14N Nuclear Quadrupole Coupling in Heterocyclic Compounds, Part 2. A Comparison of ab initio Calculations and Experimental Results for the Azoles and Azines

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Abstract
Ab initio calculations of double zeta quality have been reported for the conjugated 5- and 6- membered ring heterocycles containing NH and NMe groups. The electric field gradients were calculated and the derived nuclear quadrupole coupling constants (NQCC) compared, both in magnitude and direction, with those obtained by microwave spectroscopy and by nuclear quadrupole resonance. A comparison of the results with those of the corresponding O- and S-heterocycles, and with acyclic compounds has been given, and it was shown that aromaticity considerations were of no relevance to the NQCC values at pyridine-like N; planarity of the N atom at N-1 in the systems 2 c-9 c is responsible for the very low magnitude of Xπ, and this is clearly associated with the level of π-donation from N-1; this is a function of position of the other ring N atoms, and appears to become saturated as the ring positions become predominantly N. In particular three contiguous N atoms seem to have less effect than two N atoms

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