Hydrogen bonding in aggregates of dialkyl-substituted diphosphonic acids and monofunctional analogues

Hydrogen bonding of dialkyl-substituted diphosphonic acids in nonpolar (toluene and CCl4) and alcohol (I-decanol) solutions is examined. The compounds are monomeric in l-decanol and dimeric or higher in nonpolar organic diluents. Infrared spectroscopy and molecular mechanics calculations suggest that the dimers of P,P'-di(2-ethylhexyl) methanediphosphonic acid (H2DEH[MDP]) and its straight-chain isomer, P,P'-dioctyl methanediphosphonic acid (H2DO[MDP]), adopt rigid highly hydrogen-bonded structures such as C or D. The homologous P,P'-di(2-ethylhexyl) ethane- and butanediphosphonic acids, H2DEH[EDP] and H2DEH[BuDP], respectively, adopt structures that are also intermolecularly hydrogen-bonded but more flexible. The effect on the P=O stretching vibration of increasing l-decanol concentration in the solvent differs for these compounds. In the case of H2DEH[MDP] and H2DO[MDP],;he frequency remains constant until all CCl4 has been replaced by the alcohol, then the P=O stretching frequency shifts to a lower energy. In the case of H2DEH[EDP] and H2DEH[BuDP], a gradual shift to higher energy occurs as the alcohol concentration increases. The magnitude of the difference in the P=O stretching frequencies between CCl4 and I-decanol solution is greatest for organophosphonic acids, less for organodiphosphonic acids, and least for organophosphoric acids.