Inter- or intramolecular N...H-O or N-H...O hydrogen bonding in 1,3-amino-alpha/beta-naphthols: an experimental NMR and computational study.

The existence of intermolecular or intramolecular N...H-O or N-H...O hydrogen bonding in three series (series 1, substituted 1-aminoalkyl-2-naphthols: R = H, Me, Et, Pr, i-Pr; series 2, substituted 1-alpha-aminobenzyl-2-naphthols: H, p-OMe, p-F, p-Cl, p-Br, p-NO2, p-Me; series 3, substituted 2-alpha-aminobenzyl-1-naphthols: R = H, p-Me, p-F, p-Br, p-OMe, m-NO(2), m-Br) are studied by NMR spectroscopy and computed at the DFT level of theory [B3LYP/6-311+G(d,p)]. The correct nature of the H-bond was assigned unequivocally both experimentally and computationally by potential energy scans rotating the involved dihedral angles. We investigated the effects of substituents on the strength of the H-bond by evaluating the corresponding hyperconjugative stabilization energy n(lonepair) --> sigma*(X-H) and Hammett substituent constant plots. By this means, steric and electronic substituent effects could be easily quantified and separated.