Difference between 2JC2H3 and 2JC3H2 spin–spin couplings in heterocyclic five‐ and six‐membered rings as a probe for studying σ‐ring currents: a quantum chemical analysis

Adequate analyses of canonical molecular orbitals (CMOs) can provide rather detailed information on the importance of different σ -Fermi contact (FC) coupling pathways (FC term transmitted through the σ -skeleton). Knowledge of the spatial distribution of CMOs is obtained by expanding them in terms of natural bond orbitals (NBOs). Their relative importance for transmitting the σ -FC contribution to a given spin-spin coupling constants (SSCCs) is estimated by resorting to the expression of the FC term given by the polarisation propagator formalism. In this way, it is possible to classify the effects affecting such couplings in two different ways: delocalisation interactions taking place in the neighbourhood of the coupling nuclei and ‘round the ring’ effects. The latter, associated with σ -ring currents, are observed to yield significant differences between the FC terms of 2 JC2H3 and 2 JC3H2 SSCCs which, consequently, are taken as probes to gauge the differences in σ -ring currents for the five-membered rings (furan, thiophene, selenophene and pyrrol) and also for the six-membered rings (benzene, pyridine, protonated pyridine andN-oxide pyridine) used in the present study. Copyright c � 2010 John Wiley & Sons, Ltd.