Theoretical and spectroscopic (IR and Raman) study of different 7-hydroxy-4-methylcoumarin (mendiaxon) systems (mend - , mendNa, mendCu, mendH and mendH.2H 2 O) were performed at B3LYP/6-31G(d) and B3LYP/6-31++G(d,p) levels of theory. The geometric and electronic structures as well as the vibrational behavior of the systems studied were discussed: (1) as to the changes that occurred in the anion coumarin ring upon the mend - -X + (X + =Na + , Cu + , H + ) interactions and (2) as to the changes that occurred in mendH due to hydrogen bondings in mendH.2H 2 O. The largest bond length changes in the anion coumarin ring were obtained for mendH and the smallest ones for mendNa. The bond length changes were mainly produced from the electrostatic effect of the positive charge of X. The induced polarization of the C O bond upon the mend - -X + interactions was found to be opposite to the basic one and it led to shorter C O bond lengths (higher ν(C O) frequencies) in the order: mendNa, mendCu and mendH. Conversely, upon the hydrogen bonding the induced polarization of the C O bond was found in the same direction as the basic one and it produced elongation to the C O bond length (lower ν(C O) frequency). On the basis of the correlations found, the ν(C O) positions in mendNa, mendCu, mendH and mendH.2H 2 O were explained.