Structural feature

Structural variation in non-oxo (a), 0x0- (b) and dioxovanadium complexes (c) with coordination number cw = 4 (I), 5 (II), 6 (III), 7 (IV) and 8 (V). For cn = 6, structure types Ilia and Ilia, the antiprismatic (octahedral) and prismatic arrangements are additionally highlighted. 

Non-oxo pentacoordinated vanadium complexes can adopt the stmcture of a trigonal bipyramid [VF5 in accord with valence shell-electron pair repulsion (VSEPR) theory], or that of a square pyramid [predicted for ( 113)5]. ] 

Note that we are dealing with octahedral geometry, and not with octahedral symmetry (point group Of,)-[V(H20)g] +, a (f system, has lower than Oj, symmetry due to Jahn-Teller distortion. 

In dinuclear oxovanadium complexes, the two oxovanadium centres may be bridged through one or two ligands. These can be 0 (jjl—0), halides, in particular CP, or alcoholate functions OR. The doubly bonded oxo group also can act as a bridging ligand (with a reduction in bond order as discussed above), mainly so if the mononuclear units are based on d/oxovanadium species. The bridging mode can be symmetric in 

Configurations of the central V=0(/tt—0) 2 and (V=0)2/t-—O cores in binuclear oxovanadium complexes. The classifications follow the orientations of the terminal oxo gronps with respect to the plane (edge-sharing units) or the V-O-V fragment (corner-sharing nnits), partly following ref. 36. 

Another structural feature of 1 that favors transannulation is the already virtual planarity of the bridgehead nitrogen. A conformational change in the cage upon deprotonating protonated 7 results in substantial steric repulsions according to density functional calculations carried out in other laboratories, and this effect is believed to account for the weak acidity of this cation [77]. 

Basicity/Nucleophilidty of Pro- and Quasi-Azaphosphatranes with Reactive Z Groups 

The related imidophosphorane wherein Z = PhCH2N, and its acyclic analogue PhCH2N=P(NMe2)3, are very efficient catalysts for the protective acylation of alcohols in the presence of enol esters [96]. Acid-labile groups (such as acetal and epoxide) survive and groups such as TBS and disulfide [which undergo 

The protonated species in Fig. 6 wherein Z=H2N is apparently partially transannulated, in contrast to what would be expected for Z = HN [97]. The acychc H2N=P(NMe2)3+ cation, structured as the acetate salt by others, also revealed an ionic structure, with an ion pair being formed via a bifurcated 3-center hydrogen bond [NH( - 0)2] from a proton on the imido nitrogen to both oxygens of an acetate ion [98]. 

In competitive deprotonation experiments in KO-t-Bu/DMSO, the relative proton acidities of cations la, lb, and Ig were estimated from P-NMR equilibrium measurements to be between 27 and 30 [72]. More recently, the pKa values of 2b, 2d, 2e, 2f, and 10a were measured in acetonitrile and they are listed in Table 1 along with values for other non-ionic bases for comparison. Although bases 1 are about ten orders of magnitude weaker in basicity than P4-t-Bu or Pj-t-Bu in acetonitrile, they are ten to twenty orders of magnitude stronger than nitrogen bases in this solvent. It should also be recalled that, unlike phosphazene bases, pro-azaphosphatranes protonate at the phosphorus rather than at one of the two types of nitrogens in these molecules. 

While discussing the structural features special emphasis shall be laid only to those molecules that are 

Examples (a) meta-orientation about an aromatic ring, and 

The above two instances are sufficient to insulate chromophores from each other totally, 

These bands will be discussed briefly here with regard to the structural features. 

It is quite evident that the conjugated systems might fail to display the expected conjugated bands due to the following two reasons, namely  

In spite of numerous efforts, we are still far from being able to offer any satisfactory answer to question 1 posed in Sect. 1.1. One obvious starting point in this direction 

A more profound structural property of benzenoid systems is the following 

Theorem 1. Let C be a cycle of a benzenoid system. The size of C is necessarily an even integer. If the size of C is divisible by four, then in the interior of C there is an odd number of vertices. Otherwise, in the interior of C there are either no vertices or their number is even. 

This result has long been known (e.g. see [6]), but its complete proof was offered quite recently [10]. One of its proper consequences is that all conjugated circuits in all benzenoid systems have sizes not divisible by four (see pp. 85-87 in [3]). Another consequence of Theorem 1 is 

Corollary 1.1. Catacondensed benzenoid systems do not possess cycles whose sizes are divisible by four. 

