The functional M05-2X together with the SMD solvent model have been used to calculate hydrogen bonding association constants in dimethylsulfoxide (DMSO) solution. Data of equilibrium constants in DMSO for the case of electrochemically generated dianions interacting with methanol have been considered to test the reliability of the chemistry theoretical approach. From this approach, it was found that the successive association constants involved in the formation of the complexes depend on a linear combination of three quantum chemistry indexes which are the ionization energy, the electron affinity, and the charge on the oxygen atom receiving the methanol molecule. Under this perspective, the stoichiometry of all the dianion-methanol complexes was explained on the basis of the relative strength of the hydrogen bonding interaction compared to that of the methanol-DMSO and methanol dimer complexes. This linear combination seems to be valid regardless of the nature of the dianion structure and the number of methanol molecules in the complex, which is a relevant finding to generalize the applicability of both the functional M05-2X and the SMD solvent model, to calculate association constants for any other neutral or anionic molecules interacting by hydrogen bonding with proton donors.