Ionic resonance structures

The main lesson from the analysis given above is that the activation free energy of the reaction is strongly correlated with the stabilization of the ionic resonance structure by the protein-active site. The generality of this concept will be considered in the following chapters. 

From the enthalpies of formation from Roth for the fulvenes and from Pedley for the other hydrocarbons in equations 37 and 38, we find the former reaction is exothermic by 12 kJmol-1 while the latter is endothermic by 12 kJmol-1. Ionic resonance structures analogous to 128 are expected to be of less importance for the ring alkylated species than for the parent species 103 negatively charged carbon is destabilized by adjacent... 

The completely ionic resonance structure in the lower-right-hand panel of Fig. 3.78 might also be depicted as... 

Whether of nM 7tL or n- -OL (off-axis) type, pi-backbonding interactions correspond to partial inclusion of ionic resonance structures featuring a metal-ligand double bond, as illustrated schematically in (4.79) for phosphine ligands ... 

The empirical valence bond (EVB) method of Warshel [19] has features of both the structurally and thermodynamically coupled QM/MM method. In the EVB method the different states of the process studied are described in terms of relevant covalent and ionic resonance structures. The potential energy surface of the QM system is calibrated to reproduce the known experimental... 

Compounds with a cyclopentadienyl-silicon cr-bond adopt a variety of bonding arrangements, which can be classified on the basis of the hybridization of the corresponding cyclopentadienyl carbon atom, as displayed in Scheme 1. In species of type 1, the silicon atom is bound to an sp3 -hybridized carbon atom, i.e. to an ally lie carbon within a cyclopentadiene unit. The silicon atom is bound to an sp2-hybridized carbon atom in species of type 2-6, including silylcyclopentadienyl radicals (2), ionic silylcyclopentadi-enide species (3), and silylcyclopentadienyl fragments bound in a /j5 -fashion to a metal centre (4). Species 5 represents the ionic resonance structure of a silafulvene, and structure 6 stands for the vinylic isomers of a cr-cyclopentadienylsilane (type 1), but these two types of compound are not examined in detail in this article. 

Carboxylic acids. Even in quite dilute solution in non-polar solvents, acids exist essentially as dimeric species, which may be readily explained on the basis of the electronic structure of the carboxyl group. Powerful hydrogen bonding between the molecules, and the strength of these bonds, has been accounted for on the basis of a large contribution of an ionic resonance structure. 

The exceptional strength of the hydrogen bonding is explained on the basis of the large contribution of the ionic resonance structure. Because of the strong bonding, a free hydroxyl stretching vibration (near 3520 cm-1) is observed only in very dilute solution in nonpolar solvents or in the vapor phase. 

Fig.2 shows the state correlation digrams for the three identity S 2 reactions, and the major numerical results derived from Fig.2 are reported in Table 3. In both Fig.2 and Table 3, AE denotes the reaction barrier, B represents the coupling between two covalent resonance structures a and b, and T measures the magnitude of the participation of the ionic resonance structure c. 

When two molecules interact, there is often a small amount of electron flow from one to the other. For example, in the equilibrium geometry of the linear water dimer HO— H... OH2, the water molecule that is the proton acceptor has transferred about 0.05e to the proton donor water (9,10). The attractive energy associated with this charge transfer is the charge transfer energy and can be thought of as a mixing of an ionic resonance structure... 

This heteroannular dienone system causes rings A and B to assume a half-chair conformation. Carbon atoms 1, 3, 4, 5, 6, 8, 9, 10, 11 are all in one plane while carbon atom 2 projects above and carbon atom 7 below the plane. The rest of the molecule is below the plane. Addition of the second unsaturation to the A -3-ketone enone system provides an additional electron-rich unsaturated residue, i.e., it enhances the electron delocalization. The result will be an enlarged contribution of ionic resonance structures such as... 

The reactant may be considered as a polarised carbonyl bond, reflected in contributions of covalent and ionic resonance structures. As the reaction proceeds, the contribution of both of these is replaced by the structure at the right of the diagram. Because of the greater concentration of positive charge on carbon in the reactant, donor substituents stabilise reactants more than transition state. In summary, donor substituents deactivate carbonyls. Cieplak acknowledged that the reactivity effect of a donor might be different from its stereochemical effect. 

The reason for this energetic preference for fluorine accumulation at the same sp center is selfstabilizing by formation of possible ionic resonance structures, as already discussed in Section 1.4.2. 

Figure 8.1. Energetics of the proton transfer between an acid (A) and a base (B) by valence bond resonance structures. The reaction is described in terms of a covalent resonance structure (A-H B) and an ionic resonance structure (A H-B+). The energies of the valence bond structures are as given in the text and depend on the coordinates of the reacting atoms, R, and the coordinates of the rest of...

Although the molecular geometries of 2 - and 2 -phosphinins are nearly identical (no significant deviations from planarity or bond alternation are observed with either system), the A -phosphinines as well as the mono-, di-, tri- and tetra-aza-A -phosphinines, the A A -diphosphinines and the A A A -triphosphinines are certainly not Hiickel-aromatic systems. Their description as cyclic phosphorus ylides with ionic resonance structures (26) is probably close to the truth. The H and C NMR evidence is particularly strong on this point. 

The degree of ionic character depends on the nature of the metal, the medium, and the substituents on the carbanionic carbon atom. For simple alkyls (those in which resonance stabiHzation of the negative charge is not expected), the nature of the metal is especially important. The percent ionic character (alternatively, the percent contribution of the ionic resonance structure to the hybrid) increases with increasing difference in electronegativity of the two atoms. 

Lewis structure models incorporating radical character are useful in the exposition of photochemical reactions of carbonyl compounds, alkenes, dienes, and aromatic compounds that are not substituted with polar groups. When heteroatoms are substituted onto an aromatic ring, considerable charge transfer character can be introduced into both the ground state and the excited state, and ionic resonance structures become more suitable as models for reactivity. 

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