title: Local aromaticity of the six-membered rings in pyracylene. A difficult case for the NICS indicator of aromaticity.
abstract: In this work, we have analyzed the local aromaticity of the six-membered rings (6-MRs) of planar and pyramidalized pyracylene species through the structurally based harmonic oscillator model of aromaticity (HOMA), the electronically based para-delocalization index (PDI), and the magnetic-based nucleus independent chemical shift (NICS) measurements, as well as with maps of ring current density. According to ring currents and PDI and HOMA indicators of aromaticity, there is a small reduction of local aromaticity in the 6-MRs of pyracylene with a bending of the molecule. In the case of NICS, the results depend on whether the NICS value is calculated at the center of the ring (NICS(0)) or at 1 A above (NICS(1)(out)) or below (NICS(1)(in)) the ring plane. While NICS(1)(out) values also indicate a slight decrease of aromaticity with bending, NICS(0) and NICS(1)(in) wrongly point out a large increase of aromaticity upon distortion. We have demonstrated that the NICS(0) reduction in the 6-MRs of pyracylene upon bending is due to (a) a strong reduction of the paratropic currents in 5-MRs and (b) the fact that, due to the distortion, the paratropic currents point their effects in other directions.
authors.affiliation: Institut de Química Computacional and Departament de Química, Universitat de Girona, E-17071 Girona, Catalonia, Spain.
authors.forename: Jordi
authors.initials: J
authors.lastname: Poater
authors.forename: Miquel
authors.initials: M
authors.lastname: Solà
authors.forename: Rosario G
authors.initials: RG
authors.lastname: Viglione
authors.forename: Riccardo
authors.initials: R
authors.lastname: Zanasi