Intramolecular hydrogen bonding (IHB) interactions and molecular structures of 2-nitrosophenol, nitrosonaphthols, and their quinone-monooxime tautomers were investigated at ab initio and density functional theory (DFT) levels. The geometry optimization of the structures studied was performed without any geometrical restrictions. Possible conformations with different types of the IHB of the tautomers were considered to understand the nature of the HB among these conformers. The effect of solvent on hydrogen bond energies, conformational equilibria, and tautomerism in aqueous solution were studied. Natural bond orbital analysis was performed to study the IHB in the gaseous phase and in aqueous medium. The NMR H-1, C-13, N-15, and O-17 chemical shifts in the gaseous phase and in solution for the studied compounds were calculated using the gauge-including atomic orbitals approach implemented in the Gaussian 03 program package. The optimized geometrical parameters and H-1 NMR chemical shifts are in good agreement with previous theoretical and experimental data.