¹³C and some ¹⁵N N.M.R. spectra of heteroaromatic molecules containing bridgehead nitrogen atoms

Abstract

The carbon-13 and (in part) nitrogen-15 chemical shifts of 1,2,3- triazaindolizine and seven three ringed analogues of indolizine have been analysed and enable deductions to be made concerning the extent that the free pair of electrons on the bridgehead nitrogen atom contributes to the delocalized electronic structures. Analogies between these molecules and indolizine are noted in that all the systems possess a high degree of aromaticity. Additivity relationships for nitrogen and methyl substitution are compared with those previously reported for other heteroaromatic molecules.

Comparison of previously published 100-MHz and the present-work 270-MHz proton n.m.r. results enabled ¹³C spectral assignments to be confirmed by selective ¹H irradiation experiments, since coupling constant analyses did not provide unambiguous assignments in all cases. One-, two-, three- and four-bond ¹³C-¹H coupling constants are reported for most carbon atoms where first-order spectral analysis was possible. Several examples of non-first-order behaviour are presented and attempts to reconcile the occurrence of this behaviour are outlined.