been noted from vibration spectroscopy (133). 

The higher conversion in the presence of Ti-beta is probably a result of the higher temperature (343 v.v. 323 K). Diffusional constraints cannot account for the observed differences in selectivity. Ti-beta and TS-1 are distinctly more selective than the mesoporous material. Recalling that tetrapodal titanium sites are more predominant in the former two molecular sieves although tripodal titanium sites are the major surface species over the latter mesoporous material (Section II), we infer that the data indicate that high epoxidation selectivity is probably correlated with the presence of tetrapodal structures in these two molecular sieves. This correlation is discussed in Section VI. 

The majority of the titanium ions in titanosilicate molecular sieves in the dehydrated state are present in two types of structures, the framework tetrapodal and tripodal structures. The tetrapodal species dominate in TS-1 and Ti-beta, and the tripodals are more prevalent in Ti-MCM-41 and other mesoporous materials. The coordinatively unsaturated Ti ions in these structures exhibit Lewis acidity and strongly adsorb molecules such as H2O, NH3, H2O2, alkenes, etc. On interaction with H2O2, H2 + O2, or alkyl hydroperoxides, the Ti ions expand their coordination number to 5 or 6 and form side-on Ti-peroxo and superoxo complexes which catalyze the many oxidation reactions of NH3 and organic molecules. 

In the titanosilicate system, cyclic voltametric measurements had indicated (Section III.D) that the electron density at the tripodal sites is higher than at the tetrapodal sites. Hence, by analogy with the chromium and manganese complexes, we may expect the tripodal sites to favor hydrogen abstraction and allylic CH oxidation, although electron transfer and epoxidation occur preferentially on the tetrapodal sites. 

Significant progress has been achieved in the preceding few years in the study of titanosilicate molecular sieves, especially TS-1, TS-2, Ti-beta, and Ti-MCM-41. In the dehydrated, pristine state most of the Ti4+ ions on the surfaces of these materials are tetrahedrally coordinated, being present in either one of two structures a tetrapodal (Ti(OSi)4) or a tripodal (Ti(OSi)3OH) structure. The former predominates in TS-1, TS-2, and Ti-beta, and the latter is prominent in Ti-MCM-41. The Ti ions are coordinatively unsaturated and act as Lewis acid sites that coordinatively bind molecules such as H20, NH3, CH3CN, and H202. Upon interaction with H202 or H2 + 02, the Ti ions form titanium oxo species. Spectroscopic techniques have been used to identify side-bound hydroperoxo species such as Ti(02H) and superoxo structures such as Ti(02 ) on these catalysts. 

Figure 4. Bright - and annular dark field image of a CdSe tetrapod and an adjacent gold particle recorded in a TEM. The line profiles demonstrate Z-contrast in the dark field image.

---