(3M4P) and proton donors [1-4]. Experimental frequency shifts of (C=O), (C=O), (C=O) and (N 1 =C) modes of 3-methyl-4-pyrimidone, which are sensitive to Hbonding, the values of the H-bond interaction energy (ΔE c) and the internuclear and intermolecular distances computed at the DFT(B3LYP)/6-31+G(d) level of theory have been shown that H-bonds between weak acids such as methanol are formed preferentially at the carbonyl group, whereas for stronger acids such as hydrochloric acid the complexation occurs on the N 1 atom of the ring. Theoretical calculations were carried out using the DFT/B3LYP/6-31+G(d) methodology in an attempt to predict the preferred interaction site of a polyfunctional heterocyclic base 3-methyl-4pyrimidone molecule with a series of proton donors of different acidic strength, i.e. water, methanol, phenol, 1-naphtol, 2,4,5 trichlorophenol, pentachlorophenol, picric acid and hydrogen chlordide. Computed H-bond interaction energies (∆E c), internuclear and intermolecular distances r(O … H) and r(O … O), infrared frequency shifts (C=O) and ((OH) are proved to be reliable parameters for predicting the preferred interaction site of 3-methyl-4-pyrimidone. These computational data suggest that the O-H … O=C complex is preferred with water, methanol, phenol, 1-naphtol, 2,4,5 trichlorophenol and pentachlorophenol. However, for H-bonding with stronger acids such as picric acid or hydrochloric acid, the computational data suggest that the H-bonding occurs at the N 1 ring atom of 3-methyl-4-pyrimidone. In the O-H … O=C complex, where the H-bond at the carbonyl O-atom can be oriented "anti" (H a) and "syn" (H b) with respect to the N 3 atom, the same computational data suggest a higher stability of the "anti-O" compared to the "syn-O" orientation. In spite of these exciting results, no other, detailed studies on reliable theoretical parameters on the preferred interaction site of the polyfunctional heterocyclic base 3M4P are available in the literature. In order to identify these parameters, it is necessary to widen the range of proton donors complexing 3M4P characterized by different acidic strengths. 1-Naphthol is used here because of its rotameric properties and its possibility to form a complex with a stacking structure supported by Hbonding. Ortho-substituted chlorophenols exhibit relatively strong intramolecular OH … Cl H-bonds, apart from their stacking ability. Therefore, in this case, there is a competition between intramolecular and intermolecular H-bonds in the 3M4P complex that can reduce the stability of the complex. In both cases the prediction of the preferred interaction site due to only the acidity of the proton donor can be modified.