An Energetic Measure of Aromaticity and Antiaromaticity Based on the Pauling–Wheland Resonance Energies

Abstract

Various criteria based on geometric, energetic, magnetic, and electronic properties are employed to delineate aromatic and antiaromatic systems. The recently proposed block-localized wave function (BLW) method evaluates the original Pauling-Wheland adiabatic resonance energy (ARE), defined as the energy difference between the real conjugated system and the corresponding virtual most stable resonance structure. The BLW-derived ARE of benzene is 57.5 kcal mol(-1) with the 6-311+G** basis set. Kistiakowsky's historical experimental evaluation of the stabilization energy of benzene (36 kcal mol(-1)), based on heats of hydrogenation, seriously underestimates this quantity due to the neglect of the partially counterbalancing hyperconjugative stabilization of cyclohexene, employed as the reference olefin (three times) in Kistiakowsky's evaluation. Based instead on the bond-separation-energy reaction involving ethene, which has no hyperconjugation, as well as methane and ethane, the experimental resonance energy of benzene is found to be 65.0 kcal mol(-1). We derived the "extra cyclic resonance energy" (ECRE) to characterize and measure the extra stabilization (aromaticity) of conjugated rings. ECRE is the difference between the AREs of a fully cyclically conjugated compound and an appropriate model with corresponding, but interrupted (acyclic) conjugation. Based on 1,3,5-hexatriene, which also has three double bonds, the ECRE of benzene is 36.7 kcal mol(-1), whereas based on 1,3,5,7-octatetraene, which has three diene conjugations, the ECRE of benzene is 25.7 kcal mol(-1). Computations on a series of aromatic, nonaromatic, and antiaromatic five-membered rings validate the BLW-computed resonance energies (ARE). ECRE data on the five-membered rings (derived from comparisons with acyclic models) correlate well with nucleus-independent chemical shift (NICS) and other quantitative aromaticity criteria. The ARE of cyclobutadiene is almost the same as butadiene but is 10.5 kcal mol(-1) less than 1,3,5-hexatriene, which also has two diene conjugations. The instability and high reactivity of cyclobutadiene thus mainly result from the sigma-frame strain and the pi-pi Pauli repulsion.

How resonance assists hydrogen bonding interactions: An energy decomposition analysis

“…The magnitude of resonance in aromatic rings will be significantly increased in comparison with linear systems because of the strong coupling (or aromaticity 49 ) from isoenergetic resonance structures, but the general features are similar to those in 3-hydroxypropenal. Figure 5 shows the EDD maps in 2-pyridinone (4) and 2-hydroxpyridine (5).…”

“…Species with aromatic rings (3-5) exhibit very high resonance energies, in accord with their extraordinary stability or aromaticity. 49 However, we are interested in the redistribution of electron density due to resonance, as delocalization significantly changes the dipole moments which will subsequently influence the interaction between monomers. With the resonance of electrons turned off, we conducted the energy analysis and results on the nonresonance-assisted hydrogen bonds (nonRAHBs) are listed in Table 2.…”

A comparison of approaches to estimate the resonance energy

“…That has been defined in valence bond as a theoretical resonance energy (TRE) [7]-the energy difference between the resonating, multi-structure VB wave function used for the description of benzene (real molecule) and the single structure VB wave function that describes its non-aromatic counterpart 1,3,5-cyclohexatriene in a relaxed D 3h symmetry. In contrast, in the block-localised wave function (BLW) [11][12][13] approach, the resonance energy is defined as the energy that is gained by the delocalisation of the localised constituent parts (blocks), computed using a BLW wave function (non-aromatic counterpart), to the delocalised molecule, calculated using the Hartree-Fock wave function (real molecule). In the BLW approach, the delocalisation energy is thus calculated.…”

Cyclobutadiene: The Antiaromatic Paradigm?

“…In view of all the above it is not surprising that recent opinions vary enormously on how much CBD is thermochemically destabilized solely by virtue of the cyclic conjugation of its four p electrons; estimations range from 230 kJ mol À1 (Deniz et al [24] ) to 44 kJ mol À1 (Mo and Schleyer [13] ). If even this lower number is attributed to p repulsion, one is left with the prediction that CBD, in spite of its enormous destabilization, must be considered as essentially nonaromatic, as based on its thermochemical properties.…”

“…If even this lower number is attributed to p repulsion, one is left with the prediction that CBD, in spite of its enormous destabilization, must be considered as essentially nonaromatic, as based on its thermochemical properties. In fact, Schleyer concludes in his above-cited paper [13] that "instead of the conventional interpretation of CBD as the antiaromatic paradigm, it should be regarded as a unique molecule", a view that I tend to share.…”