We conduct quantum studies of adsorption of diazine heterocycles on graphene to discuss experimental thermodynamics of gas-phase adsorption of pyridazine, pyrimidine and pyrazine on graphitized thermal carbon black, as reported previously. Using Born-Oppenheimer molecular dynamics and density functional studies, we characterize structural and electronic tendencies of the heterocycles on graphene. The theoretical studies predict the adsorption of pyridazine, pyrazine and pyrimidine to cause electronic perturbations of dipole, quadrupole and mixed spatial characters, respectively, resulting in a red shift of the electronic components of the heterocycles to modulate graphene electronics upon admixing of diazine orbital components with the πz states of the substrate. Investigating the thermodynamics of adsorption further involves calculating Henry's constant with the expression of the uniform surface limit: using experimental data, we estimate binding energies and force derivatives with respect to the surface normal. The extracted association energies agree with the results of Lennard-Jones potential calculations. Together, the reported pyridazine anomalous retention required the association force constant to be lower compared with values for the other diazines. Exploring energies of intermolecular relations, we ascribe the pyridazine anomalous retention to possibility of the formation of pyridazine dimers: when on the surface, only for pyridazine, the computed benefit of pairing is larger than the energy of molecular association with graphene.