Energy bond, delocalized bonds

Bond Energies and Distances in Compounds Containing Delocalized Bonds... 

There is on the other hand a great deal of evidence showing that the electrochemical reduction of 1,2-dihalides to olefins can occur via a concerted pathway, i.e., via a transition state (39) in which both carbon-halogen bonds are partially broken and the carbon-carbon double bond is partially formed. An important, indeed critical, point of evidence supporting the conclusion that reduction is concerted lies in the remarkable ease with which vicinal dihalides are reduced. For example, the half-wave potentials of ethyl bromide and 1,2-dibromoethane are -2.08 V and -1.52 V (vs. s.c.e.), respectively 15 >46) those of ethyl iodide and /J-chloroethyl iodide are -1.6 V and -0.9 V, respectively 47). These very large differences must reflect the lower energy of delocalized transition state 39 relative to the transition state for reduction of an alkyl monohalide. 

Since a is the energy of a 7r electron in a 2p a.o. and since (3 refers to an electron in a bond, both of these quantities are negative, and the energies above therefore occur in increasing order. The energy associated with the delocalized bond in the molecular ground state is... 

The energy associated with the 4 electrons in their atomic orbitals is 4a and so the dissociation energy of the delocalized bond in butadiene is 4.4/3. Each orbital contributes an amount of energy ej = a + m.j/3 and it is said to be bonding, anti-bonding or non-bonding for positive, negative or zero values of mr... 

Thus, the RE determined from the energy of an isodesmic reaction of bond separation is in fact QMRE-like and represents an estimate of various effects of electron delocalization. By contrast, the use of the homodesmotic reaction leads to a Dewar-type RE (75TCA121) allowing the evaluation of the contribution by precisely the cyclic electron (bond) delocalization. 

By contrast, the Dewar resonance energy represents solely the contribution coming from the cyclic electron (bond) delocalization since the model reference structure is represented not by a system of isolated 7r-bonds, but by a hypothetical cyclic polyene with the number of tr- and tr-bonds equal to that in a given molecule. Making use of the additivity of bond energies in acyclic polyenes (65JA692), one may calculate the total energy... 

The energy of the homodesmotic reaction does not exclusively reflect the effect of cyclic (bond) delocalization. The reference structure is hypothetical and one cannot write the equation of a reaction, where a cyclic and an acyclic structure participate, for which the difference between the energies of products and reactants was determined by a single factor, namely, aromatic stabilization (antiaromatic destabilization) (75TCA121). 

The HSE values estimate the contribution by cyclic (bond) delocalization, whereas the AE values for the isodesmic reaction (ISE) [76JCS(P2)1222] refer to the stabilization energy associated with conjugation as a whole clearly, the latter values turn out appreciably larger, cf. HSE (4), (6) and ISE (3), (5). 

The aromatic stabilization of a molecule is the energy contribution due to the cyclic bond delocalization. This contribution is defined as the resonance energy (RE)... 

The problem in determining resonance energies is to single out of the total energy of the molecule the contribution from the cyclic bond delocalization. 

Localized MP2 (LMP2) models have already been shown to provide results which are nearly indistinguishable from MP2 models for both thermochemical calculations (see Chapter 12) and for calculation of conformational energy differences (see Chapter 14). Activation energy calculations provide an even more stringent test. Transition states necessarily involve delocalized bonding, which may in turn be problematic for localization procedures. 

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