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

Hydrogen bonding of dialkyl-substituted diphosphonic acids in nonpolar (tol uene and CCl4) and alcohol (I-decanol) solutions is examined. The compounds are monomeric in l-decanol and dimeric or higher in nonpolar organic dilue nts. Infrared spectroscopy and molecular mechanics calculations suggest tha t 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 h omologous P,P'-di(2-ethylhexyl) ethane- and butanediphosphonic acids, H2DEH [EDP] and H2DEH[BuDP], respectively, adopt structures that are also intermo lecularly hydrogen-bonded but more flexible. The effect on the P=O stretchi ng vibration of increasing l-decanol concentration in the solvent differs f or these compounds. In the case of H2DEH[MDP] and H2DO[MDP],;he frequency r emains constant until all CCl4 has been replaced by the alcohol, then the P =O stretching frequency shifts to a lower energy.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 co ncentration increases. The magnitude of the difference in the P=O stretchin g frequencies between CCl4 and I-decanol solution is greatest for organopho sphonic acids, less for organodiphosphonic acids, and least for organophosp horic acids.