TSLS complex structure

The microporosity of a new tubular silicatelayered silicate nanocomposite formed by the intercalation of imogolite in Na -montmorillonite has been characterized by nitrogen and m-xylene adsorption. The nitrogen adsorption data yielded liquid micropore volume of -0.20 cm g as determined by both the t-plot and the Dubinin-Radusikevich methods. The t-plot provided evidence for a bimodal pore structure which we attributed to intratube and intertube adsorption environments. The m-xylene adsorption data indicated a much smaller liquid pore volume (-0.11 cm g ), most likely due to incomplete filling of intratubular pores by the planar adsorbate. The FTIR spectrum of pyridine adsorbed on the TSLS complex established the presence of both Bronsted and Lewis acid sites. The TSLS complex was shown to be active for the acid-catalyzed dealkylation of cumene at 350 C, but the complex was less reactive than a conventional alumina pillared montmorillonite. 

We have been investigating the use of imogolite as a pillaring agent for smectite clays with layer lattice structures ". The regular intercalation of the tubes within the layered host results in the formation of a tubular silicate-layered silicate (TSLS) complex. These new nanocomposite materials may be viewed as pillared clays in which the pillars themselves are microporous. Significantly, the TSLS structure is thermally stable up to 450 C when montmorillonite is selected as the layered host . 

Figure 3. Structure of a TSLS complex formed by intercalation of an imogolite monolayer in the galleries of Na" -montmorillonite.

FIGURE 29. Optimized geometries for the reactants, transition structures and products in the sequence of reactions between the hthium enolate (LiEn) derived from acetaldehyde and formaldehyde. The total reaction sequence is LiEn- -CH20->complex-l- TSl->-Pl->-complex-2->-TS2->-P2 (Figure 28)->-TS3 (Figure 30)->-P3 (Figure 28). Theoretical calculations were carried out at the HF/6-31H—l-G level. Reprinted with permission from Reference 29. Copyright 1998 American Chemical Society... 

The origin of the enantiodiscrimination appears to be strongly dependent on the structure of the HCLA employed. For HCLA bases of type A (53 to 56), stereoselectivity has been empirically deduced to arise [in the transition state (TS)] from the difference of energy between the two diastereoisomeric 1/1 HCLA/oxirane complexes TSl and TS2 (Scheme 27). Indeed, the steric repulsions between cyclohexene oxide and the pyrrolidinyl substituents in TS 1 favor TS 2, as proposed by Asami in 1990 for enantioselective rearrangement of cyclohexene oxide by HCLA 53 (Scheme 26) . ... 

Figure 2, Structure of tt-complex and transient state in the coordinated (TSl) addition of ozone to ethylene.

In [20], the RHF calculations showed that the PEP has one saddling point in the TS region this point corresponds to symmetrical TS with Rj= R2 = 2.165 A. The TS structure is similar to that of the Ti-complex schematically, it is shown in Figure 2 (TSl). Consequently, for the intreraction of ozone with ethylene considered in the one-determinant approximation (where = 0), the reaction pathway through the formation of symmetrical TS is more energy-advantageous. 

Structures of the nine TS are shown in Scheme 1.24. At the level of theory used, TSl and TS2 appear to be very similar. Three parameters are of interest as to whether or not these structures are derived from the bromo—sigma complex. The fint is the distance between bromide ion and the bromine atom associated to the benzene molecule, while the second is the Br—C distance of the latter. The third parameter recorded on the images of the structures is the Br-C-H angle at the C atom to which Br is attached. The importance of the Br—Br distance relative to the question of whether or not the Wheland intermediate is involved relates to the separation of charge in the bromine molecule. This requires considerable energy and is not expected to take place without compensation from bond formation of the resulting Br moiety. The distribution of charge on the... 

Figure 2.2). Further C-N bond formation is the stereoselective stage. The structurally similar TSl(i ) and TS(S) are nicely discriminated by 5.2 kcal/mol due to the unavoidable hindrance between the carboxy-methyl substituent in 12 and BOC substituent in 21 that is absent in 19 (Figure 2.2, Scheme 2.5). However, the structurally different mode of the substrate bifimctional activation available in the H-boimd complex 17 and realized in the adduct 20 results in the possibility of the formahon of the S-product via TS2 which is only 2.2 kcal/mol higher in energy than the TSl(B) (Scheme 2.5, Figure 2.2).

It is seen that, after 12 fs, the wave packet splits into two parts, moving to the left and to the right, respectively. The part on the left-hand side represents the molecules that isomerize to the ethylene complex. These parts are absorbed by a complex absorbing potential (CAP) that simulates decay of the complex, and they disappear for t > 80 fs. The part that moves along the right-hand side of the barrier runs into the local minimum of the P-agostic structure. It evolves further to the TS2 barrier, where it is reflected at 144 fs. Subsequently, the wave packet returns to the TSl barrier, where a part of it overcomes the barrier and is thus... 

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