GIAO-HF/DFT calculation of C-13 and N-15 chemical shifts for studying tautomerism and intramolecular hydrogen bonding in 2,3-disubstituted quinoxalines

Both the C-13 and N-15 chemical shifts of a number of quinoxalines substitu ted in position 2 with the pi-electron excess 2'-benzo[b]furanyl substituen t which has in position 3' a hydroxy or amino group could be satisfactorily calculated by the GIAO method on the basis of HF and DFT ab initio structu res. Thereby both the presence of the intramolecular hydrogen bond -C2=N .. . H-O-C3'- and -C2=N ... H-NH-C3'-, respectively, strongly dominating the c ommon plane of resonance of the two heterocyclic moieties, and the preferre d enol tautomer could be studied in detail. Copyright (C) 2000 John Wiley & Sons, Ltd.