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Conformational flexibility of the ring as a new dimension of aromaticity: role of the structural factors

Work number - M 67

Author: Omelchenko I. V., Ph. D.

Presented by the SSI STC “Institute for Single Crystals” NAS of Ukraine.

The work is devoted to study of the effect of basic structural factors like the cycle size, the presence of heteroatoms and substituents, on the interrelation between aromaticity and conformational flexibility of the unsaturated rings.

Series of fundamental results were obtained. The dependence of changes of aromaticity and ring flexibility with respect to the type of heteroatom was systematized for monoheterocyclic analogues of benzene and cyclopentadienyl amione. Special features of cyclic conjugated system in heterocycles with heteroatoms of III-IV period were revealed.

The difference between donor and acceptor substituents was found studying effect of the substituent on aromaticity of monosubstituted benzenes. A quantitative dependence was established between the substituent constants and energies of deformation of corresponding aromatic rings.

For polysubstituted benzenes with nitro- and amino groups, the forming of intramolecular resonance-assisted hydrogen bonds was determined as a key factor of the changes in the cyclic π-conjugation that may lead to loosing of the aromaticity of benzene ring. At the time it was shown that, in nitro- and amino substituted monoheterocycles, the push-pull effect is the main factor of aromaticity changes.

A detailed analysis of the dependence of the properties of five-membered monoheterocycles on the electronic structure of heteroaromatic group revealed the principal difference between the heterocycles with different ring size.

New approach to the theoretical analysis of conformational characteristics of aromatic rings at the nonzero temperature and including entropy factor was introduced. The energy of deformation of the ring was shown as a new structural-dynamic aromaticity index. Unusually high conformational flexibility and low degree of aromaticity was revealed studying aromatic rings at the room temperature, as compare to their behavior at 0 K. It was shown that the entropy factor plays a critical role in such cases, and its influence on high symmetrical benzene molecule is much more than on low symmetrical molecules of heterocycles.

A basis for prognostic models was created that predict change of different physico-chemical parameters of aromatic rings as a function of their element composition and structural surrounding. New approaches with using of molecular dynamic methods allow predicting conformational characteristics of the cycles at nonzero temperature and related spectral features. Quantitative parameters obtained during the research can be used for modeling of complex biological systems and functional materials.

There are 61 publications totally (46 papers, 43 of them were published in referred scientific journals from the Scopus list, and 15 conference theses) cited 218 times (h-index is 7). Results of this work are presented in 15 publications, including 7 papers in referred scientific journals. These works were cited 93 times. (All data is presented according to Google Scholar).