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 at surfaces.