Abstarct: In this research, molecular structure, vibrational frequencies and the chemical shifts of 1HNMR of 2-Hydroxy-5-Nitrobenzaldehyde (2H5N) and 2-Hydroxypropiophenone (OHPP) were investigated by means of density functional theory (DFT) calculations and then the results were compared with the calculated parameters of 2- hydroxybenzaldehyde (OHBA). The effect of nitro and ethyl groups on the intramulecular hydrogen bonding strength studied by the natural bond orbital (NBO) analysis. The ratio of vibrational frequencies OH/OD and OH/OD for 2H5N were calculated at 1.372 and 1.363, and for OHPP at 1¬.371 and 1.360, respectively, which were consistent with the calculated geometry and proton chemical shift results. The results of NBO analysis indicated that the electron-withdrawing effect of nitro group decreased the hydrogen bond strength, but the resonance increased the strength of this bond. In OHPP, the strength of the bond was increased because of the electron-donating effect of ethyl group along with the steric effects respect to OHBA.