Electron Energy and Aromaticity

In this section, we have dealt only with ionization energies assigned to removal of a 7t electron. Removal of electrons from a orbitals is also observed. If the impinging photons are of X-radiation frequencies, the inner shell electron ioiuzation energies are seen. Quantum-chemical treatments of these ionization energies are possible using theoretical methods described later. 

There is a theoretical parallel to this. The HMO energy for butadiene is 4a + 4.472. For a pair of isolated double bonds, we double the energy of ethylene to obtain 4a -I- 4. Therefore, the HMO method indicates that the conjugated double bonds are stabilized by 0.472y6. 

Because the tt electrons in butadiene are delocalized over all three C-C bonds, this 0.472)6 has often been referred to as delocalization energy. It was common practice for many years to equate this theoretical delocalization energy for a molecule to its experimentally measured extra stability (e.g., for butadiene, 0.472 p = 3.5kcal/mole). 

TABLE 8-2 jr-Bond Types and Effective Binding Energies for Carbon-Carbon Double 

Bond type Effective binding energy in units of p 

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As we have seen, the important zero energy difference which measures aromatic reactivity contains a term involving rr-electron energies, and rvith the transition state model there will also be a contribution from... 

Energy-resolved rate constant measurements near the threshold for diplet methylene formation from ketene have been used to provide confirmation of the fundamental hypothesis of statistical transition state theory (that rates are controlled by the number of energetically accessible vibrational states at the transition state).6 The electronic structure and aromaticity of planar singlet n2-carbenes has been studied by re-election coupling perturbation theory.7 The heats of formation of three ground-state triplet carbenes have been determined by collision-induced dissociation threshold analysis.8 The heats of formation of methylene, vinylcarbene (H2C=CHCH), and phenylcarbene were found to be 92.2 3.7, 93.3 3.4, and 102.8 33.5 kcal mol-1, respectively. 

The value of k was fixed at 0-5 and the n electron energy when the orbital representing the attacking reagent was positioned near to a particular position in the aromatic nucleus was computed, using values of h var3nng from — 3 to +3. 

As discussed in the theoretical section (4.04.1.2.1), electrophilic attack on pyrazoles takes place at C-4 in accordance with localization energies and tt-electron densities. Attack in other positions is extremely rare. This fact, added to the deactivating effect of the substituent introduced in the 4-position, explains why further electrophilic substitution is generally never observed. Indazole reacts at C-3, and reactions taking place on the fused ring will be discussed in Section 4.04.2.3.2(i). Reaction on the phenyl ring of C- and A-phenyl-pyrazoles will be discussed in Sections 4.04.2.3.3(ii) and 4.04.2.3.10(i), respectively. The behaviour of pyrazolones is quite different owing to the existence of a non-aromatic tautomer. 

NMR and, 3, 951 aromaticity, 3, 945 delocalization energy, 3, 959 deprotonation, 3, 972 disulfones reactions, 3, 970 double bond character, 3, 945 electronic energy levels, 3, 946 electrophilic reactions, 3, 965 electrophilic substitution, 3, 960 half-wave potential, 3, 968 NMR, 3, 952 H NMR, 3, 951 nucleophilic reactions, 3, 969 oxidation, 3, 967 oxides... 

For aromatic hydrocarbon molecules, in particular, the main acceptor modes are strongly anharmonic C-H vibrations which pick up the main part of the electronic energy in ST conversion. Inactive modes are stretching and bending vibrations of the carbon skeleton. The value of Pf provided by these intramolecular vibrations is so large that they act practically as a continuous bath even without intermolecular vibrations. This is confirmed by the similarity of RLT rates for isolated molecules and the same molecules imbedded in crystals. 

Aromaticity is usually described in MO terminology. Cyclic structures that have a particularly stable arrangement of occupied 7t molecular orbitals are called aromatic. A simple expression of the relationship between an MO description of stmcture and aromaticity is known as the Hiickel rule. It is derived from Huckel molecular orbital (HMO) theory and states that planar monocyclic completely conjugated hydrocarbons will be aromatic when the ring contains 4n + 2 n electrons. HMO calculations assign the n-orbital energies of the cyclic unsaturated systems of ring size 3-9 as shown in Fig. 9.1. (See Chapter 1, Section 1.4, p. 31, to review HMO theory.)... 

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