6533b7d0fe1ef96bd125a419

RESEARCH PRODUCT

On the relations between aromaticity and substituent effect

subject

description

Aromaticity/aromatic and substituent/substituent effects belong to the most commonly used terms in organic chemistry and related fields. The quantitative description of aromaticity is based on energetic, geometric (e.g., HOMA), magnetic (e.g., NICS) and reactivity criteria, as well as the properties of the electronic structure (e.g., FLU). The substituent effect can be described using either traditional Hammett-type substituent constants or characteristics based on quantum-chemistry. For this purpose, the energies of properly designed homodesmotic reactions and electron density distribution are used. In the first case, a descriptor named SESE (energy stabilizing the substituent effect) is obtained, while in the second case cSAR (charge of the substituent active region), which is the sum of the charge of the ipso carbon atom and the charge of the substituent. The use of the above-mentioned characteristics of aromaticity and the substituent effect allows revealing the relationship between them for mono-, di-, and polysubstituted π-electron systems, including substituted heterocyclic rings as well as quasi-aromatic ones. It has been shown that the less aromatic the system, the stronger the substituent influence on its π-electron structure. In all cases, when the substituent changes number of π-electrons in the ring in the direction of 4N+2, its aromaticity increases. Intramolecular charge transfer (a resonance effect) is privileged in cases where the number of bonds between the electron-attracting and electron-donating atoms is even. Quasi-aromatic rings, when attached to a truly aromatic hydrocarbon, simulate well the “original” aromatic rings, alike the benzene. For larger systems, a long-distance substituent effect has been found.