Delocalization energy. See

Topological jt-resonance energy. The concept of the ji-resonance energy, or, in other words, the delocalization energy (see p. 24), of conjugated molecules has served for many years as a simple criterion of aromaticity. The jx-resonance energy (RE) in p units is defined as... 

The lability of thieno[3,4-6]thiophene (3) and other iso-annelated systems, such as benzo[c]thiophene and benzole] furan, may be due to the strain effect (Mills-Nixon effect see also Zwanenburg et alP and references therein) in the condensed five-membered ring. The stability of the iso-annelated dithienothiophenes 7—9 is noteworthy. Simple LCAO MO method calculations on benzo[c]thiophene indicate that its instability is due to low specific delocalization energy and high free valence index at position 1. 

Delocalization energies (DE) are readily calculated from the total ir-electronic energies. The following empirical relationship has been found32 between the experimental and theoretical delocalization energies of compounds of various types (see also ref. 3) ... 

Ignoring O (see eqn (10-5.1)), the 7r-electron energy for the ground state will be 6a+80 which, when compared with the 71-electron energy of three ethylene molecules (6 a+60), shows that the delocalization energy of benzene is 2/ . 

Substituents in the ortho position, almost without exception, reduce the reactivity of benzoic acid derivatives a significant rate increase is a prima facie case for intramolecular catalysis (see p. 201). This effect is associated in the case of the esterification of or/fio-substited benzoic acids with a somewhat lower enthalpy of activation, and a greater decrease in the entropy of activation. The first factor may be due to a secondary steric effect145, whereby the energy of the initial state is raised because the carboxyl group is twisted out of the plane of the ring, to reduce non-bonded interaction with the or/fio-substituent (see p. 178), and delocalization energy is lost. The lower entropy term is presumably accounted for by the bulk effect of the substituent. 

Remembering that p is negative, we see that n bonding has stabilized the molecule by 8p. Energies expressed in units of / are not very informative, however, unless we can estimate the value of p. We shall next turn to the calculation of the delocalization energy of benzene in units of /i. Since the delocalization energy may be estimated experimentally, we shall then be able to evaluate p. It is not practicable to do this by computation. 

Further examples of the jeopardy involved in casually dismissing d orbitals participation are the findings of Haddon and coworkers28 that rf-orbitat participation is especially important m S4F4, which is nonplanar, and ako that it accounts for about one-half of the delocalization energy in the one-dimensional conductor (SN)4. In the latter case, the low electronegativity of the d orbitals (see Chapter 5) increases the ionicily of the S—N bond and stabilizes the structure. 

This geometry precludes the possibility of two equivalent VB structures, as for benzene, because, as you will see if you try to make a ball-and-stick model, 25b is highly strained and not energetically equivalent to 25a at all. Thus we can conclude that the delocalization energy of cyclooctatetraene is not large enough to overcome the angle strain that would develop if the molecule were to become planar and allow the tt electrons to form equivalent tt bonds between all of the pairs of adjacent carbons. 

The data of Figure 21-13 provide a rationale for the instability of cyclobutadiene and cyclooctatetraene. For cyclobutadiene, we can calculate that four 77 electrons in the lowest orbitals will lead to a predicted 77-electron energy of 2(a + 2/3) + 2(a) = 4a + 4/3, which is just the 77-electron energy calculated for two ethene bonds (see Figure 21-3). The delocalization energy of the 77 electrons of cyclobutadiene therefore is predicted to be zero ... 

Influences of substituents on the positions of isomeric equilibria that have been calculated116 by the HMO route do not agree with experimental findings (see Section III,B,1). A correlation of the acidity of 3H-pyrrolizine(l)with the difference in HMO delocalization energies between the pyrrolizine anion and the neutral molecule has been attempted but showed limited success.115... 

The delocalization energy for a planar benzene molecule (see Coulson, 1961) is 2jS. For distortions as above this will be reduced to 2/J(l — f 02) with a consequent decrease in stability. 

Delocalization energy (this is related to aromaticity and antiaromaticity) Transition state structures and energies (see the hedge below)... 

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