Theor Chem Acc (2007) 118:89–97 DOI 10.1007/s00214-007-0247-0 REGULAR ARTICLE Magnetotropicity of phosphole and its arsenic analogue Stefano Pelloni · Paolo Lazzeretti Received: 21 November 2006 / Accepted: 13 December 2006 / Published online: 1 February 2007 © Springer-Verlag 2007 Abstract Spatial ring current models for the phosp- hole molecule and its arsenic parent have been con- structed. Diatropism of these molecules is quite peculiar and fundamentally different from that of benzene as shown by stagnation graphs of current density field. Maps of shielding density are helpful for interpreting the effect of electronic currents on nuclear shielding. Con- strained planarity increases the degree of diatropicity quantitatively specified by magnetic descriptors, which implies that ring currents go together with π -electron distortivity. 1 Introduction The meaning of expressions like “aromatic character” or “aromatic behaviour” is quite vague despite their cur- rent use in the chemical literature. On the one hand, the word “aromaticity” seems to connote a commonly accepted idea, or even an essential category of the chem- ical thinking, on the other, no simple and indisputable definition has so far been provided in rigorous quantum mechanical terms. “Aromaticity indicators” that should help understand this elusive quality are frequently proposed [1]. There is also a general consensus that aromaticity can be “mea- sured” by geometric, energetic, and magnetic quanti- fiers. However, extended analyses proved that widely Contribution to the Fernando Bernardi Memorial Issue. S. Pelloni · P. Lazzeretti (B ) Dipartimento di Chimica, Università degli Studi di Modena e Reggio Emilia, Via Campi 183, 41100 Modena, Italy e-mail: lazzeret@unimo.it employed descriptors “do not speak with the same voice” and that “the phenomenon of aromaticity is regarded as statistically multidimensional” [2]. None- theless, several studies indicate that different aromaticity measures may collide, providing divergent information. For instance, the most stable fused heterobicycles are not the most aromatic positional isomers [3]. Non disputable, agreed definitions of aromaticity should be arrived at in terms of measurable molecu- lar properties [4], therefore magnetic response appears to offer most reliable criteria. Investigations on bis- heteropentalenes C 6 H 4 X 2 showed that the [3,4c] iso- mer is the least stable, but most diatropic, compound for X = NH, O, S, and PH [5–8]. Analogously, stronger dia- tropicity is apparently compatible with smaller energetic stabilisation in tetraazanaphthalenes [9]. We can possi- bly claim that the word aromaticity does not convey a generally accepted semantic value, despite its frequent use. Therefore, throughout this paper, “aromaticity” will be used as a colloquial term, whereas “magnetotropic- ity” is assumed to stand on more firm ground, as it can be related to experimentally accessible quantities, say, magnetic susceptibility and nuclear magnetic shielding. 2 Aromaticity of heterocyclic pentatomics containing phosphorous and arsenic The aromaticity of planar phosphole and its arsenic ana- logue – both saddle points of the inversion motion – is significantly larger than that of the non planar structures corresponding to the minimum on the energy hyper- surface. This has early been established for phosphole by Mislow and coworkers [10, 11]. More recently, the aromaticity of the planar structure was assessed by a