HYDROGEN BONDING IN AGGREGATES OF DIALKYL-SUBSTITUTED DLPHOSPHONIC ACIDS AND MONOFUNCTIONAL ANALOGUES∗

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Abstract
Hydrogen bonding of dialkyl-substituted diphosphonic acids in nonpolar (toluene and CC14) and alcohol (1-decanol) solutions is examined. The compounds are monomeric in 1-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=0 stretching vibration of increasing 1-decanol concentration in the solvent differs for these compounds. In the case of H2DEH[MDP]and H2DO[MDP], the frequency remains constant until all CC14 has been replaced by the alcohol, then the P=0 stretching frequency shifts to a lower energy.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=0