Intra-tetrahedral

In these calculations averaged charges on the intra-tetrahedral lattice cation positions were used. The difference between the two heats of formation due to ionic bonding is added to the heat of formation due to covalent bonding resulting from the simple Extended Huckel Method for zeolitic silicas in order to arrive at the total heat of formation of the zeolite structure as a function of the amount of aluminum. 

The observed bands were classified by Flanigen et al. [112] into two types, namely internal modes of the TO4 tetrahedra ( intra-tetrahedral modes, cf. Sect 5.2) and external modes ( inter-tetrahedral modes) of the zeolite fi-amework. Table 1 summarizes the zeolite infrared assignment according to the FKS correlation. The internal vibrations represent structure-insensitive modes, and no distinction has been made between the modes of Si04 and AIO4 tetrahedra. The bands of external modes were observed to be sensitive to the structure, and their positions in the spectra are shifted in dependence on the framework topology and on the nsi/n i ratio. Even though this classification proved to be very successful in many applications, from a theoretical point of view such a division into... 

The two main categories of bands observed in mid infrared, i.e., intra-tetrahedral and inter-tetrahedral vibrations, are indicated by (1) solid and (2) broken lines, respectively. 

The most prominent bands ascribed to intra-tetrahedral modes were assiuned to occur in the ranges from (ij) 1250 to 950 cm k (i2) 790 to 650 cm, and (ij) 500 to 420 cm. Proposed assigmnents were as follows (cf. also Table 1 T=framework cation, e.g.. Si or Al) ... 

In a similar way, bands between 650 and 500 cm were attributed to inter-tetrahedral vibrations of double four-membered rings (D4R, as, e.g., in A-type zeolites) or double six-membered rings (D6R as, e.g., in X- and Y-type faujasite). Again, it must be stressed that the assigmnent of inter-tetrahedral modes to ring vibrations (ej, 02 see Table 1) soon became a matter of debate and deviating interpretations were advanced (cf. Sect 2.5 and Refe. [113,240]). The intra-tetrahedral vibrations were shown to be essentially structure-insensitive. This became evi-... 

Fig. 11. Zeolite framework vibrations according to the interpretation of Flanigen et al. (adopted from [ 112 ]) 1, solid lines intra-tetrahedral vibrations 2, broken lines inter-tetrahedral vibrations...

Tetrahedral coordination of all Sn atoms. No 233 intra- or intermolecular interactions between Sn and N atoms are present. 

There are several types of -class CAs i.e., a-CA I-VII, reported in the literature, out of which the human carbonic anhydrase II (HCA II), the most extensively studied carbonic anhydrase, has an exceptionally high CO2 hydration rate and a wide tissue distribution 107). The HCA II comprises a single polypeptide chain with a molecular mass of 29.3 kDa and contains one catalytic zinc ion, coordinated to three histidine residues, His 94, His 96, and His 119. A tetrahedral coordination geometry around the metal center is completed with a water molecule, which forms a hydroxide ion with a pK value of 7.0 108). Quigley and co-workers 109,110) reported that the inhibition of the synthesis of HCO3 from CO2 and OH- reduces aqueous humor formation and lowers intra-ocular pressure, which is a major risk factor for primary open-angle glaucoma. 

Complexes belonging to type 468 (M = Be, Zn, Cd, Hg, Pb) are very important subjects of study in the field of stereochemical nonrigid tetrahedral structures [851,852], Using the method of spin labels (a method in dynamic NMR), it is possible to determine the kinetic parameters, separately as intra- or intermolecular processes, for the stereoisomerization reactions in solution. 

Non-tetrahedral structures of organic derivatives of the silicon subgroup elements are often caused by inter- or intra-molecular coordination interaction X M. This takes place in compounds where there is a nucleophilic substituent at the central M atom. An electronegative X atom, which has at least one unshared electron pair (X = N, P, O, S, halogen) and is directly bonded to M, can be such a substituent. Compounds of this kind tend to be involved in inter-molecular coordination. There can be also a heteroatom X as part of the organic substituent at M in this case an intra-molecular coordination usually occurs . Such compounds, which contain five- or six-membered coordination rings, include, for example, draconoides (4) , their analogues (5) , metalloatranes (6) and others. The stability of a coordination bond X —> M increases with the atomic number of M Si < Ge < Sn < Pb. 

Tetrahedral coordination of solvent molecules The final idea checked here is whether incorporating a tetrahedral coordination structure for water molecules in liquid water significantly improves the prediction of cavity formation free energy. We used a cluster Poisson model to accomplish this (Neyman and Scott, 1956). The physical picture is of tetrahedral clusters of water molecules with prescribed intra-cluster correlations but random positions and orientations as suggested in Fig. 8.4. 

Other ionic liquids that show electronic transitions in the visible region of light are based on transition metals and lanthanides. For example, the magnetic ionic liquid [C4mim][FeCl4] shows an intense yellow-brownish colour which comes from the intra-configurational d-transition Fe3+ in a tetrahedral ligand field (Fig. 1) [16]. 

For tin(IV), the most frequent coordination geometries are based on tetrahedral, trigonal-bipyramidal, octahedral, and pentagonal-bipyramidal polyhedra." In organometallic tin(IV) complexes there is a tendency for the tin atoms to enhance their coordination number either via intra-molecular coordination or inter-molecular association. The final coordination number depends strongly on the number of organic substituents attached to the tin atom and the nature of the donor atoms of the ligands. ... 

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