Structural effect

Such effects are likely to be of special interest to a physical organic chemist since they are closely related to the transition state structure. Ring-size effects have been examined in order to determine the transition state geometry which is a function of the state of hybridisation of the a-carbon atom. Moreover, ring-substituent effects in the aromatic series are expected to depend closely on 

A similar conclusion has been reached from ring-size effects on rates of base-catalysed (NEt3 in DMF/D20) ionisation of phenyl cycloalkyl ketones [38] for which Shechter et al., (1962) found that the methine proton reactivities have the following relative values (isobutyrophenone chosen as a reference)  

Obviously, these results cannot be explained by a very enol-like transition state. This does not mean that enol stability does not affect reactivity, but that it probably does so to a lesser extent than at first expected. Such a conclusion runs counter to the first assertions and to what is usually assumed (see e.g. Lamaty, 1976), but is in agreement with the data cited above concerning Bronsted a-exponents. Indeed, the a-value of 0.74 observed for acid-catalysed enolisation of cyclohexanone (Lienhard and Wang, 1969) corresponds to a relatively early transition state since the Bronsted / for base-promoted proton abstraction from the hydroxycarbenium ion intermediate [see eqn (3)] equals 1 — 0.74, or 0.26. As pointed out above, some data on the stereochemistry of ketonisation were accounted for by assuming an enol-like transition state. Clearly, these interpretations need to be re-examined. 

Kinetic and thermodynamic data for acid-catalysed enolisation of cycloalkanones 104 H0./dm3 mol-1 s-1 AH /kcal mol 1 

4 Calculated from the first-order rate constants observed (Shechter et at., 1962) in the presence of HC1 (0.05 mol dm-3), by assuming that the hydronium ion is the only effective catalyst 425°C Dubois et al., 1981 

Most of the effects induced by the presence of fluorine atoms in a molecule come from both the structure and the fundamental atomic properties of the fluorine atom (Table 1.1). Because of its electronic structure ls 2s 2p, fluorine has very specific properties, as indicated by the extreme values of the atomic parameters given in Table 1.1.  

The very high ionization potential and the low polarizability of the fluorine atom imply that fluorinated compounds have only weak intermolecular interactions. Thus, perfluoroalkylated compounds have very weak surface energies, dielectric constants, and refracting indexes. 

The dipolar nature of the C—F bond in lightly fluorinated molecules gives a polar character to these molecules . Consequently, their physico-chemical properties can be quite different from those of hydrocarbon compounds and from those of the corresponding perfluorinated compounds. 

In brief, the effects of fluorination on the molecular properties stem from the combination of the atomic properties of the fluorine atom strong electronegativity, small size, excellent overlap of the 2s or2p orbitals with the corresponding orbitals of carbon, and very strong bond with carbon. 

Atom Ionization Potential (kcal/mol) Electron Affinity (kcal/mol) Atom Polarizability (A ) Van Der Waals Radii (A) Pauling s Electronegativity Xp 

We have also made detailed geometric comparisons between the structures of dimethoxymethane from the MM4 and BC calculations. Ab initio calculations in general give an approximation to the equilibrium (r ) structures for molecules. To compare these with experimental structures, the effects of vibrational motion have to be taken into account (Chapter 2). From a study of the basis set/correlation truncation errors from BC calculations, the CO bond needs a correction from the raw value (an 

TABLE 7.4. Geometry (re) and Energy (kcal/mol) Data for Dimethoxymethane  

Compared with other metal nuclei, the Li chemical shift scale is rather small and the NMR signals of Li encompass not more than ca 6 ppm for salt solutions and about 12 ppm for organolithium compounds. This can be attributed to the relatively small paramagnetic contribution to the shielding constant of Li which leads to a near cancellation of the diamagnetic term [46]. In addition, Li shifts are sensitive to solvent effects, viscosity. 

There is a remarkable shielding effect, generally classified as ring current effect on the Li resonance in polyhapto lithium compounds of organic n-systems [57-66], where lithium is situated above the plane of the n-cloud (Table 4). Low-frequency and high-frequency shifts indicate diatropic and paratropic properties, respectively. The shielding effect is most pronounced for situations where Li is sandwiched between two ic-systems [61, 62, 66], but smaller than expected in dianions of condensed benzenoid aromatics. 