Si-0-Si bond angle of 143° is much more open than the usual tetrahedral bonding (occurring at and can invert through the linear 

The Ugands give rise to complexes with very similar coordination geometries because of their rigid and predefined geometry, which is a result of three common attributes  

2) We have also synthesized several complexes with an indenyl instead of cyclopentadienyl ring and some of them showed exciting polymerization behavior. However, in solution the catalysts suffered from insufficient stability over a longer polymerization time, especially at higher temperatures. 

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Figure 1.8 Optimization discards many structural features, leaving an optimized structure.

Alternative superstructures to those in Figs. 16.26 and 16.27 can be developed. On the one hand, it is desirable to include many structural options to ensure that all features which are candidates for an optimal solution have been included. On the other hand, including more and more structural features increases the computational load dramatically. Thus care should be taken not to include unnecessary features in the superstructure. 

The structure in the reflectivity can be understood in tenns of band structure features i.e. from the quantum states of the crystal. The nonnal incident reflectivity from matter is given by... 

It is possible to identify particular spectral features in the modulated reflectivity spectra to band structure features. For example, in a direct band gap the joint density of states must resemble that of critical point. One of the first applications of the empirical pseudopotential method was to calculate reflectivity spectra for a given energy band. Differences between the calculated and measured reflectivity spectra could be assigned to errors in the energy band... 

Outline the laboratory preparation of a sample of dinitrogen tetroxide. Describe and explain what happens when it is heated from 290 K to 900 K. Suggest electronic structures for dinitrogen tetroxide and the other nitrogen-containing molecules formed from it on heating to 900 K. Point out any unusual structural features. 

Reactions can be considered as composite systems containing reactant and product molecules, as well as reaction sites. The similarity of chemical structures is defined by generalized reaction types and by gross structural features. The similarity of reactions can be defined by physicochemical parameters of the atoms and bonds at the reaction site. These definitions provide criteria for searching reaction databases [23],... 

Structural keys describe the chemical composition and structural motifs of molecules represented as a Boolean array. If a certain structural feature is present in a molecule or a substructure, a particular bit is set to 1 (true), otherwise to 0 (false). A bit in this array may encode a particular functional group (such as a carboxylic acid or an amidelinkage), a structural element (e.g., a substituted cyclohexane), or at least n occurrences of a particular element (e.g., a carbon atom). Alternatively, the structural key can be defined as an array of integers where the elements of this array contain the frequency of a specific feature in the molecule. 

Two approaches to quantify/fQ, i.e., to establish a quantitative relationship between the structural features of a compoimd and its properties, are described in this section quantitative structure-property relationships (QSPR) and linear free energy relationships (LFER) cf. Section 3.4.2.2). The LFER approach is important for historical reasons because it contributed the first attempt to predict the property of a compound from an analysis of its structure. LFERs can be established only for congeneric series of compounds, i.e., sets of compounds that share the same skeleton and only have variations in the substituents attached to this skeleton. As examples of a QSPR approach, currently available methods for the prediction of the octanol/water partition coefficient, log P, and of aqueous solubility, log S, of organic compoimds are described in Section 10.1.4 and Section 10.15, respectively. 

Descriptors have to be found representing the structural features which are related to the target property. This is the most important step in QSPR, and the development of powerful descriptors is of central interest in this field. Descriptors can range from simple atom- or functional group counts to quantum chemical descriptors. They can be derived on the basis of the connectivity (topological or... 

On the other hand, techniques like Principle Component Analysis (PCA) or Partial Least Squares Regression (PLS) (see Section 9.4.6) are used for transforming the descriptor set into smaller sets with higher information density. The disadvantage of such methods is that the transformed descriptors may not be directly related to single physical effects or structural features, and the derived models are thus less interpretable. 

The correction term in Eq. (9) shows that the basic assumption of additivity of the fragmental constants obviously does not hold true here. Correction has to be appHed, e.g., for structural features such as resonance interactions, condensation in aromatics or even hydrogen atoms bound to electronegative groups. Astonishingly, the correction applied for each feature is always a multiple of the constant Cu, which is therefore often called the magic constant . For example, the correction for a resonance interaction is +2 Cj, or per triple bond it is -1 A detailed treatment of the Ef system approach is given by Mannhold and Rekker [5]. 

A series of monographs and correlation tables exist for the interpretation of vibrational spectra [52-55]. However, the relationship of frequency characteristics and structural features is rather complicated and the number of known correlations between IR spectra and structures is very large. In many cases, it is almost impossible to analyze a molecular structure without the aid of computational techniques. Existing approaches are mainly based on the interpretation of vibrational spectra by mathematical models, rule sets, and decision trees or fuzzy logic approaches. 

