Structures 2222-type

Fluorides 11 LaFa LaFj LaFa LaFa LaFa LaFa LaFa LaFa 

Trihalides are known for most of the actinides and form the basis for comparison with the lanthanides. Table 10.2 lists the known structures adopted by the trihalides. 

The actinide trihalides display a similar pattern of structure to those of the lanthanide trihalides. However, comparing the coordination numbers for Ln + and An + ions with the same number of f electrons ( above one another in the Periodic Table ), it can be seen that the coordination number of the lanthanide halides decreases sooner than in the actinide series, a reflection of the fact that the larger actinide ions allow more halide ions to pack around them. Table 10.3 gives comparative coordination numbers for the trihalides of the lanthanides and actinides. 

---

There are two hydrate structures, types 1 et 11, each composed of two cavity sizes. Only the lightest of the hydrocarbons can form hydrates. Table 4.20 gives the hydrates formed by the most common compounds. 

Meier W M, Olson D FI and Baerlocher Ch 1996 Atias ofZeoiite Structure Types 4th revised edn (London Elsevier)... 

As shown in Figure 27, an in-phase combination of type-V structures leads to another A] symmetry structures (type-VI), which is expected to be stabilized by allyl cation-type resonance. However, calculation shows that the two shuctures are isoenergetic. The electronic wave function preserves its phase when tr ansported through a complete loop around the degeneracy shown in Figure 25, so that no conical intersection (or an even number of conical intersections) should be enclosed in it. This is obviously in contrast with the Jahn-Teller theorem, that predicts splitting into A and states. 

The user is often more interested in the contents than in the technical organization of databases. The wide variety of data allows the classification of databases in chemistry into literature, factual (alphanumeric), and structural types (Figure 5-10) [12, 13). 

The organic chemical structural types believed to be characteristic of coals include complex polycyclic aromatic ring systems with connecting bridges and varied oxygen-, sulfur-, and nitrogen-containing functionalities. 

The structural types described m this section are but a tiny fraction of those pos sible The chemistry of heterocyclic aromatic compounds is a nch and varied held with numerous applications... 

At first glance splitting may seem to complicate the interpretation of NMR spectra In fact It makes structure determination easier because it provides additional information It tells us how many protons are vicinal to a proton responsible for a particular signal With practice we learn to pick out characteristic patterns of peaks associating them with particular structural types One of the most common of these patterns is that of the ethyl group represented m the NMR spectrum of ethyl bromide m Figure 13 15... 

Section 14 14 Transition metal complexes that contain one or more organic ligands offer a rich variety of structural types and reactivity Organic ligands can be bonded to a metal by a ct bond or through its it system Metallocenes are transition metal complexes m which one or more of the ligands is a cyclopentadienyl ring Ferrocene was the first metallocene synthesized Its electrostatic potential map opens this chapter... 

Saccharin sucralose and aspartame illustrate the diversity of structural types that taste sweet and the vitality and continuing development of the in dustry of which they are a part ... 

Lipids differ from the other classes of naturally occurring biomolecules (carbohy drates proteins and nucleic acids) in that they are more soluble m nonpolar to weakly polar solvents (diethyl ether hexane dichloromethane) than they are m water They include a variety of structural types a collection of which is introduced m this chapter... 

In spite of the number of different structural types lipids share a common biosyn thetic origin m that they are ultimately derived from glucose During one stage of car bohydrate metabolism called glycolysis glucose is converted to lactic acid Pyruvic acid IS an intermediate... 

The enzyme catalyzed reactions that lead to geraniol and farnesol (as their pyrophosphate esters) are mechanistically related to the acid catalyzed dimerization of alkenes discussed m Section 6 21 The reaction of an allylic pyrophosphate or a carbo cation with a source of rr electrons is a recurring theme m terpene biosynthesis and is invoked to explain the origin of more complicated structural types Consider for exam pie the formation of cyclic monoterpenes Neryl pyrophosphate formed by an enzyme catalyzed isomerization of the E double bond m geranyl pyrophosphate has the proper geometry to form a six membered ring via intramolecular attack of the double bond on the allylic pyrophosphate unit... 

Analogous processes involving cyclizations and rearrangements of carbocations derived from farnesyl pyrophosphate produce a rich variety of structural types m the... 

Crystal Structure. The structure type of crystals may be added in parentheses and in italics after the name the latter should be in accordance with the structure. When the typename is also the mineral name of the substance itself, italics are not used. 