Recently, deuterium-induced isotope effects on Li chemical shifts in organolithium compounds have been observed [51] and based on this observation, the isotopic fingerprint method was developed as a tool for structural investigations in the field of organolithium compounds. With this method, typical Li multiplets, which are characteristic of the aggregation state. 

Given a dimeric structure of type 1, like that of phenyllithium (R = C6Hs) in the presence of tetramethylethylene diamine (TMEDA) [71], and a 1 1 mixture of deuterated and non-deuterated ligands R (d and h, respectively), the Li environments 2-4 exist  

Hn THF relative to external aqueous I M LiQ, if not otherwise stated. OEP- octaethylporphyrine. Rdative to external 0.3 M LiCl in MeOH. CP isodicyclopentadienyl. Relative to external I M LiBr in THF. 

Proton-conducting materials have rather special structures , ion-exchange and acid-base properties It is necessary to take into account the presence of either cation vacancies (such as vacancies) and 

The structural chemistry in these materials is essentially that of H bonds, that is to say the behaviour of different pairs of bases towards H for example HjO and CIO towards in 

This leads not only to the partial or total internal ionization of the material but also influences the conduction mechanism. 

The crystal structures of proton conductors depend on (a) the anionic part of the material, (b) the cationic part and (c) the defects. Anions can be (1) monomeric spheres such as CIO (2) polymeric layers as in 3-alumina (3) polymeric channelled skeletons as in antimonic add hydrates and (4) drowned oxoanion clusters in water, as in heteropolyadd hydrates. 

Note that most of the proposed combinations are written as ionic formulae. It must be pointed out (Table 1.1), however, that, in acid hydrates, the acidic function is often not entirely ionized. This factor is taken into account as and when necessary, 

The introduction of isotopes into a compound alters the coupling pattern and the chemical shifts of the observed spectrum. As shovm in Eigure 1.18, deuterium-induced chemical shift variations have allo ved the estimation of the ratio of isomers 90a-d formed in Eq. (2) vhen R = Ph, R = CHjCOOH, and DCOOD/ Et3N is used for the hydrogen transfer [136]. The three sp -carbons Cl, C2 and C3 each afford a distinct singlet for the four possible isotopomers 90a-d (replacement of by shifts the resonances of the adjacent carbon nuclei to lo ver frequency) [137]. 

Since the rate-determining stage in an electrophilic addition reaction often involves the attack of the electrophile upon the unsaturated system, factors which affect the electronegativity of the atom being attacked will influence the rate of the reaction. In the acid-catalysed hydration of olefins, which in dilute solutions follows the simple kinetic form 

RELATIVE RATES OF HYDRATION OF OLEFINES. CH2=C(Me)R, in 29.6% perchloric ACID AT 38°C 

Here the reaction is assisted by conjugative and hyperconjugative effects, and is retarded by the inductive effect which withdraws electrons from the reaction site. A similar effect is found in the addition of bromine to olefins (RCH=CH2) in acetic acid at 25°, viz- 

Structural effects in the reactant also have an influence upon the reaction 

The mechanisms of electrophilic addition reactions to unsaturated systems has been the subject of a recent monograph for this reason it is not proposed to discuss the criteria for adopting a mechanism in any detail in the present work, although it must not be thought that all such proposed mechanisms have been demonstrated incontravertibly. 

The molar rotation [ ] caused by a given asymmetric carbon atom in compounds of the general type R— CH(CH3)—(CH2) —CHa is only slightly influenced by distant neighboring groups (Table 4-8). The measurable optical rotation depends on the sensitivity of the polarimeter and on any special experimental conditions used. For example, L-malic acid rotates to the left in dilute aqueous solution and to the right in concentrated solution. The optical rotation is zero at a certain concentration in water, although L-malic acid is chiral. All optically active systems are therefore chiral. Whether a chiral system is optically active or not depends on the conditions. 

