Delocalized bonding interactions

The other C=N systems included in Scheme 8.2 are more stable to aqueous hydrolysis than are the imines. For many of these compounds, the equilibrium constants for formation are high, even in aqueous solution. The additional stability can be attributed to the participation of the atom adjacent to the nitrogen in delocalized bonding. This resonance interaction tends to increase electron density at the sp carbon and reduces its reactivity toward nucleophiles. 

As noted previously, arylamines are generally less basic than alkylamines. Anilinium ion has pKa = 4.63, for instance, whereas methylammonium ion has pfCa = 10.64. Arylamines are less basic than alkylamines because the nitrogen lone-pair electrons are delocalized by interaction with the aromatic ring tt electron system and are less available for bonding to H+. In resonance terms, aryl-amines are stabilized relative to alkylamines because of their five resonance forms. 

The amide bond that links different amino acids together in peptides is no different from any other amide bond (Section 24.3). Amide nitrogens are non-basic because their unshared electron pair is delocalized by interaction with the carbonyl group. This overlap of the nitrogen p orbital with the p orbitals of the carbonyl group imparts a certain amount of double-bond character to the... 

Fig. 5a-c Through-bond interactions in the triplet state of 1,3-diradical, a Mechanism of electron delocalization and polarization of a-spin electrons, b Cyclic orbital interaction, c Orbital phase continuity... 

The molecules (or atoms, for noble gases) of a molecular solid are held In place by the types of forces already discussed In this chapter dispersion forces, dipolar interactions, and/or hydrogen bonds. The atoms of a metallic solid are held in place by the delocalized bonding described in Section 10-. A network solid contains an array of covalent bonds linking every atom to its neighbors. An ionic solid contains cations and anions, attracted to one another by electrical forces as described in Section 8-. 

Notably, the bicyclic radical 41 is localized, in sharp contrast to the allylic-type delocalization of cyclotrigermenyl radical 39 and cyclotetrasilenyl radical 40. The obvions reason for snch a distinction is the absence of the neighboring to the Ge-radical center tt-bond necessary for the effective throngh-bond delocalization of the unpaired electron in the radical 41, whereas the through-space radical-C=C bond interaction is not sufficiently strong to indnce the effective delocalization of the unpaired electron. 

To determine the BEs (Eq. 1) of different electrons in the atom by XPS, one measures the KE of the ejected electrons, knowing the excitation energy, hv, and the work function, electronic structure of the solid, consisting of both localized core states (core line spectra) and delocalized valence states (valence band spectra) can be mapped. The information is element-specific, quantitative, and chemically sensitive. Core line spectra consist of discrete peaks representing orbital BE values, which depend on the chemical environment of a particular element, and whose intensity depends on the concentration of the element. Valence band spectra consist of electronic states associated with bonding interactions between the... 

The low BE region of XPS spectra (<20 — 30 eV) represents delocalized electronic states involved in bonding interactions [7]. Although UV radiation interacts more strongly (greater cross-section because of the similarity of its energy with the ionization threshold) with these states to produce photoelectrons, the valence band spectra measured by ultraviolet photoelectron spectroscopy (UPS) can be complicated to interpret [1], Moreover, there has always been the concern that valence band spectra obtained from UPS are not representative of the bulk solid because it is believed that low KE photoelectrons have a short IMFP compared to high KE photoelectrons and are therefore more surface-sensitive [1], Despite their weaker intensities, valence band spectra are often obtained by XPS instead of UPS because they provide... 

However, in this structure the two positive charges are also completely delocalized and Te62+ uses the complete set of bonding interactions described... 

There are two other mechanistic possibilities, halogen atom abstraction (HAA) and halonium ion abstraction (EL), represented in Schemes 4.4 and 4.5, respectively, so as to display the stereochemistry of the reaction. Both reactions are expected to be faster than outer-sphere electron transfer, owing to stabilizing interactions in the transition state. They are also anticipated to both exhibit antiperiplanar preference, owing to partial delocalization over the C—C—Br framework of the unpaired electron in the HAA case or the electron pair in the EL case. Both mechanisms are compatible with the fact that the activation entropies are about the same as with outer-sphere electron donors (here, aromatic anion radicals). The bromine atom indeed bears three electron pairs located in two orthogonal 4p orbitals, perpendicular to the C—Br bond and in one s orbital. Bonded interactions in the transition... 

Generalized valence bond interaction energies were computed for mono/poly-nitrogenous five- and six-membered heterocycles.203 Results that diverged from those obtained by other methods were obtained only for poly-nitrogenous systems such as pyridazine, benzotriazole, and tetrazole, which may confirm Bird s earlier finding123 204 that electron delocalization is not a stand-alone and direct measure of aromaticity for nitrogenous heterocyclic compounds. 

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