Double bond bond energy

The predictions of relative stability obtained by the various approaches diverge more widely when nonbenzenoid systems are considered. The simple Hiickel method using total n delocalization energies relative to an isolated double-bond reference energy (a + fi) fails. This approach predicts stabilization of the same order of magnitude for such unstable systems as pentalene and fulvalene as it does for much more stable aromatics. The HMO, RE, and SCF-MO methods, which use polyene reference energies, do much better. All show drastically reduced stabilization for such systems and, in fact, indicate destabilization of systems such as butalene and pentalene (Scheme 9.2). 

The energy required for the hydrogenation of cyclohexene to cyclohexane is —28.6 kcal/mol. Therefore, in the case of cyclohexadiene, where there are two double bonds, the energy required for the hydrogenation can be calculated as 2 x —28.6 = —57.2kcal/mol. In practice, the experimental value is quite close to this calculated value, and is —55.4kcal/mol. In this... 

Unfortunately, there were two errors in Heicklen and Knight s analysis. Subsequent work66 has shown that their quantum yields were too low by a factor of two. Second, at the lowest pressures used (0.6 torr) some of the excited mercury will disappear by fluorescence, thus giving lower quantum yields than would otherwise have been obtained. Both of these errors would give too large a value for E0 it should be smaller than 2.2 kcal/mole. Consequently, the double bond dissociation energy in C3Fe should be between 112.7 and 114.9 kcal/mole. 

Values given are in units of kcal mole per carbon atom. c Based on bond strengths for average carbon-carbon double bond. a Energy per carbon atom based on JANAF Table data for C, F . 

In 6-dimethylaminofulvene, the parent compound of (7), and in 6-dimethylamino-6-methylfulvene, for the rotation about the A5 6 double bond, activation energies of 92.5 kJ (22.1 kcal) and 68.6 kJ (16.4 kcal) respectively were measured5. Starting from (la), the adequate form (7 b) should be reached in a thermal equilibrium quite likely on heating. 

The overall stabilization of the molecule as the result of conjugation is estimated. Remember from the resonance concept in VB theory that conjugation is generally associated with additional stabilization (see Section 1.1.7). In HMO theory this stabilization is expressed as the difference between the energy of the conjugated system and the same number of isolated double bonds. The energy of an isolated double bond in the HMO method is equal to 2a+ 2(3, so for 1,3,5-hexatriene, a stabilization of 0.988(3 is computed. For benzene, the computed stabilization is 2(3 ... 

The presence of resonance (or delocalization) is universally an energy-reducing effect, and this is pronounced in the case of benzene. From experimental measurements of the heat of combustion or heat of hydrogenation, benzene has been found to have 36 kcal/mol less energy than expected for a molecule with three real double bonds. This energy value is commonly referred to as resonance energy, but it needs to be remembered that this is energy that a system does not have rather than what it has. 

Linear polymers that possess only single bonds in their backbones between atoms, C-C, or C-0, or C-N, can undeigo rapid conformational changes. Also, ether, imine, or ct>double bonds reduce energy barriers and, as a result, soften the chains, causing the polymer to become more rubbery and nlore soluble in various solvents. The opposite is true of cyclic structures in the backbones. Actually, cyclic structures not only inhibit conformational changes but can also make crystallization more difficult. 

The band structure is shown in Fig. 3.4. e = 2t at fc = 0 and e = 2 t at A = 7r/2a. Thus, the band gap is 45t, while the full band width is the same as the undimerized chain, namely 4t. Notice that as 6 is increased from 0 to 1 the band structure evolves from that of the undimerized chain (with a folded dispersion), to localized orbitals on the double bonds, with energies of 2t. 

Figure 14-16 HOMO of propene in two conformations. Methyl group (a) eclipses and (b) staggers the double bond. The energy is lower in (a).

Liu J, Zhou D, Xu M, Jing X, Wang P (2011) The structure-qtroperty relationship of organic dyes in mesoscopic titania solar cells only one double-bond difference. Energy Environ Sci 4 3545-3551... 

Figure 2.8 Equivalent bent (or banana) bond description of the double bond ciated energy levels are degenerate ...

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