P═O Moiety as an Ambidextrous Hydrogen Bond Acceptor

Hydrogen bond patterns of crystals of phosphinic, phosphonic, and phosphoric acids and their cocrystals with phosphine oxides were studied using 31P NMR and single-crystal X-ray diffraction. Two main factors govern these patterns and favor or prevent the formation of cocrystals. The first one is a high proton-accepting ability of the P═O moiety in these acids. As a result, this moiety effectively competes with other proton acceptors for hydrogen bonding. For example, this moiety is a stronger proton acceptor than the C═O moiety of carboxylic acids. The second factor is the inclination of the P═O moiety of both the acids and the oxides to form two hydrogen bonds at once. The peculiarity of these bonds is that they weaken each other to a little degree only. In order to highlight this point, we are using the term “ambidextrous”. These two features should govern the interactions of P═O moiety with water and other proton donors and acceptors in molecular clusters, the active sites of enzymes, soft matter, and ...