Molina and Hammer studied the size of small Au particles dispersed on the MgO(lOO) surface by DFT. The structure shown in Fig. 2.21b is found to be more stable by 0.1 eV 

A decrease in particle size increases the relative ratio of surface atoms and atoms which have lower coordination numbers. Therefore, the reactivity of these particles tends to increase. Interestingly, often the activity per surface atom also changes. For supported catalysts, the turnover number (TON) typically goes through a maximuml . Smaller particles lead to increased coordinatively unsaturated sites. The adsorption energies at these sites are typically the highest. Dissociative addition reactions, therefore preferentially occur on such sites also for reactions that are positioned to the left of the Sabatier maximum. This is consistent with an increase in TON. Second, the active sites are sometimes positioned at the interface of metal particle and catalyst suppmrt. With a decrease in particle size, this interface between the metal and support increases, which should also increase the overall activity. 

Results of DFT calculations predict that Pd, as well as Pt, on the ideal MgO(lOO) surface will tend to form clusters rather then isolated ionsl . Metals that lie closer to the right in the periodic table, such as Cu, form much weaker metal-metal bonds. Copper, silver and gold tend to prefer isolated ions which tend wet the surface. For a detailed review on metal-support interactions we refer to the review by Campbelll . 

The adsorbate bond energy increases with increase in the degree of coordinatively unsaturated metal atoms. This is due to the decrease in the localization energy of electrons on the Au surface atoms for structures with fewer neighboring atoms. 

As we will discuss in more detail in Chapter 3, the delocalization of electrons is proportional to the square root of the number of coordinating atomsl l. One would therefore expect adsorbate binding energies to increase with decreasing particle size, owing to the increased number of coordinatively unsaturated surface atoms. The reactivity of these particles with respect to cluster size will then depend the position of the adsorbate bond energy with resp ect to the Sabatier curve maximum. 

As noted above, only a qualitative analysis can be done in view of the different approaches to defining the Tip, and, in general, the use of Tip or Tdem instead of the LCST as a uniform and unique temperature to define the phase transition 

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Chemical Structures Effective in Improving the Gasoline Octane Number... 

The greater positive character hence the increased acidity of the O—H proton of 2 2 2 tnfluoroethanol can be seen m the electrostatic potential maps displayed m Figure 1 8 Structural effects such as this that are transmitted through bonds are called indue tive effects A substituent induces a polarization m the bonds between it and some remote site A similar inductive effect is evident when comparing acetic acid and its trifluoro derivative Trifluoroacetic acid is more than 4 units stronger than acetic acid... 

The carbonyl carbon of a ketone bears two electron releasing alkyl groups an aldehyde carbonyl group has only one Just as a disubstituted double bond m an alkene is more stable than a monosubstituted double bond a ketone carbonyl is more stable than an aldehyde carbonyl We 11 see later m this chapter that structural effects on the relative stability of carbonyl groups m aldehydes and ketones are an important factor m then rel ative reactivity... 

J. Hine, Structural Effects on Equilibria in Organic Chemistyff ohxi Wiley Sons, Inc., New York, 1975, p. 259, and references cited therein. 

Mine, J. "Structural Effects on Equilibria in Organic Chemistry", Wiley-Interscience New York, N.Y., 1975, p. 287. 

These techniques have very important applications to some of the micro-structural effects discussed previously in this chapter. For example, time-resolved measurements of the actual lattice strain at the impact surface will give direct information on rate of departure from ideal elastic impact conditions. Recall that the stress tensor depends on the elastic (lattice) strains (7.4). Measurements of the type described above give stress relaxation directly, without all of the interpretational assumptions required of elastic-precursor-decay studies. 

Cathodoluminescence, CL, involves emission in the UV and visible region and as such is not element specific, since the valence/conduction band electrons are involved in the process. It is therefore sensitive to electronic structure effects and is sensitive to defects, dopants, etc., in electronic materials. Its major use is to map out such regions spatially, using a photomultiplier to detect all emitted light without... 

It is difficult to attribute the capacity reduction which occurs above 700°C to structural effects because the structure of the samples is not changed significantly, as indicated by our X-ray diffraction measurements on the CRO samples. On the other hand, the hydrogen content of the samples is dramatically reduced over this temperature range. To investigate the importance of the hydrogen content, we made a series of cells from the other samples. 

