Diamagnetic susceptibilities and susceptibility exaltations of some 7-substituted cycloheptatrienes

Abstract

The diamagnetic susceptibilities of several 7-substituted cycloheptatrienes have been measured and used in conjunction with estimated values of their susceptibilities (Haberditzl increment system) to obtain the susceptibility exaltations of these compounds. These exaltations were found to be very large and to vary as a function of the size of the C7 substituent. There was no apparent correlation of the exaltation of a compound with the electronic properties of its substituent. The effect of a substituent on the susceptibility exaltation is interpreted in terms of conformational changes in the seven-membered ring. The magnitude of the susceptibility exaltations of these cycloheptatrienes, for example 7-tert-butylcycloheptatriene (Λ = 14.8), in comparison to that of benzene (Λ = 13.7) would suggest that the presence of an induced diamagnetic ring current is not a good criterion of its aromaticity.

The homotropylium ion and homoaromaticity

“…There has been extensive discussion on the most appropriate criterion and it will not be repeated here.6 Suffice it to say that the presence of an induced ring current in a molecule, whether it be detected in its NMR spectrum or determined directly with diamagnetic susceptibility measurements,14 is but one and indeed a relatively poor criterion of aromaticity.15 For example, several years ago we determined the volume diamagnetic susceptibilities and susceptibility exaltations of 7-substituted cycloheptatrienes. 16 The disturbing feature of these results was that the exaltations were in some cases larger (more negative) than those found for benzene itself! Thus 7-teri-butylcycloheptatriene had an exaltation of-14.8 X 6 cm3/mol as compared to that of benzene of -13.7 X 1CT6 cm3/mol.…”

Unified theory of resonance energies, ring currents, and aromatic character in the (4n + 2).pi.-electron annulenes

“…This is particularly true of the magnetic or ring current criterion of aromaticity.2-4•6•26-40 Of all the criteria this one is most often employed, as some measure of the ring current is usually available from the 1H NMR (and sometimes 13C NMR41-43) chemical shifts4 of the compound, and measurements of diamagnetic susceptibility exaltations44 seem to be becoming quite routine. 45 In fact the magnetic criterion is often the sole piece of evidence for the aromatic character of new compounds, as resonance energies are extremely difficult to measure experimentally and are usually inferred from the observed chemical stability (which is not necessarily directly related to ground-state quantities such as resonance energy). There is now strong evidence to suggest that there is no relationship among these different criteria of aromatic character in the case of nonalternant hydrocarbons and heteroatomic systems.26-30 Even for the alternant hydrocarbons and annulenes there is as yet no demonstrated relationship between ring currents and resonance energies, although the qualitative connection seems soundly based.…”