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Difference between 2 J C 2 H 3 and 2 J C 3 H 2 spin–spin couplings in heterocyclic five‐ and six‐membered rings as a probe for studying σ‐ring currents: a quantum chemical analysis
Author(s) -
Contreras Rubén H.,
dos Santos Francisco P.,
Ducati Lucas C.,
Tormena Cláudio F.
Publication year - 2010
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.2669
Subject(s) - chemistry , thiophene , ring (chemistry) , atomic orbital , pyridine , computational chemistry , protonation , aromaticity , spin (aerodynamics) , coupling constant , molecular physics , chemical physics , molecule , electron , quantum mechanics , physics , organic chemistry , ion , thermodynamics
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 J C 2 H 3 and 2 J C 3 H 2 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 and N ‐oxide pyridine) used in the present study. Copyright © 2010 John Wiley & Sons, Ltd.