σ−π Energy Separation in Modern Electronic Theory for Ground States of Conjugated Systems

Jug, Karl; Hiberty, Philippe C.; Shaik, Sason

doi:10.1021/cr990328e

Cited by 118 publications

(177 citation statements)

References 138 publications

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“…31 Berry 32 used this suggestion to account for the observed increased frequency of the b2u Kekule vibrational mode when going from the ground 1 A1g to the first 1 B2u excited state. The same idea was reinforced later on by the work of Haas and Zilberg, 33,34 and especially by that of Hiberty, Shaik, and co-workers 35,36 and others 37,38 Finally, more recently Pierrefixe and Bickelhaupt [39][40][41] showed that the regular geometry of benzene is a consequence of how  and overlaps depend on bond distances. In aromatic annulenes the authors confirmed that bond equalisation is due to the  electrons whereas the -electron system favours doublebond localisation.…”

Section: Introductionmentioning

confidence: 88%

Quantifying aromaticity with electron delocalisation measures

et al. 2015

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Aromaticity cannot be measured directly by any physical or chemical experiment because it is not a well-defined magnitude. Its quantification is done indirectly from the measure of different properties that are usually found in aromatic compounds such as bond length equalisation, energetic stabilisation, and particular magnetic behaviour associated with induced ring currents. These properties have been used to set up the myriad of structural-, energetic-and magnetic-based indices of aromaticity known to date. Cyclic delocalisation of mobile electrons in two or three dimensions is probably one of the key aspects that characterise aromatic compounds. However, it has not been until the last decade that electron delocalisation measures have been widely employed to quantify aromaticity. Some of these new indicators of aromaticity such as the PDI, FLU, ING, and INB were defined in our group. In this paper, we review the different existent descriptors of aromaticity that are based on electron delocalisation properties, we compare their performance with indices based on other properties, and we summarise a number of applications of electronic-based indices for the analysis of aromaticity in interesting chemical problems.2

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Section: Introductionmentioning

confidence: 88%

Quantifying aromaticity with electron delocalisation measures

et al. 2015

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“…Due to second order Jahn-Teller effect [42], electronic states close to each other may interact and for n sufficiently large a structure with bond alternation may be energetically advantageous even for the higher annulenes satisfying the Hückel rule [43,44]. Following early recognitions of the propensity of π electrons to distortivity [18,45], the rather surprising statement was advanced in a series of important papers [17,[46][47][48][49][50][51][52][53]. The delocalized structure of benzene was seen as intermediate configuration between the localized K A and K B structures in the bond exchange process K A → K B along the distortion mode where CC stretchings alternate with CC shrinkings.…”

Section: π-Electron Approximationmentioning

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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“…In the case of π-electron systems, changes in the molecular geometry are usually considered as solely caused by modifications of the π-electron structure, but this approach is still a subject of vivid disputation. [6][7][8] Aromaticity as an electronic structure dependent property of π-electron systems 9,10 is well described for families of similar molecules by geometric, 11 magnetic 12 and energetic 13 characteristics. However, for a more differentiated set of molecular systems, no good correlation between these different parameters is found.…”

Section: Introductionmentioning

confidence: 99%

Metal Complexation and H-bonding Effects on Electronic Structure of Cytosine Studied in the Gas Phase

Stasyuk¹,

Szatyłowicz²,

Krygowski³

2014

Croat. Chem. Acta

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Abstract.The influence of H-bonding and complexation with cations (probed by HF, F -, Li + , Na + and K + ) on structural and π-electron changes in the six most stable cytosine tautomers has been studied in the gas phase using the B3LYP/6-311++G(2d,2p) computational level. The presence of two exo-groups (amino/imino and carbonyl/hydroxyl) in cytosine tautomers significantly increases their sensitivity to structural changes due to intra-and intermolecular interactions. These interactions induce large changes in aromaticity of the rings and in the CX (X = N, O) bond lengths of exocyclic groups. Three types of H-bonds, considering their strength, could be distinguished: (i) charge-assisted X -···HF, X = N or O, as the strongest, (ii) neutral X···HF, where X is the nitrogen atom of the ring or imino group or the keto form oxygen atom and (iii) also neutral X···HF, where X being either amino N or alternatively hydroxylic O. Hydrogen bond energy decreases approximately twice in the above listed sequence of interactions. Structural consequences of H-bonding and metal complexation have been observed not only in the immediate region of the interaction but also in other parts of the molecule (the shape of the amino group, changes in CO and CN bond lengths). Complexation of the cytosine tautomers with cations leads to monotonic changes in aromaticity in line with an increase of their ionic radii.

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σ−π Energy Separation in Modern Electronic Theory for Ground States of Conjugated Systems

Cited by 118 publications

References 138 publications

Quantifying aromaticity with electron delocalisation measures

Quantifying aromaticity with electron delocalisation measures

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

Metal Complexation and H-bonding Effects on Electronic Structure of Cytosine Studied in the Gas Phase

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