The Heat of Hydrogenation of (a) Cyclohexatriene

Beckhaus, Hans‐Dieter; Faust, Rüdiger; Matzger, Adam J.; Mohler, Debra L.; Rogers, Donald W.; Rüchardt, Christoph; Sawhney, Ajai K.; Verevkin, Sergey P.; Vollhardt, K. Peter C.; Wolff, Stefan

doi:10.1021/ja001274p

Cited by 53 publications

(50 citation statements)

References 28 publications

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“…However, also for these two similarly distorted benzenes the situation is more complicated. It was recently shown that the central benzene ring of the former lacks any conjugation [8], whereas the latter still possesses all the regular benzene properties [9]. Thus, the occurrence of bond alternation is not necessarily indicative of a lack or reduction of either aromaticity or resonance [10].…”

Section: Introductionmentioning

confidence: 99%

1,3,5-Cyclohexatriene captured in computro; the importance of resonance

Lenthe

Havenith

Dijkstra

et al. 2002

Chemical Physics Letters

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The geometry and energy of 1,3,5-cyclohexatriene, the reference molecule for the determination of the extra stabilization of benzene, have been calculated using an Ab Initio Valence Bond method. The theoretical resonance energy, according to Dewar, calculated as the energy difference between two-structure benzene and single-structure 1,3,5-cyclohexatriene, both with completely optimized geometries and orbitals, is only )12.05 kcal/mol. Resonance energies of )25.37 (local orbitals), )19.82 (delocal orbitals) and )44.13 (breathing orbitals) kcal/mol are found using the Pauling definition. The concept of the vertical resonance energy, however, is proven to be not tenable beyond a minimal basis. Ó

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Section: Introductionmentioning

confidence: 99%

1,3,5-Cyclohexatriene captured in computro; the importance of resonance

Lenthe

Havenith

Dijkstra

et al. 2002

Chemical Physics Letters

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“…[57][58][59] Beckhaus et al reported that the strain-corrected heat of hydrogenation of a benzannulated species ([4]phenylene) with a cyclohexatriene-like geometry was nearly the same as that of three cyclohexenes. [60] As discussed below, this was evidently due to the absence of antiaromatic cyclobutadiene character in the substituents. In contrast, computational studies of deformed benzenes, for example, with substantial alternation of the bond lengths, have not revealed any such appreciable decrease in the cyclic electron delocalization.…”

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confidence: 98%

“…The residual conjugation energy (VRE) at the optimal geometry of localized 1,3,5-cyclohexatriene with 1.314 and 1.522 alternating CC bond lengths is 40.2 kcal mol À1 , 44 % of the VRE at the bond-equivalent geometry! The behavior of tris(benzocyclobutadieno)benzene [60] as well as tris(bicyclo[2.1.1]hexeno)benzene [58] evidently originates from the inhibition of conjugation in order to avoid benzocyclobutadiene-like antiaromaticity or to reduce the strain of the benzannulated groups.…”

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confidence: 99%

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

Schleyer

2006

Chemistry A European J

149

211

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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.

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“…6,8,13,15 After the synthesis of [N]phenylenes of various topologies, [16][17][18][19][20][21][22][23][24][25][26][27][28] the experimental confirmation of the theoretical data has become possible. The cyclohexatriene nature of the central ring of (1) in the ground state has been demonstrated experimentally by the chemical reactivity of this compound 29,30 and the X-ray diffraction data. 19 31 Therefore, an analysis of the vibronic structure of the fluorescence and absorption spectra of [N]phenylenes is necessary.…”

Section: Introductionmentioning

confidence: 90%