Local aromaticity in polyacenes manifested by individual proton and carbon shieldings: DFT mapping of aromaticity

Magdalena Gajda; Łukasz Gajda; Teobald Kupka; Tapas Kar

doi:10.1002/mrc.4967

Local aromaticity in polyacenes manifested by individual proton and carbon shieldings: DFT mapping of aromaticity

Magn Reson Chem. 2020 Feb;58(2):145-153. doi: 10.1002/mrc.4967. Epub 2019 Nov 21.

Authors

Magdalena Gajda¹, Łukasz Gajda², Teobald Kupka², Tapas Kar³

Affiliations

¹ Institute of Biotechnology, University of Opole, Opole, Poland.
² Faculty of Chemistry, University of Opole, Opole, Poland.
³ Department of Chemistry and Biochemistry, Utah State University, UT.

PMID: 31713900
DOI: 10.1002/mrc.4967

Abstract

Exponential dependencies between locally calculated geometric and magnetic indexes of aromaticity, harmonic oscillator model of aromaticity (HOMA) and nucleus independent chemical shifts (NICS)(0), NICS(1) and NICS(1)zz, and the number of conjugated benzene rings in linear acenes, from benzene to decacene were observed at B3LYP/6-311+G** level of theory. Correlations between HOMA and NICS indexes showed exponential dependencies and were fitted with simple three-parameter function. Similar correlations between both indexes of aromaticity and proton and carbon nuclear isotropic shieldings of individual acene rings were observed. Contrary to proton data, the predicted ¹³ C nuclear isotropic shieldings of carbon atoms belonging to inner rings in polyacenes were less shielded, indicating lower aromaticity and therefore, higher reactivity.

Keywords: HOMA; NICS; aromaticity; molecular modeling; nuclear shielding; polyacenes.