Spatial magnetic properties subject to lone pair and π electron delocalization in benzenoid and quinoid structures. Are quinoid tautomers really nonaromatic?

Abstract

The spatial magnetic properties, through-space NMR shieldings (TSNMRS), of benzenoid and quinoid tautomeric structures such as benzodifurantrione and phenazine-type molecules have been calculated using the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept of Paul von Ragué Schleyer and visualized as iso-chemicalshielding surfaces (ICSS) of various size and direction. The TSNMRS values were employed to quantify and visualize the partial aromaticity of the studied compounds. In the case of the surprisingly more stable quinoid tautomers, the aromaticity-synonymous with stability due to the conjugation of π electrons and lone pairs-was not found to be particularly reduced.

Synthesis, Photochemical Properties, and Cytotoxicities of 2H-Naphtho[1,2-b]pyran and Its Photodimers

“…On the other hand, the location of the proton at H-9 in the lowest energy confirmation of 8b is estimated to be near the top of the carbon at C-1a′. Because the distance between H-9 and C-1a′ is estimated to be 4.68 Å, the proton at H-9 will be located in the shielding region of space inside the aromatic dialkoxynapthalene ring (rings A′ and B′) . Thus, the upfield shift of H-9 in 8b can be explained by through-space shielding effects due to the aromatic dialkoxynapthalene ring …”

25 years of NICS - much more than nothing!

“…of Klod and Kleinpeter, [11][12][13] they used NICS points around interesting molecules and structural motives to plot the "iso-chemical-shielding surfaces" (ICSS) investigating through space NMR shieldings and making them visible. 14 It is worth mentioning that an analogous one-dimensional approach, also known as the NICSscan method, is widely addopted. 15 More straightforward was the concept of separating π-and σ-influences of the NICS values.…”