How Different are the Diamagnetic and Paramagnetic Contributions to Off-Nucleus Shielding in Aromatic and Antiaromatic Rings?

Autor: Karadakov PB; University of York, Chemistry, Heslington, YO10 5DD, York, UNITED KINGDOM.
Jazyk: angličtina
Zdroj: Chemphyschem : a European journal of chemical physics and physical chemistry [Chemphyschem] 2023 Feb 22, pp. e202300038. Date of Electronic Publication: 2023 Feb 22.
DOI: 10.1002/cphc.202300038
Abstrakt: The spatial variations in the diamagnetic and paramagnetic contributions to the off-nucleus isotropic shielding, σiso(r) = σisod(r) + σisop(r), and to the zz component of the off-nucleus shielding tensor, σzz(r) = σzzd(r) + σzzp(r), around benzene (C6H6) and cyclobutadiene (C4H4) are investigated using complete-active-space self-consistent field wavefunctions. Despite the substantial differences between σiso(r) and σzz(r) around the aromatic C6H6 and the antiaromatic C4H4, the diamagnetic and paramagnetic contributions to these quantities, σisod(r) and σzzd(r), and σisop(r) and σzzp(r) are found to behave similarly in the two molecules, shielding and deshielding, respectively, each ring and its surroundings. The different signs of the most popular aromaticity criterion, the nucleus-independent chemical shift (NICS), in C6H6 and C4H4 are shown to follow from a change in the balance between the respective diamagnetic and paramagnetic contributions. Thus, the different NICS values for antiaromatic and antiaromatic molecules cannot be attributed to differences in the ease of access to excited states only; differences in the electron density which determines the overall bonding picture also play an important role.
(© 2023 Wiley-VCH GmbH.)
Databáze: MEDLINE
Externí odkaz: