Critique of the resonance energy concept with particular reference to nitrogen heterocycles, especially porphyrins

George, Philip

doi:10.1021/cr60293a004

Cited by 54 publications

(46 citation statements)

References 16 publications

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“…Resonance energies of large aromatic systems such as porphyrins are difficult to assess experimentally [98] and consequently, relative stabilities of porphyrinoid systems are normally estimated using highlevel quantum chemical methods. It should be noted that the effect of aromatic stabilization (antiaromatic destabilization) is less pronounced for large macrocyclic aromatics than for their smaller congeners.…”

Section: Aromaticity Criteriamentioning

confidence: 99%

Figure Eights, Möbius Bands, and More: Conformation and Aromaticity of Porphyrinoids

2011

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The aromatic character of porphyrins, which has significant chemical and biological consequences, can be substantially altered by judicious modifications of the parent ring system. Expansion of the macrocycle, which is achieved by introducing additional subunits, usually increases the so-called free curvature of the ring, leading to larger angular strain. This strain is reduced by a variety of conformational changes, most notably by subunit inversion and p surface twisting. The latter effect creates a particularly convenient access to Möbius aromatic molecules, whose properties, predicted over 40 years ago, are of considerable theoretical importance. The conformational processes occurring in porphyrin analogues are often coupled to other chemical phenomena, and can thus be exploited as a means of constructing functional molecular devices. In this Review, the structural chemistry of porphyrinoids is discussed in the context of their conformational dynamics and p-electron conjugation.

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Section: Aromaticity Criteriamentioning

confidence: 99%

Figure Eights, Möbius Bands, and More: Conformation and Aromaticity of Porphyrinoids

2011

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“…Empirical estimates of resonance energies are obtained by means of isodesmic and homodesmotic reactions [66,67] or heats of formation [68,69]. The isodesmic reaction C 6 H 6 + 6CH 4 → 3(H 2 C = CH 2 ) + 3(H 3 C − CH 3 ), having equal numbers of formal single and double CC bonds in reactants and products, gives an approximate determination of ARE (C 6 H 6 ) since it compares the energy of benzene with that of three ethylene molecules.…”

Section: Quantum Mechanical and Thermochemical Resonance Energymentioning

confidence: 99%

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

Gellini

Salvi

2010

Symmetry

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The paper introduces general considerations on structural properties of aromatic, antiaromatic and non-aromatic conjugated systems in terms of potential energy along bond length alternation and distortion coordinates, taking as examples benzene, cyclobutadiene and cyclooctatetraene. Pentalene, formally derived from cyclooctatetraene by cross linking, is also considered as a typical antiaromatic system. The main interest is concerned with [n]annulenes and model [n]annulene molecular systems, n ranging from 10 to 18. The rich variety of conformational and configurational isomers and of dynamical processes among them is described. Specific attention is devoted to bridged [10]-and [14]annulenes in the ground and lowest excited states as well as to s-indacene and biphenylene. Experimental data obtained from vibrational and electronic spectroscopies are discussed and compared with ab initio calculation results. Finally, porphyrin, tetraoxaporphyrin dication and diprotonated porphyrin are presented as annulene structures adopting planar/non-planar geometries depending on the steric hindrance in the inner macrocycle ring. Radiative and non-radiative relaxation processes from excited state levels have been observed by means of time-resolved fluorescence and femtosecond transient absorption spectroscopy. A short account is also given of porphycene, the structural isomer of porphyrin, and of porphycene properties.

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“…Hence, there are two possible forms of bonding to metal atoms: N-end for pyrrole and O-end for furan lone-paired -orbital bonding and -orbital bonding. Although five-membered heterocyclic ligands are poorer electron donors than benzene and pyridine, many examples can be found in the literature of such complexes in the condensed phase [33], and gas phase studies [34]. Previous studies carried out in condensed phases have shown that pyrrole can N-bond to metal centers [35][36][37][38].…”

Section: Bonding and The Collision-induced Fragmentation Patterns Of mentioning

confidence: 99%

Gas phase studies of metal dimer complexes, M2Ln+, where M=Zn and Mn, and L=pyrrole and furan, for n in the range 1–5

Stace

2006

International Journal of Mass Spectrometry

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Critique of the resonance energy concept with particular reference to nitrogen heterocycles, especially porphyrins

Cited by 54 publications

References 16 publications

Figure Eights, Möbius Bands, and More: Conformation and Aromaticity of Porphyrinoids

Figure Eights, Möbius Bands, and More: Conformation and Aromaticity of Porphyrinoids

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

Gas phase studies of metal dimer complexes, M2Ln+, where M=Zn and Mn, and L=pyrrole and furan, for n in the range 1–5

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