Spectral features and their corresponding molecular descriptors are then applied to mathematical techniques of multivariate data analysis, such as principal component analysis (PCA) for exploratory data analysis or multivariate classification for the development of spectral classifiers [84-87]. Principal component analysis results in a scatter plot that exhibits spectra-structure relationships by clustering similarities in spectral and/or structural features [88, 89]. 

Caleulations by the more rigorous proeedure yield, in MM3, a sum of (a) bond energies, (b) steric energy, (c) vibrational zero point and thermal energies, and (d) structural features POP and TORS. Energies (a), (b), and (d) are calculated as before. Bond energy parameters appear to be quite different from those of the default MM3 calculations canied out so far because zero point and thermal energies are not included in the parameters but are added later. 

Recognition of functional groups or gross structural features. 

Determination of structural features. The ultraviolet spectrum has been of value in the determination of the structure of several vitamins. Thus the presence of an a-naphthoquinone system in vitamin K was first detected by this means. Also the 4-methylthiazole and the 2 5-dimethyl-6-aminopyridine system was first identified in vitamin Bj (thiamine), a- and /3-Ionones can be distinguished since the former contains two conjugated chromophores and the latter three conjugated chromophores. 

We have considered the following structural features affecting the choice of antitheticaf steps starting from simple target molecules ... 

This case history presents only a simple account of one of R.B. Woodward s adventures based on ingenious undentanding of structural features and experimental findings described in the literature. The hydrogenation of porphyrins is still one of the most active subjects in heterocyclic natural products chemistry, and the interested reader may find some modem developments in the publications of A. Eschenmoser (C.Angst, 1980 J.E. Johansen, 1980). 

The structural features of methane ethane and propane are summarrzed rn Ergure 2 7 All of the carbon atoms have four bonds all of the bonds are srngle bonds and the bond angles are close to tetrahedral In the next sectron we 11 see how to adapt the valence bond model to accommodate the observed structures... 

The structural feature that Figures 3 2 and 3 3 illustrate is the spatial relationship between atoms on adjacent carbons Each H—C—C—H unit m ethane is characterized by a torsion angle or dihedral angle which is the angle between the H—C—C plane... 

The first widely used molecular mechanics pro gram was developed by Professor N L Allinger of the University of Georgia and was known in its various versions as MM2 MM3 and so on They have been re fined to the extent that many structural features can be calculated more easily and more accurately than they can be measured experimentally... 

Because so many factors contribute to the net intermolecular attractive force it is not always possible to predict which of two compounds will have the higher boiling point We can however use the boiling point behavior of selected molecules to inform us of the relative importance of various intermolecular forces and the structural features that influence them... 

Certain structural features can sometimes help us determine by inspection whether a mol ecule IS chiral or achiral For example a molecule that has a plane of symmetry or a cen ter of symmetry is superimposable on its mirror image and is achiral... 

In this example addition to the double bond of an alkene converted an achiral mol ecule to a chiral one The general term for a structural feature the alteration of which introduces a chirality center m a molecule is prochiral A chirality center is introduced when the double bond of propene reacts with a peroxy acid The double bond is a prochi ral structural unit and we speak of the top and bottom faces of the double bond as prochiral faces Because attack at one prochiral face gives the enantiomer of the com pound formed by attack at the other face we classify the relationship between the two faces as enantiotopic... 

At this point It s useful to compare some structural features of alkanes alkenes and alkynes Table 9 1 gives some of the most fundamental ones To summarize as we progress through the series m the order ethane ethylene acetylene... 

Thus with dihalocarbenes we have the interesting case of a species that resem bles both a carbanion (unshared pair of electrons on carbon) and a carbocation (empty p orbital) Which structural feature controls its reactivity s Does its empty p orbital cause It to react as an electrophile s Does its unshared pair make it nucleophilic s By compar mg the rate of reaction of CBi2 toward a series of alkenes with that of typical electrophiles toward the same alkenes (Table 14 4) we see that the reactivity of CBi2... 

The structural features especially the very polar nature of the carbonyl group point clearly to the kind of chemistry we will see for aldehydes and ketones in this chapter The partially positive carbon of C=0 has carbocation character and is electrophilic The planar arrangement of its bonds make this carbon relatively uncrowded and susceptible to attack by nucleophiles Oxygen is partially negative and weakly basic... 

Certain structural features can make the keto-enol equilibrium more favorable by stabi hzmg the enol form Enolization of 2 4 cyclohexadienone is one such example... 

The two most important structural features that stabilize the enol of a (3 dicarbonyl compound are... 

The structural features of the carboxyl group are most apparent m formic acid Formic acid IS planar with one of its carbon-oxygen bonds shorter than the other and with bond angles at carbon close to 120°... 

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