1H, 13C and 15N NMR study and molecular modelling of 2,3-di-substituted quinoxalines with sterically hindered aromatic and heteroaromatic substituents

Isolation, X-ray structure and properties of an unusual pentacarbonyl(2,2′-pyridyl-quinoxaline) tungsten complex

“…Unfortunately, N 1 and N 4 cannot be assigned unequivocally, as the longrange connectivity of both N 1 and N 4 with H 3 and correspondingly with H 8 and H 5 are not particularly enlightening, because H 8 and H 5 are NMR equivalent. Nevertheless, it has been reported that the chemical shift of N 4 is relatively insensitive to the different heteroaromatic substituents at the 2-position of the quinoxaline ring [20], while the 15 N chemical shift of N 1 is found strongly dependent on these substituents. On this basis, we assign N 4 at 331 ppm, as this chemical shift is closer to corresponding ones of other quinoxaline derivatives.…”

Pentacarbonyl[2‐(2′‐pyridyl)quinoxaline‐κN4]tungsten: A Combined Study of Its Conformational and Electronic Structure Based on Experimental and DFT‐TDDFT Data

“…This is illustrated by the positions of the 15 N NMR chemical shifts that are characteristic of sp 2 hybridized nitrogen atoms. [19] [20] while the 15 N NMR chemical shift of N 1 is found to be strongly dependent on these substituents. On this basis, the most downfield shifted nitrogen atom is N 4 (δ = 331 ppm), followed by N 1 (δ = 319 ppm) and N 1Ј (δ = 311 ppm), this is in accordance with the semiempirically calculated partial charges on the nitrogen atom of pq.…”

GIAO-HF/DFT calculation of13C and15N chemical shifts for studying tautomerism and intramolecular hydrogen bonding in 2,3-disubstituted quinoxalines

“…The tautomeric equilibria A C and B C which were expected to be slow on the NMR time-scale have been studied already by examining the chemical shift of C-2', which should be strongly shifted to high field due to sp 3 hybridization in C; thus C was excluded from the tautomeric equilibrium present. 1,13 The tautomeric equilibrium A B, however, which is expected to be fast on the NMR time-scale, cannot be examined adequately by NMR spectroscopy because there is no unequivocal proof. Only chemical shifts, e.g.…”

“…1 If additional substituents are introduced into position 3 of quinoxaline and position 3' of the annelated heterocycle, the common plane of resonance between the two ring systems will be more or less sterically hindered, limiting p-electron delocalization at the same time. The 13 C and 15 N chemical shifts of the nuclei involved readily indicate both the amount of pelectron delocalization and the dihedral angle about the C2-C2' bond.…”

“…The corresponding barrier to rotation could not be studied for several reasons: either it is too low to make the dynamic process slow on the NMR timescale or the population difference of the two rotamers is so strongly different that the minor one could not be detected. 1 However, this restricted rotation could be studied in detail by ab initio MO calculations simulating this rotational process by calculating both the dihedral angledependent global minima structures and on their basis (by the GIAO method) the corresponding 13 C and 15 N chemical shifts of the nuclei involved. 2 Both global minima structures and 13 C and 15 N chemical shifts indicate impressively the steric/electronic state of this kind of compound.…”