Aromatic Stabilization Energy ASE

Aromatic compounds are unusually stable. The classic demonstration of this stability is with the heat of hydrogenation of benzene, = -49.2 kcal mol (Reaction 3.15) and the heat of hydrogenation of cyclohexene, = -28.3 kcal mol  

As with the evaluation of RSE discussed previously, evaluating the stabilization afforded by aromaticity requires comparison with some sort of nonaromatic reference. In the following discussion, all computed energies were done at the G2MP2 level, which is the same method that we used in the previous section to obtain 

Computational evaluation of the stability of aromatic compounds really began with Roberts report of Hiickel delocalization energy for a series of aromatic hydrocarbons. The delocalization energy tends to increase with the size of the compound, and some compounds predicted to have large delocalization energies turn out to be unstable. The error here is not so much with the computational method, which is of course very rudimentary, but rather with the reference compound. In the Hiickel approach, the jc-energy of the aromatic compound is compared with the jt-energy of the appropriate number of ethylene molecules, that is, for benzene, the reference is three ethylene molecules. 

One can recast the use of ethylene as a reference into an isodesmic reaction (Reaction 3.17), where its reaction energy should indicate the stabilization due to aromaticity for benzene. One can use the experimental A//f or the computed energies to obtain the overall energy of this reaction. The resulting estimate for the stabilization energy ( 65 kcal mol ) is large due to additional differences between reactants and products besides just aromaticity. Isodesmic Reactions 3.18 and 3.19 might improve matters. These reactions are less exothermic. 

Recalling our discussion of isodesmic reaction in the previous section, it is clear that Reactions 3.17-3.19 contain energetic consequences for other effects besides aromaticity, including changes in hybridization. In particular, delocalization effects are not conserved. It is important to distinguish delocalization effects from resonance effects from aromatic effects. The first refers to stabilization 

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According to a semiempirical study by Malar, the different polyphospholide anions have 86—101% aromaticities of that of the cyclopentadienide anion. Chesnut and Quin reported on the basis of GIAO NMR calculations using a triple-valence quality basis set that the phospholide anion s aromaticity is 63% that of the cyclopentadienide anion. The aromatic stabilization energy (ASE) obtained by Schley-er et al. from eq 2 (X = P ) was 90% that of the cyclopentadienide anion. 

TABLE 5. Substituent effects on the structure and aromatic stabilization energy (ASE) of SigRg (31a or 32a) ... 

Table 19 Barriers (AH ) of reactions 34a, 34b, 35a, 35b, 36a, and 36b and aromatic stabilization energies (ASE) in the gas phase ...

We have used a different approach to compare the aromaticities of phosphole (8) and pyrrole (10) [23, 24], From literature data on derivatives of 8 and 9 it is known that the inversion barrier of phosphole is about 67 kJ mol-1 (70.2 kJ mol-1 at the B3LYP/aug-cc-pVTZ level) [25] while that of tetrahydrophosphole amounts to 163 kJ mol-1. This is explained by the fact that the planar transition state of 8 is highly aromatic. Pyrrole (10) is planar and pyrrolidine has a calculated inversion barrier of 15-17 kJ mol-1. Several aromaticity indices were used in this study, based on different criteria of aromaticity energetic (aromatic stabilization energy, ASE), geometric (harmonic oscillator model of aromaticity, HOMA, and /5), and magnetic (NICS). 

Another theoretical criterion applied to estimation of aromaticity of homo- and heteroaromatic ring system is aromatic stabilization energy (ASE). Based on this approach, the aromatic sequence of five-membered ring systems (ASE in kcal mol-1) is pyrrole (20.6) > thiophene (18.6) > selenophene (16.7) > phosphole (3.2) [29], According to geometric criterion HOMA, based on the harmonic oscillator model [30-33], thiophene is more aromatic than pyrrole and the decreasing order of aromaticity is thiophene (0.891) > pyrrole (0.879) > selenophene (0.877) > furan (0.298) > phosphole (0.236) [29],... 

Aromatic stabilization energy, ASE. We skirt the enormous literature on the meaning and detection of aromaticity [164] and assert that a good measure of the phenomenon is the aromatic stabilization energy, the energy change when an... 

A limited amount of energy data is available for this class of heterocycle. Table 38 shows empirical resonance energies (ERE) and Hess-Shaad resonance energies (HSRE) for a limited list of azoles. Also included in Table 38 are aromatic stabilization energies (ASEs). The ASE is a more recent measure of aromaticity based on homodesmotic and isodes-motic reactions. ASE values for a wide range of five-membered heterocycles are available <2003T1657>. 

The merits of NIGS, obtained by an upgraded computational protocol, as a measure of the aromaticity, nonaromaticity, or antiaromaticity of, inter alia, planar five-membered heterocycles (including furan) have been discussed. The subtypes NICS(0),izz and NICS(l)zz appear to perform most reliably in providing a linear relationship with aromatic stabilization energies (ASEs) <20060L863>. 

Using MP2/6-31G calculations, the aromatic stabilization energy (ASE) has been obtained from the energies of isodesmic reactions (Equation 1) for many derivatives <1995JST57>. Thus, for arsole, the heat of bond separation reaction is calculated to be 16.63 kcalmoU. This is ca. 36% of the corresponding value for pyrrole. 

Recently, it was found that poly(phospholes) provide an even better perspective for such devices (06CRV4681). Indeed, the calculated aromaticity of the unsubstituted phosphole is quite low in comparison with that of pyrrole in terms of aromatic stabilization energy (ASE) and nucleus-independent chemical shift (NICS) pyrrole, ASE = 25.5 (kcal/mol), NICS =—15.1 (ppm) phosphole, ASE 7.0 (kcal/mol), NICS = -5.3 (ppm) (06CRV4681). 

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