Electron delocalization and

The resulting radical is stabilized by electron delocalization and eventually reacts with either another inhibitor radical by combination (dimerization) or disproportionation or with an initiator or other radical. 

The aromaticity of naphthalene is explained by the orbital picture in Figure 15.12. Naphthalene has a cyclic, conjugated it electron system, with p orbital overlap both around the ten-carbon periphery of the molecule and across the central bond. Since ten 77 electrons is a Hiickel number, there is tt electron delocalization and consequent aromaticity in naphthalene. 

Scheme 13 Electron delocalization and stabilization by the interaction between the occupied and unoccupied orbitals...

Scheme 3 Electron delocalization and the cyclic interaction of the electron configurations...

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... 

Much of the Pt Mossbauer work performed so far has been devoted to studies of platinum metal and alloys in regard to nuclear properties (magnetic moments and lifetimes) of the excited Mossbauer states of Pt, lattice dynamics, electron density, and internal magnetic field at the nuclei of Pt atoms placed in various magnetic hosts. The observed changes in the latter two quantities, li/ (o)P and within a series of platinum alloys are particularly informative about the conduction electron delocalization and polarization. 

The resulting crystal proved amenable to a conclusive XRD analysis. As shown in Eq. (26), the C—C ring bond is lengthened over what it is in the structurally similar cyclopropene (1.304 A) and the B—C bonds shortened relative to the electronically analogous bond in trivinylborane (1.558 A). Thus, one can safely conclude that there is extensive 7r-electron delocalization and Hiickel aromatic character in the borirene ring. 

Noteworthy is the "B resonance in 103, which falls in the range 58-60 ppm, very little different from the signal of trivinylborane (55 ppm) therefore, any extensive ir-electron delocalization and density on boron can be ruled out. Thus both in chemical and physical properties, such systems as 103 can be considered antiaromatic. 

The dianionic salts of the 1,2-diboraatabenzene (104) and the 2,3-dibora-tanaphthalene (105) systems have recently also been synthesized.100101 These systems appear to exhibit electron delocalization and aromatic stabilization similar to that shown by 102. 

Noodleman, L., C. Y. Peng, D. A. Case, and J. M. Muesca. 1995. Orbital Interactions, Electron Delocalization and Spin Coupling in Iron-Sulfur Clusters. Coordination Chemistry Reviews 144, 199. 

During the last few years, both neutral and cationic 1,3,2-diazaphospholes and NHP have been studied extensively by computational methods. The best part of these studies focused on a discussion of n-electron delocalization and their implication on chemical reactivities and stabilities, the explanation of the unique ionic polarization of exocyclic P-X bonds noted for some species, and the evaluation of structural and spectroscopic properties with the aim of helping in the interpretation of experimental data. 

The observed planarity and bond length equalization in 1,3,2-diazaphospholenium cations likewise suggest that these compounds have substantial n-electron delocalization and possess possibly aromatic character. Several studies were undertaken to quantify the degree of n-delocalization by computational calculations using the interpretation of population analyses, ELF calculations, evaluation of magnetic criteria [nucleus independent chemical shift (NICS) values], and the... 

Pyran-4-one (56a) and its benzo derivative (chromone) show chemical properties in agreement with substantial jr-electron delocalization and consistent with a betaine structure 56b (Scheme 27). Experimental data have therefore generated numerous theoretical studies on the aromaticity of pyranones, which have been extensively reviewed.219 Earlier studies suggested that chemical shifts and coupling constants... 

When it comes to metal-rich compounds of the alkaline earth and alkali metals with their pronounced valence electron deficiencies it is no surprise that both principles play a dominant role. In addition, there is no capability for bonding of a ligand shell around the cluster cores. The discrete and condensed clusters of group 1 and 2 metals therefore are bare, a fact which leads to extended inter-cluster bonding and results in electronic delocalization and metallic properties for all known compounds. 

Roth and coworkers23 reported NMR data of the orthogonal butadiene (Z,Z)-3,4-dimethylhexa-2,4-diene. (Z,Z)-13 having the planes of the double bonds at a dihedral angle not far from 90°. This diene serves as the model for conjugated diene lacking rr-electron delocalization and for the transition state for interconversion of antiperiplanar (trans) and synperiplanar (cis or gauche) butadiene. 

The second chapter, by Jan Sandstrom, deals with stereochemical features of push-pull ethylenes. The focus is on rotational barriers, which span a large range of values. The ease of twisting is partly a matter of electron delocalization and partly a matter of steric and solvent effects. Electronic structure and such related items as dipole moments and photoelectron spectra for these systems are discussed. The chapter also deals with the structure and chiroptical properties of twisted ethylenes that do not have push-pull effects, such as frans-cyclooctene. 

As to the cation-radical version of this isomerization, there are testimonies on the transition of the norcaradiene carcass into the cycloheptatriene skeleton. Calculations at the B3LYP level shows that cycloheptatriene cation-radical is more stable than norcaradiene cation-radical by ca. 29 kJ mol (Norberg et al. 2006). Hydrocarbon ion-radicals with strained ring structures have a tendency to undergo facile rearrangement to enforce the unpaired electron delocalization and release their strain energy. 

The first nitrile ylide stable enough to be isolated (i.e., 1) has been prepared by the carbene/nitrile method (1). For this dipole, the anionic component is stabilized by electron delocalization and the nitrilium component by the steric bulk of the adamantyl group to such an effect that it has a melting point of 230 °C. The X-ray structure showed that the nitrile ylide moiety is close to linear and much like the resonance structure shown below. 

Fig. 6. Schematic partial density of states scheme for an NaCl-type (binary) compound (with UN as an example) with f electrons delocalized and unhybridized. Uranium is on the left and nitrogen on the right. In ascending order nitrogen valence band f-band tied to the Fermi level the d conduction band. The Fermi level is at zero on the energy scale. The unhybridized band centres, Qi, are shown on the right. This unhybridized model corresponds to the fully ionic model...

Only a small number of monocyclic 1,2,3-triazines are known, so knowledge of the structure of these compounds is poor. The available data on the parent compound (1) and its methyl derivatives do not permit an evaluation of the degree of electron delocalization and aromaticity of (1) or its alkyl derivatives. Attempts at a crystallographic analysis of (1) were unsuccessful as the crystal was unstable under X-ray irradiation. 

---