The molecular and crystal structures of the title compound, C19H18N2O, were determined and characterized by single crystal X-ray diffraction and spectroscopic methods. Details of the molecular geometry imply that there is a mesomeric effect between the electron-withdrawing N atoms of nitrile substituents and electron-donating O atom. Formally, single central O-C-ar bond lengths are considerably different from each other. O-C-ar distance to phthalonitrile ring is shorter than the other O-C-ar distance due to mesomeric effect under discussion. In addition to structural and spectral evidences, possible results from mesomerism of the compound were investigated by topological analysis on the electronic properties using quantum theory of atoms in molecules (QTAIM) approach. It is inferred from topological analysis that the nitrile group in para position has slightly stronger mesomeric effect than that in meta position due to diffusive double charge separation property during meta mesomerism of the compound. Mesomeric effect revealing itself by differences in delocalization indices between certain bonded atom pairs results in considerable decrease in aromaticity of phthalonitrile ring.