T-sites

ATP is synthesized at the T site and released from the O site. Two additional passes through this cycle produce two more ATPs and return the enzyme to its original state. 

Recall that difference patterns are simply the space-time patterns of the difference between two evolutions of the same transition rule starting from two different starting configurations. For example, the value of the T site at time t of a difference pattern for a k = 2 global rule and two different initial global states cti(0) and... 

For simplicity, let p = m/n be the probability that a site contains a marker, and assume it is the same for all sites. Let the probability that there are s markers among t sites bo given by p(s, t), The ensemble average may then be written as... 

In order to derive activity-composition relationships for ThMgAl206 in clinopyroxene, Landwehr et al. (2001) used a mixing-on-sites model assuming complete short-range order between M and T sites. The activity of ThMgAl206 in clinopyroxene is thus given by ... 

Since both analyses assumed random occupancy of T sites, the NMR data substantiate the model of random distribution of silicon and aluminum in ZSM-4. 

A1 3Q MAS NMR spectroscopy and DFT/MM calculations were employed to study the A1 siting in eleven differently synthesized samples of the ZSM-5 zeolite. The results of our study reveal that the occupation of the distinguishable framework T sites by A1 and the concentration of A1 in these T sites are neither random nor controlled by a simple rule. They both depend on the conditions of the zeolite synthesis. We further found that at least 10 out of the 24 distinguishable framework T sites are occupied by A1 in our ZSM-5 samples. 

Theoretical calculations of the energies of A1 atoms in individual T sites revealed only small differences leading to conclusions that the A1 siting was either largely random or kinetically controlled during the synthesis [1-6],... 

In this study, the distribution of Al atoms over the individual T sites of the ZSM-5 framework is examined by a combined experimental and computational approach. Eleven differently prepared samples of ZSM-5 were studied by 27Al 3Q MAS NMR spectroscopy. To interpret the observed NMR spectra, we calculated the isotropic shift of Al occupying all the distinguishable T sites of ZSM-5 and used the calculated isotropic shifts to assign the observed resonances to individual distinguishable T sites. 

The estimated distribution of A1 atoms is shown in Figure 2. Both the siting as well as therelative concentration of A1 in the individual T sites significantly vary. Our QM-Pot calculations of ZSM-5 structures (PI symmetry) containing one A1 atom per unit cell resulted in the 24 structures corresponding to A1 substitution into the 24 distinguishable T sites of the monoclinic ZSM-5 and yielded 24 values of shieldings which were converted into 24 values of isotropic chemical shift. 

Figure 2 clearly indicates a large variability in both the sites occupied by Al atoms and the concentration of the Al atoms in these sites for the ZSM-5 samples studied. Even samples with similar framework aluminum concentration exhibit very different Al sitings as well as Al concentrations in the individual occupied T sites. This fact clearly rules out that the Al siting in the zeolite framework is random or is controlled by thermodynamic stability. It rather shows that the conditions of the synthesis are responsible for the Al siting in the framework. 

We have demonstrated that a combined experimental (27A1 3Q MAS NMR) and theoretical (QM-Pot employing the bare framework model) approach represents a powerful tool for the determination of the local geometry of framework A104 tetrahedra, the prediction of27A1 isotropic chemical shifts in hydrated silicon rich zeolites, and the identification of A1 siting in the framework of silicon-rich zeolites. Experimental evidence is provided for the occupation of at least 10 out of 24 distinguishable framework T sites by A1 atoms in silicon-rich ZSM-5. The conclusion is reached that the A1 distribution over the framework T sites is neither random nor controlled by a simple rule, but depends on the conditions of the zeolite synthesis. 

Figure 1 Binuclear iron core inserted over a) Z" and b) Z"0h clusters used to represent the 5MR portion of the ZSM-5 zeolite framework. T site numbers follow the ZSM-5 nomenclature. Pink balls are Al, grey balls are Si, blue balls are Fe, red balls are O and white balls are H.

Table 1. Relative energies computed, at the B3LYP/TZVP level theory, for [Fe(p-0)(p-0l I)Fe 1 inserted in two types of Z cluster (Z- and Z-OH). T-sites correspond to different Al-site locations over which the hinuclear iron core is inserted.

Fig. 15. Annealing behavior of BC and T site components of H in Si determined by ion channelling together with the annealing behavior of IR absorption lines in hydrogen-implanted Si. [Reprinted with permission from The American Physical Society, Bech Nielsen, B. (1988). Phys. Rev. B 37, 6353.]...

Bech Nielsen et a/. s experimental channeling data for the (100) axial channels is shown in Fig. 14. Together with 111 planar data, which showed a pronounced flux peak, these data clearly indicate a near bond-center site for the 2H. According to Bech Nielsen s analysis, the best fit to the data was obtained with 87% of the 2H atoms in the sample assigned to near BC sites and the rest to T sites. However, the attribution of the minority component could be influenced by radiation effects during the analysis, as will be discussed later. 

Fig. 14. Axial channeling scans from Nielsen et al. (1988) showing the yield from the (3He, ap) reaction with 2H in B—H pairs and the Si crystal host dip. The solid lines show a fit to the experimental data with the Statistical Equilibrium model for 87% of the 2H in a near bond-center site and the remainder in a T site.

After prolonged irradiation, Bech Nielsen et al. found an increased proportion of the 2H atoms at T sites, which they attribute to trapping of the deuterium at an interstitial, as in the work on implanted deuterium described earlier. 

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