Comparison of the intramolecular hydrogen bond strength in F, Cl, Br, NO2, and OMe substitutions of naphthazarin

The hydrogen bond strength, geometry optimization and 1H NMR for naphthazarin (NZ) and its F, Cl, Br, NO2 and OMe substitutions have been calculated at the B3LYP/6-31G** theoretical level. The calculated chemical shifts of the chelated proton for all molecules, using GIAO method, are well correlate with the calculated geometrical parameters results. According to the calculated results, in general, substitutions of Cl and Br near the hydroxyl groups and near the carbonyl groups increases and decreases the strength of the hydrogen bond, respectively. The hydrogen bond strength of NZ decreased by substitution of F and NO2 groups in the different positions of NZ and increased by substitution OMe groups. By natural bond orbital (NBO) method, the effect of substitution on the hydrogen bond strength, the charge distributions, steric effects, and electron delocalization in the studied compounds were investigated. The natural bond orbital analysis very well explains the calculated results of substitutions on the hydrogen bond strength.