Aromaticity and ring currents.

Accepting a commission to review progress in the subject embodied in our title is, perhaps, to take up something of a ‘poisoned chalice’. One of us, contributing on this same topic more than 20 years ago at the 1979 International Symposium on Aromaticity in Dubrovnik, wrote1 “A cynic would say that there are actually only two difficulties in discussing the subject of ‘aromaticity’ and ‘ring currents’sdeciding what is meant by ‘ring current’, and assigning a meaning to the term ‘aromaticity’!” That comment, though ostensibly facetious, had serious intent: it did encapsulate, with only a modicum of exaggeration, the problems that inherently beset any assessment such as the one attempted at Dubrovnik1 and in the present review. At the heart of the matter lies the undeniable fact that neither ring currents nor aromaticity are physical observables. Nevertheless, the intervening period has seen the ring-current idea, at least, become generally less controversial and more accepted than it once was. At the time of our opening quotation, one of us and Haigh had just published an exhaustive review2 of the ring-current concept covering the period up to about 1980sthe end of what might now be regarded as the era of semiempirical calculations in this field.2 This review2 (1979/1980) concluded that “the ‘ring current’ picture has proved itself ... to have great power in rationalising, at least qualitatively, the magnetic properties of π-electron systems. It is so pictorial that one can almost feel what is happening when a [conjugated] molecule is subjected to a magnetic field. Whatever advances the future may bring, it may be that the favourite habitat of the ‘ring current’ will be that in which it was born and brought up, namely, that of semi-empirical π-electron theory”. In other words, these authors were sanguine that, at the time (ca. 1980), the ring-current idea was gently coming to the end of its natural, useful life. However, as recently as 1997, when reviewing progress concerning the status of the ring-current model during the decade and a half or so after 1980s a period in this field that we have dubbed3 ‘the ab initio era’sthe present authors3 were initially somewhat surprised to find themselves concluding that “the ‘ring-current’ idea has well survived the first 15 years of the ab initio era.” Lazzeretti’s subsequent magnum opus4 on ring currents has more than confirmed this. By contrast, the sheer fact that in 2001sthe very first year of the 21st centurysthe American Chemical Society has seen fit to run this particular issue of Chemical Reviews shows that the concept of Aromaticity is as elusive as it ever was (see section II). There are three reasons for our not feeling obliged or inclined to present, in this review, an exhaustive, systematic, or historical critique of the ring-current concept itself. First, as we have just claimed, the idea of a ring current seems more secure now than it was 20 years ago, and it would appear that less apology or justification is needed for invoking it. Second, we ourselves have, in any case, only recently updated 1349 Chem. Rev. 2001, 101, 1349−1383