Bond energies resonance effect

It has been pointed out4 that bonds between non-identical atoms may be considered to resonate between a covalent and an ionic structure, the bond in this way having partially covalent and partially ionic character. The resonance energy of this effect, which is usually essentially the same for a given bond in different molecules, is included in the values given for the bond energies in the nonresonating molecules discussed. 

The postulated transition state of the tt—a bond conversion occurs when the plane of the rotating benzene molecule is approximately at 45° to the catalyst surface. The electronic hybridization changes involved in this process (Fig. 4) are similar to those proposed by Melander 22) for homogeneous substitution reactions and show how resonance interactions may achieve a lowering in the activation energy. This aspect has previously been discussed in terms of the Lennard-Jones theory where the activation energy is lowered more effectively by resonance effects... 

For such situations we have developed a different approach. The parameters calculated by our methods are taken as coordinates in a space, the reactivity space, A bond of a molecule is represented in such a space as a specific point, having characteristic values for the parameters taken as coordinates. Figure 6 shows a three-dimensional reactivity space spanned by bond polarity, bond dissociation energy, and the value for the resonance effect as coordinates. 

Figure 6, Reactivity space having bond polarity, Q, bond dissociation energy, BDE, and resonance effect parameter, R, as coordinates ...

The entire set of molecules contained 782 bonds out of which 111 a-bonds were selected. The parameters were calculated by our methods to build a reactivity space with electronegativity difference, resonance effect parameter, bond polarizability, bond polarity, a-charge distribution, and bond dissociation energy as six coordinates. 

Next, supervised-learning pattern recognition methods were applied to the data set. The 111 bonds from these 28 molecules were classified as either breakable (36) or non-breakable (75), and a stepwise discriminant analysis showed that three variables, out of the six mentioned above, were particularly significant resonance effect, R, bond polarity, Qa, and bond dissociation energy, BDE. With these three variables 97.3% of the non-breakable bonds, and 86.1% of the breakable bonds could be correctly classified. This says that chemical reactivity as given by the ease of heterolysis of a bond is well defined in the space determined by just those three parameters. The same conclusion can be drawn from the results of a K-nearest neighbor analysis with k assuming any value between one and ten, 87 to 92% of the bonds could be correctly classified. 

The basis of the scheme developed particularly by Alfrey and Price is the assumption that the activation energies of the propagation reactions, and hence the related rate constants and reactivity ratios, are governed primarily by resonance effects and by the interaction of the charges on the double bonds of the monomers with those in the active radicals. Accordingly, the rate constant of the reaction between a radical and a monomer is represented by ... 

This book is about atomic charges, chemical bonds, and bond energy additivity. However, nuclear magnetic resonance, inductive effects, zero-point and heat content energies, and other topics are an integral part of this study, to achieve... 

There can be resonance between covalent and ionic states. In the molecule H H a complete shift of the electron pair to the left would have the effect to make the left-hand H atom a negative ion, leaving the right-hand one as a positive ion. Next to the state H H there will be two others, H H+ and H+H , which closely resemble the electrostatic model for the H2 molecule. Since the three states are resonating, the states H H+ and H+H will make a contribution to the bonding energy, too in this case, however, their contribution will be relatively small, because the energy of the covalent state H H certainly is much lower than that of the ionic states. H H+ and H+H-. 

Jprgensen57 has referred to this tendency of fluoride ions to favor further coordination by a fourth fluoride (the same is true for hydrides) as symbiosis." Although other factors can work to oppose the symbiotic tendency, it has widespread effect in inorganic chemistry and helps to explain the tendency for compounds to be symmetrically substituted rather than to have mixed substituents. We have seen (Chapter 5) that the electrostatic stabilization of C—F bonds (ionic resonance energy) will be maximized in CF4, and similar arguments can be made for maximizing hard-hard or soft-soft interactions. 

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