Titanium explanations

NakayamaS however, has suggested that, for rutile, which is tetragonal in structure, the strong bond between metal and oxide results from the favourable spacing between titanium ions in the rutile lattice and those in the metal structure. This explanation, however, does not account for the fact that other oxides of titanium, such as brookite, which is orthorhombic, and anatase, which is tetragonal, are also protective . 

The elucidation of the factors determining the relative stability of alternative crystalline structures of a substance would be of the greatest significance in the development of the theory of the solid state. Why, for example, do some of the alkali halides crystallize with the sodium chloride structure and some with the cesium chloride structure Why does titanium dioxide under different conditions assume the different structures of rutile, brookite and anatase Why does aluminum fluosilicate, AljSiCV F2, crystallize with the structure of topaz and not with some other structure These questions are answered formally by the statement that in each case the structure with the minimum free energy is stable. This answer, however, is not satisfying what is desired in our atomistic and quantum theoretical era is the explanation of this minimum free energy in terms of atoms or ions and their properties. 

The stereoselectivity of this reaction also depends on the titanium reagent used to prepare the enolate.104 When the substituent is benzyloxy, the 2,2 -anti-2,3-syn product is preferred when ( -PrO)TiCl3 is used as the reagent, as would be expected for a chelated TS. However, when TiCl4 is used, the 2,2 -syn-2,2-syn product is formed. A detailed explanation for this observation has not been established, but it is expected that the benzyloxy derivative would still react through a chelated TS. The reversal on use of TiCl4 indicates that the identity of the titanium ligands is also an important factor. 

The way in which aluminum alkyls and titanium halides combine together to form propagating centers have been discussed in depth for the last 15 years without any one mechanism taking precedence over another (6). The simplest explanation is that aluminum alkyl alkylates the transition metal in the crystal lattice to give a transition metal alkyl center. Polymerization takes place by... 

The formation of complexes between olefins and metal halides is particularly well documented for titanium tetrachloride [10, 11, 12] thus my theory can be applied with some confidence to systems which involve this metal halide. I will show that it provides a simple qualitative explanation for observations which have so far remained obscure and affords also a quantitative interpretation which is open to testing once the necessary... 

Spin trapping by PBN has also been employed to detect radical formation in a photo-Kolbe reaction in which acetic acid is irradiated (A > 360 nm) in the presence of platinized titanium dioxide powder (Kraeutler et al, 1978). The nitroxide observed was considered to be (PBN—Me ), but the published spectrum clearly shows the presence of a second species spectral overlap might therefore be an alternative to solvent polarity as an explanation of the discrepancy between the observed splitting parameters and those previously reported for this species. Where poor resolution obtains, it is important that... 

My own credentials are mixed. On the credit side is a refusal to predict concerted rrans-addition less impressive is a clear explanation of facile free radical loss from titanium tetramethyl complexes, shortly before evidence began to accu-mulate that such species decompose by internal or mutual abstraction. 

Thus, for similar values of ns, the entropy at infinite dilution for TiMo/H2 is about 3.4 eu/H more negative than that for 0-Ti/H2. An explanation for this difference might be that the molybdenum atoms in the metal lattice block potentially available interstitial sites for hydrogen occupation, resulting in nonrandom occupation of sites at low hydrogen content. Rudman (24) proposed such a role for aluminum in titanium. We presently are gathering more data for alloys of varying molybdenum composition to test this hypothesis. 

A final point of interest is that in two of the oils, titanium is considerably enriched relative to other elements, compared with the ratios in the original coal. A plausible explanation is that the titanium is present in organometallic combination in these oils. 

As was mentioned in Section V.C.3.b, when competitive oxidation of 1-octene and -hexane is carried out, the alkene is preferentially oxidized. Correspondingly, alkenes react at lower temperatures than alkanes. It is therefore surprising that under noncompetitive reaction conditions, the rate of oxidation of n-hexane is higher than that of 1-octene (Huybrechts et al., 1992). One possible explanation for this observation is that the reaction conditions were different (Clerici et al., 1993b). At 373 K titanium peroxo compounds decompose, thereby giving rise to radical chain reactions that are negligible at lower temperatures. Thus there could be a different mechanism for low- and high-temperature oxidations made more complex by secondary uncatalyzed oxidation of initial products (Spinace et al., 1995). 

Another allyl compound which reacts stoichiometrically with carbon dioxide is (fj5-C5H5)2Ti(l-methylallyl) (120). The titanium acetate complex which is formed is interesting in that the carbon dioxide carbon atom is attached to the substituted end of the allyl. It seems unlikely, then, that the product is the result of C02 insertion into the -methylallyltitanium bond in view of the fact that methyl-substituted allyls tend to form fj -complexes in which the metal is bonded to the least substituted end of the allyl. One possible explanation offered by the authors is that the allyl is bonded to titanium at the methylene carbon, but that rearrangement occurs subsequent to adduct formation [Eq. (49)]. 

A material prepared by anchoring titanium(IV) on to the walls of a high-area, crystalline mesoporous silica (MCM41) has been used as an alkene epoxidation catalyst with alkyl hydroperoxides.204 The effect of replacing one of the three O—Si= groups to which the Ti(IV) is bound by an O—Ge= group is reported to lead to an increase in catalytic activity of up to 18% in die epoxidation of cyclohexene, although no explanation is provided and it is notable diat the selectivity towards the formation of cyclohexene oxide (versus cyclohexenol and cyclohexane-1,2-diol) was inferior to that with the non-modified system.205... 

These results also support the theoretical explanation of the synthesis of titanium nitride and aluminum nitride given above. They show that metal chlorides can be decomposed and metals deposited in the intense low pressure plasma under similar conditions as those employed for the synthesis of nitrides. The interpretation of the experimental results given in Ref.33 can now be considered as the initial step in the development of the ideas presented in this review (see also30 ). 

The structure of the titanium-tartrate derivatives has been determined,25,26,31 37 and based on these observations together with the reaction selectivity, a mechanistic explanation has been proposed (Scheme 9.3).38 The complex 1 contains a chiral titanium atom through the appendant tartrate ligands. The intramolecular hydrogen bond ensures that internal epoxidation is only favored at one face of the allyl alcohol. This explanation is in accord with the experimental observations that substrates with an a-substituent (b = alkyl a = alkyl or hydrogen) react much slower than when this position is not substituted (b = hydrogen). 

A consensus on or explanation for the influence of the oxidation state of titanium on olefin polymerisation activity has not been reached. The absence of any insertion of the coordinating ethylene into the Ti-C bond in Ti(II) species is noteworthy instead, two ethylene molecules, which coordinate at two coordination sites at Ti(II) species, undergo an oxidative addition, and thus the respective metallacycle, titanacyclopentane, is formed [305], Such a reaction for dimethyltitaniumcomplexed by l,2-bis(dimethylphosphione)ethane [Dmpe] is as follows [305] ... 

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