Antiaromaticity in fluorenylidene dications. Experimental and theoretical evidence for the relationship between the HOMO/LUMO gap and antiaromaticity

Nancy S. Mills, Amalia Levy, Benjamin F. Plummer

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38 Scopus citations

Abstract

The relationship between the calculated energy of the HOMO-LUMO gap, where (εLUMO - εHOMO)/2 is defined as ΔHL, and of the longest wavelength transition in the UV-visible spectrum, ΔE, was examined for a series of aromatic and antiaromatic cations and dications. TD-DFT calculations accurately modeled the energies of a series of dications including fluorenylidene dications whose UV-visible spectra are reported, as well as the energies of a series of aromatic and antiaromatic monocations whose spectra were previously reported. There is a linear correlation of the energy of the longest wavelength transition, ΔE calc, with ΔHL. There is no linear relationship between ΔEcalc and the sum of the NICS values, but there is a linear relationship between ΔEcalc multiplied by the number of atoms in the conjugated system, ΔEcalc(T), and the sum of the NICS values. There is also an approximate linear relationship between the average 1H NMR shift and the sum of the NICS values. These relationships give further support to the suggestion that the magnetic and energetic criteria of aromaticity and antiaromaticity are related. Furthermore, the data suggest that species that have ΔEcalc(T) < 20 are antiaromatic whereas those with ΔEcalc(T) > 30 are aromatic.

Original languageEnglish
Pages (from-to)6623-6633
Number of pages11
JournalJournal of Organic Chemistry
Volume69
Issue number20
DOIs
StatePublished - 1 Oct 2004
Externally publishedYes

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