These structural effects are also found by MO calculations. Calculations at die MP4/6-311++G level have been performed on the ally cation and indicate a rotation barrier of 36-38 kcal /mol. ... 

Provide a rationalization of this structural effect in terms of MO theory. Construct a qualitative MO diagram for each conformation, and point out the significant differences that can account for the preference for the eclipsed conformatiptL... 

The electrostatic solvait effects discussed in the preceding paragraphs are not the only possible modes of interaction of solvent with reactants and transition states. Specific structural effects may cause either the reactants or the transition state to be particularly stroi ly solvated. Figure 4.12 shows how such solvation can affect the relative energies of the ground state and transition state and cause rate variations from solvent to solvent. 

Any structural effect which reduces the electron deficiency at the tricoordinate carbon will have flie effect of stabilizing the caibocation. Allyl cations are stabilized by delocalization involving the adjacent double bond. 

Another significant structural effect that inrparts high nucleophilicity is the alpha effect. It is observed that atoms which are directly bonded to an atom with one or rhore unshared pairs of electrons tend to be stronger nucleophiles than would othermse be expected. Examples in Table 5.7 include H02, which is more nucleophilic than HO , and... 

Structural effects on the rates of deprotonation of ketones have also been studied using veiy strong bases under conditions where complete conversion to the enolate occurs. In solvents such as THF or DME, bases such as lithium di-/-propylamide (LDA) and potassium hexamethyldisilylamide (KHMDS) give solutions of the enolates in relative proportions that reflect the relative rates of removal of the different protons in the carbonyl compound (kinetic control). The least hindered proton is removed most rapidly under these... 

Three-dimensional potential energy diagrams of the type discussed in connection with the variable E2 transition state theory for elimination reactions can be used to consider structural effects on the reactivity of carbonyl compounds and the tetrahedral intermediates involved in carbonyl-group reactions. Many of these reactions involve the formation or breaking of two separate bonds. This is the case in the first stage of acetal hydrolysis, which involves both a proton transfer and breaking of a C—O bond. The overall reaction might take place in several ways. There are two mechanistic extremes ... 

Both the language of valence bond theory and of molecular orbital theory are used in discussing structural effects on reactivity and mechanism. Our intent is to illustrate both approaches to interpretation. A decade has passed since the publication of the Third Edition. That decade has seen significant developments in areas covered by the text. Perhaps most noteworthy has been the application of computational methods to a much wider range of problems of structure and mechanism. We have updated the description of computational methods and have included examples throughout the text of application of computational methods to specific reactions. 

Deviations from this generalization may have several sources, including charge repulsion, steric effects, statistical factors, intramolecular hydrogen bonding, and other structural effects that alter electron density at the reaction site. Hague - ° P has discussed these effects. 

Why should the cores of most globular and membrane proteins consist almost entirely of a-helices and /3-sheets The reason is that the highly polar N—H and C=0 moieties of the peptide backbone must be neutralized in the hydrophobic core of the protein. The extensively H-bonded nature of a-helices and /3-sheets is ideal for this purpose, and these structures effectively stabilize the polar groups of the peptide backbone in the protein core. 

So how do the foot structures effect the release of Ca from the terminal cisternae of the SR The feet that join the t-tubules and the terminal cis-ternae of the SR are approximately 16 nm thick. The feet apparently function by first sensing either a voltage-dependent conformation change (skeletal mus-... 

Removal of the the related ethoxymethyl group from an imidazole with 6 N HCl at reflux is slow and low yielding. Small structural effects at a site seemingly remote from the MOM group can have a significant influence on the deprotection process. The MOM group in compound a is easily removed with acid, but the cleavage with HCl in compound b proved quite difficult. ... 

From the data reported in Table II it can be seen that a specific structural effect of the substrate plays a role in determining the appa-... 

IV, C, 1, d). Second, for both classes of aromatic compounds such values show a surprisingly small dependence on the nature of the attacking reagent, probably indicating the predominant role of the reorganization of the substrate toward a new state represented by structure 63 or 65. FinaUy, it may not be fortuitous that a correspondence is found between structural effects on substitution rates and on ionization constants (Section IV,C, l,a). Bond-making would in fact be the essential analogy between these phenomena [Eqs. (16) and (17)], and... 

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