DOI: 10.1002/chem.200701534 Correlation of Delocalization Indices and Current-Density Maps in Polycyclic Aromatic Hydrocarbons Stijn Fias, [a] Patrick W. Fowler, [b] Juan Luis Delgado, [b, c] Uwe Hahn, [b, d] and Patrick Bultinck* [a] Introduction The concept of aromaticity is fundamental and widespread in chemistry, [1–4] but aromaticity is not itself a directly meas- urable quantity. This absence of an immediate observable has resulted in a proliferation of approaches which have been suggested to describe and quantify the aromaticity of a molecule, typically based on energetic, [5] geometric, [6] quan- tum chemical [7] or magnetic considerations. [8–12] Lack of cor- relation between different measures has provoked sugges- tions that aromaticity should be considered a multidimen- sional phenomenon. [5,13–16] Multidimensionality has been much debated, but as all molecular properties, including ar- omaticity indices, are ultimately derived from the same mo- lecular wave function, it is hard to envisage a physical justi- fication for different well founded approaches to aromaticity yielding contradictory results. A related concept is that of electron delocalisation. There has been an upsurge of interest in ways of quantifying the extent of delocalization. Among the more popular are the Para Delocalization Index (PDI) [7,17] and the Fluctuation (FLU) [7,18] Index. Recently, one of the present authors intro- duced Multi Centre Bond Indices (MCBI) to quantify elec- tron delocalization in aromatic systems. [19–22] In fact, all three quantities were originally introduced as aromaticity in- dices, but there is an important distinction that should be kept in mind when using a delocalization index to quantify aromaticity. The distinction emerges when we examine the magnetic properties of an aromatic system. It has long been known that aromatic and antiaromatic systems sustain ring currents in the presence of a magnetic field. This forms the basis of the use of ring current (RC) maps [11,12] as an indicator of ar- omaticity and antiaromaticity. Both aromaticity and antiaro- maticity imply a delocalized electronic structure, but a mole- cule could have a delocalized system, and still not show a ring current when exposed to a magnetic field. In a pertur- bation theory approach, distribution and intensity of ring currents depends on the availability of virtual orbitals of ap- propriate symmetry. [11,23,24] In this sense, significant delocali- zation is a necessary but not a sufficient condition for ring current. Moreover, if a ring current does appear, the deloc- alization index does not reveal information on its sense: dia- Abstract: Using multicentre delocaliza- tion indices, the ring current maps of a large set of polycyclic aromatic hydro- carbons (PAH) are reconstructed and compared with ab initio computations of the same maps in the pseudo-p ver- sion of the ipsocentric approach to magnetic response. The quality of the comparison indicates that both delocal- ization and ring current approaches capture the same information about the aromatic nature of the PAH. Aro- maticity as a global property, requires knowledge of more than single circuits, but the present results suggest no need to introduce a “multidimensional char- acter” for aromaticity. Keywords: aromaticity · delocaliza- tion · NICS · ring currents [a] S. Fias, Prof.Dr. P. Bultinck Department of Inorganic and Physical Chemistry Ghent University, Krijgslaan 281 (S3), 9000 Ghent (Belgium) Fax:ACHTUNGTRENNUNG(+32)926-44-983 E-mail: Stijn.Fias@UGent.be Patrick.Bultinck@UGent.be [b] PRof. Dr. P. W. Fowler, Dr. J. L. Delgado, Dr. U. Hahn Department of Chemistry, University of Sheffield Sheffield, S3 7HF (UK) [c] Dr. J. L. Delgado Present address: Departamento de Química Orgµnica Facultad de Química, Universidad Complutense 28040 Madrid (Spain) [d] Dr. U. Hahn Present address: Departamento de Química Orgµnica Facultad de Ciencias, Universidad Autónoma de Madrid 28049 Cantoblanco (Spain) Supporting information for this article is available on the WWW under http://www.chemeurj.org/ or from the author: The SCI, summed TCI and FCI and PP-NICS [32] for each ring of the molecules. Chem. Eur. J. 2008, 14, 3093–3099 # 2008 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 3093 FULL PAPER