Aromaticity isodesmic reaction

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. 

Assuming that aromatic stabilization of 24a and 24b is of the same magnitude, and this is also true for the lone-pair repulsion in the pairs 24a/25a and 24b/25b, the energy, A , of the isodesmic reaction (1) corresponds to the contribution from greater aromatic stabilization of 25a with respect to 25b (Scheme 26). At the MP2/6-31G approximation, AE = 10.5 kJ mol (94JOC2799). Similar arguments applied to the isodesmic reaction (2) allow estimation of the energy contribution due to the repulsion of adjacent lone pairs in 25a. In MP2/6-31G approximation, A = -25.9 kJ mol ... 

The most common evaluation of aromaticity via energetic criteria is done using calculations either a type of isodesmic reaction (34) or comparison of two isomers that differ only through the aromaticity of one (3). We were interested in the possibility of evaluating stability experimentally and the electrochemical formation of dications such as 8 was attractive. In this approach, the redox potential for formation of the dication would be compared to the redox potential for formation of dications which could not be antiaromatic. If 8 was antiaromatic, its redox potential should be larger and more positive than that of the reference system. This approach was applied to the evaluation of the antiaromaticity of 9... 

It is worth mentioning at this point that the formation of a a -phosphorus from a -phosphorus resulted in a-aromatic compounds in the case of the l//-phosphirenium cation as well. Disubstitution at the phosphorus in l//-phosphirenium cation (5) resulted in the preservation of aromaticity with proper substituents (fluorine) in 36, as a result of the interaction with PF2 a -orbitals,as indicated by isodesmic reactions. A similar phenomenon has been observed also for 1,1-difluorocyclopropene, 1,1-bissilylcyclopentadiene, and 1,1-bisstannylcyclopen-tadiene. In the latter case, the phenomenon has been called hyperconjugate aromaticity . The effect of... 

The disproportionation of borirene to acetylene and diboretene (58) is slightly endothermic (by 5 kcal/mol), whereas the activation barrier of this BH transfer reaction equals 14.6 kcal/mol (86JA3960) The estimation of the aromatic stabilization energy for isodesmic reaction (59) yields values of 47.5 (6-31G //6-31G ) (86JA3960), 47.1 kcal/mol (6-31G //STO-3G), [81JA( 103)2589] which makes about 70% of the stabilization energy... 

The determination of the energy of aromatic stabilization of borepin by calculating the energies of isodesmic reactions (62) and (63) with a correction for the strain leads to the value 12.7 kcal/mol (89MI6). The same energy calculated with the same basis set (6-31G ) is for benzene... 

The MNDO calculations on sila-, germa-, and stannacyclopentadienyli-denes have shown that whereas for cyclopentadienylidene (272) the energies of the antiaromatic 47t- and the aromatic 6ir-electron structures are close in value (89UK1067), in the (273)-(275) series the 67r-electron structures are quite noticeably destabilized (Table XXIII). Unlike (272), the electronic ground state of compounds (273)—(275) correspond to minima on the PES. These results point to the diminished role of antiaromatic destabilization in the 47r-electron structure (273)—(275), as opposed to (272). It should therefore be expected that these molecules would be more stable than (272). This has indeed been confirmed by our calculation on the heats of the isodesmic reaction (85) (Table XXIII). 

According to the MNDO calculations (88MI4), the planar structure of the trigermacyclopropenyl cation (296) also corresponds to a minimum on the PES, whereas the analogous structure of (SnH)3+ (297) is a third-order saddle point. The MNDO calculation of the ISE (291) [isodesmic reaction (88)], with a correction for the strain energy determined from isodesmic reaction (89), shows the aromatic stabilization of (291) to be insignificant (1.8 kcal/mol) ... 

Evidence for aromatic Ji-delocalisation in Si(N N ) (2) included (i) the greatly enhanced (Raman) C=C stretching mode in 2 over non-aromatic reference compounds and comparisons of (ii) the enthalpies (2 > 4) of the isodesmic reactions for 2(or 4) -f SiH4... 

To explore this possibility further, quantum chemical calculations were carried out for the isodesmic reactions of the model compounds 66 and 67 with dihydrogen to give the corresponding dihydrides (Scheme 17). These calculations showed that the reaction of 66 with H2 is about 14 kcalmol-1 less exothermic than that of 67. This difference may reflect aromatic resonance energy in the unsaturated molecule 66, reducing the enthalpy of the hydrogenation reaction. 

A very different pAR applies to tropylium ion coordinated to an Fe(CO)3 group, 41. The value has not been determined experimentally but calculations for an isodesmic reaction relating 41 to the uncomplexed tropylium ion imply a pAR in the region of —5.196 The relative instability of this ion must reflect sacrifice of aromatic stabilization in the rf coordination imposed by... 

The related family of phosphonio-phospholides also belong to the five-membered aromatic heterocycles, as reviewed comprehensively in the recent work of Gudat [209] The aromaticity of the rings were concluded from stabilization in isodesmic reactions [210] and also from the structure of those rings characterized by X-ray diffraction, exhibiting equalized bond length distribution in the case of the cationic 59 [211] and also for the neutral zwitterionic 60 [212], and its r - and r 5-complexed forms [214] (Scheme 34)... 

Isodesmic reactions have been used to investigate aromatic stabilization, but there is not a unique isodesmic reaction for each problem. Write two isodesmic reactions for the ring-opening of benzene, both of which have on each side of the equation the same number of each kind of bond. Have you any reason to prefer one of the equations to the other ... 

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. 

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... 

Jenks et al. studied the effects of conjugation and aromaticity on the sulfoxide bond by means of ab initio computation <1996JOC1275>. They calculated S-O bond dissociation energies (BDEs) and found that, in a formally aromatic system such as thiophene sulfoxide, the SO BDE is decreased by as much as 25kcalmoP relative to the BDE of DMSO. Although the BDE of the formally antiaromatic thiirene sulfoxide increased by about 15 kcal moP the authors concluded, on the basis of calculated geometries and isodesmic reactions with pure hydrocarbons, that cyclic unsaturated sulfoxides are neither significantly aromatic nor antiaromatic. 

An energy difference for the isodesmic reaction [Eq. (28)] indicates a quantitative measure of the extent of electron delocalization (6-31G ) (62). The energy difference for the C, structure of the silacyclopentadienide anion is only 2.2 kcal/mol, which is much smaller than 73.4 kcal/mol for the carbon analog, a planar cyclopentadienide anion (C2 ). On the basis of these calculations, it is concluded that the ground-state structure of the silacyclopentadienide anion has only ca. 3% of the resonance stabilization exhibited by the cyclopentadienide anion. However, the previous calculation (3-21G//STO-2G) on the 2 planar structure shows that the resonance energy is 23 kcal/mol, which means ca. 25% as aromatic as the carbon analog (65). 

The electronic structure and the aromaticity of 1,2-azaphosphole, 1,2-oxaphosphole, 1,2-thiaphosphole, and 1,2-diphosphole have been theoretically investigated in terms of aromatic stabilization energies (ASEs), resonance energies (REs), magnetic susceptibility exaltations, and nucleus-independent chemical shift (NICS) indexes <2002JOC1333>, as well as based on isodesmic reactions <2003T1657>. 

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