W. M. Meier and D. H. Olson, Mtlas of Zeolite Structure Types, Juris Dmck and Vedag AG, Zurich, 1978. 

Another structural type is chromenes. Centchroman [31477-60-8]is a pyrrolidinoethoxyplienyl chromane which is a potent antiestrogen with weak estrogenic activity. In India, it is used as a weekly contraceptive pih based on its reported abiUty to inhibit the uterine preparation for the attachment of the fertilized ova to the wall of the utems (see Contraceptives) (31). 

Scheme S Illustrative 1,2-bielectrophiles classified by structural type...

Extension of the approach discussed above suggests several structural types which may be classified under this category, the main variable being the nature and hybridization of the electrophilic centers and the nature of the atoms joining these two centers. They may be conveniently divided into two groups ... 

Representative examples of these types are shown in Scheme 7. The range of structural types is greatly extended when acyl halides attached to any ring system in a vicinal relationship are considered. 

Although the wide diversity of structural types illustrated in Scheme 6 suggests that numerous examples of their application to five-membered heterocycles would be known,... 

This process is not as common as the other oxidative procedures and usually involves ring closure onto an aromatic or heteroaromatic ring. The following examples illustrate the structural types required for this cyclocondensation. 

Adsorbents Table 16-3 classifies common adsorbents by structure type and water adsorption characteristics. Structured adsorbents take advantage of their crystalline structure (zeolites and sllicalite) and/or their molecular sieving properties. The hydrophobic (nonpolar surface) or hydrophihc (polar surface) character may vary depending on the competing adsorbate. A large number of zeolites have been identified, and these include both synthetic and naturally occurring (e.g., mordenite and chabazite) varieties. 

This method is probably the most commonly used method for the cleavage of ethers because it generally gives excellent yields with a variety of structural types. BBr3 will cleave ketals. 

The fault ctirrents also develop electrodynamic foi ces, Fii, as in equation (28.4) due to the sub-transient d.c. component. These forces play an important role in the meehanical design of the interrupting device, the load-bearing and mounting structuies for the interrupter and the bus system, and the hardware used in a switchgear assembly. All such mechanical parts, supports and hardw-are should be adequate to withstand such forces when they arise, A procedure to arrive at the ideal size of the current-carrying components, mounting structure, type of supports and hardware cte, is discussed in detail in Example 28.12. 

There is some confusion in using Bayes rule on what are sometimes called explanatory variables. As an example, we can try to use Bayesian statistics to derive the probabilities of each secondary structure type for each amino acid type, that is p( x r), where J. is a, P, or Y (for coil) secondary strucmres and r is one of the 20 amino acids. It is tempting to writep( x r) = p(r x)p( x)lp(r) using Bayes rule. This expression is, of course, correct and can be used on PDB data to relate these probabilities. But this is not Bayesian statistics, which relate parameters that represent underlying properties with (limited) data that are manifestations of those parameters in some way. In this case, the parameters we are after are 0 i(r) = p( x r). The data from the PDB are in the form of counts for y i(r), the number of amino acids of type r in the PDB that have secondary structure J.. There are 60 such numbers (20 amino acid types X 3 secondary structure types). We then have for each amino acid type a Bayesian expression for the posterior distribution for the values of xiiry. 

For example, Stolorz et al. [88] derived a Bayesian formalism for secondary structure prediction, although their method does not use Bayesian statistics. They attempt to find an expression for / ( j. seq) = / (seq j.)/7( j.)//7(seq), where J. is the secondary structure at the middle position of seq, a sequence window of prescribed length. As described earlier in Section II, this is a use of Bayes rule but is not Bayesian statistics, which depends on the equation p(Q y) = p(y Q)p(Q)lp(y), where y is data that connect the parameters in some way to observables. The data are not sequences alone but the combination of sequence and secondary structure that can be culled from the PDB. The parameters we are after are the probabilities of each secondary structure type as a function of the sequence in the sequence window, based on PDB data. The sequence can be thought of as an explanatory variable. That is, we are looking for... 

Use very informative priors, where perhaps the prior could be based on the product of individual probabilities for each secondary structure type and amino acid type. 

In this case, we are looking for counts for each secondary structure type J. for each sequence x, which might be derived from PDB data by... 

At the present time, use of the Fischer convention is almost entirely restricted to carbohydrates, amino acids, and biologically important molecules of closed related structural types. The problem with more general use is that there are no adequate rules for deciding whether a diiral atom is like D-glyceraldehyde or L-glyceraldehyde when the structures are not closely similar to the reference molecules. This relationship is clear for carbohydrates and amino acids. 

The pAT values of a wide variety of organic compounds have been determined in DMSO. Some of these values are included in Table 7.2. It is not expected that the values will be numerically identical with aqueous pAT values, but for most compounds the same relative order of acidify is observed for hydrocarbons of similar structural type. 

---