Isotopic Perturbations in Aromatic Character and New Closely Related Conformers Found in [16]- and [18]Annulene

and, Cheryl D. Stevenson; Kurth, Todd L.

doi:10.1021/ja993604f

Cited by 43 publications

(55 citation statements)

References 5 publications

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“…With both functionals the D 6h geometry results to be a transition state between equivalent minima structures of C 2 symmetry (see Figure 44). The proton chemical shifts of the inner and outer hydrogen atoms calculated starting from the C 2 structure have been found in excellent agreement with experiment [6,266]. Static and dynamic crystal disorder may explain, according to this study [20], the difference with respect to the X-ray indications.…”

Section: [18]annulenesupporting

confidence: 83%

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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Section: [18]annulenesupporting

confidence: 83%

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

Gellini

Salvi

2010

Symmetry

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“…Remarkably, the 1 H NMR spectroscopic parameters match those of [18]annulene because the 1 H NMR spectrum (CDCl 3 , 213 K) of [18]annulene is characterized by quartet at d = 9.17 ppm and a quintet at d = À2.96 ppm for the external and internal protons, respectively. [25,52] Variable-temperature studies: Variable-temperature . [25] The preference for a folded geometry can be explained by a tendency to maximize the nonbonded H-1···H-4 distance to minimize the steric repulsion.…”

Section: Nmr Spectroscopic Studies Ofmentioning

confidence: 99%

“…[25,52] Variable-temperature studies: Variable-temperature . [25] The preference for a folded geometry can be explained by a tendency to maximize the nonbonded H-1···H-4 distance to minimize the steric repulsion. [24,25,53] The analogous conformational equilibrium effect was found to be instrumental in analysis of the temperature dependence of the chemical shifts detected for [16]-and [18]annulene.…”

Section: Nmr Spectroscopic Studies Ofmentioning

confidence: 99%

“…[25] The preference for a folded geometry can be explained by a tendency to maximize the nonbonded H-1···H-4 distance to minimize the steric repulsion. [24,25,53] The analogous conformational equilibrium effect was found to be instrumental in analysis of the temperature dependence of the chemical shifts detected for [16]-and [18]annulene. [25] The pyrrole rings can be tilted out of the C 5 C 10 C 15 C 20 plane in the same or opposite directions, albeit the coplanar arrangement cannot be excluded.…”

Section: Nmr Spectroscopic Studies Ofmentioning

confidence: 99%

“…[18,23] On the other hand, [18]annulene 4 itself is not just a trivial standard of aromaticity, but a fluxional molecule, affected by steric interactions of internal protons. [24,25] Although an [18]annulene complies with the spectral and structural criteria of aromaticity, as a consequence of the conformational flexibility it is a relatively reactive molecule, which decomposes at elevated temperatures with the tendency for addition/polymerization rather than electrophilic substitution reactions. [26,27] The last decade of the 20th century heralded a renaissance of annulene chemistry.…”

Section: Introductionmentioning

confidence: 99%

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A Flexible Porphyrin–Annulene Hybrid: A Nonporphyrin Conformation formeso‐Tetraaryldivacataporphyrin

Pacholska‐Dudziak¹,

Szterenberg²,

Latos‐Grażyński³

2011

Chemistry A European J

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An annulene-porphyrin hybrid, the diaaza-deficient porphyrin 5,10,15,20-tetraaryl-21,23-divacataporphyrin, has been synthesized by an extrusion of tellurium atom(s) from 5,10,15,20-tetraaryl-21,23-ditelluraporphyrin under treatment with HCl. In addition, a monoaza-deficient 5,10,15,20-tetraaryl-21-tellura-23-vacataporphyrin was formed in the same reaction. The two new members of the vacataporphyrin family were characterized by X-ray crystallography, as well as UV/Vis and NMR spectroscopy. These aromatic molecules preserve the fundamental structural and spectroscopic features of the parent tetraarylporphyrin. The X-ray crystal structures of 21,23-divacataporphyrin and 21-tellura-23-vacataporphyrin show typical porphyrin patterns. The molecules are not strictly planar and show distortion of the annulene moieties. The N22⋅⋅⋅N24 distances (5.23 and 5.09 Å) are considerably longer than in regular porphyrins. For 21,23-divacataporphyrin, variable-temperature (1)H NMR spectroscopy data allowed the identification of divacataporphyrin stereoisomers differentiated by the geometry of the butadiene bridges. The forms remain in thermodynamic equilibrium.

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A Direct Link between Annulene and Porphyrin Chemistry—21-Vacataporphyrin

2002

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Isotopic Perturbations in Aromatic Character and New Closely Related Conformers Found in [16]- and [18]Annulene

Cited by 43 publications

References 5 publications

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

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

A Flexible Porphyrin–Annulene Hybrid: A Nonporphyrin Conformation formeso‐Tetraaryldivacataporphyrin

A Direct Link between Annulene and Porphyrin Chemistry—21-Vacataporphyrin

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