Polarizable acid–acid and acid–water hydrogen bonds with H3PO2, H3PO3, H3PO4, and H3AsO4

Hypophosphorous, phosphorous, phosphoric, and arsenic acids, pure and their aqueous solutions were studied by ir spectroscopy. Strong continuous absorptions observed with the pure liquid acids and with solutions containing less than one water molecule per acid molecule demonstrate that acid–acid hydrogen bonds in these systems are easily polarizable. The discussion of the PO and AsO bands as well as the discussion of the structure of the continuum demonstrates, however, that the degree of asymmetry of the energy surfaces in these hydrogen bonds is relatively large. It is shown that extended networks of such bonds exist. Due to the polarizability of these bonds, the proton motion in these bonds is strongly correlated. With addition of water, easily polarizable acid–water hydrogen bonds (I) [Formula: see text] (II) are formed. The energy surfaces in these hydrogen bonds are similar to those in the acid–acid hydrogen bonds. With further addition of water, proton boundary structure II receives noticeable weight, i.e., the degree of asymmetry of the energy surfaces in these bonds decreases due to addition of water molecules. The degree of symmetry of the energy surfaces in the acid–water hydrogen bonds increases in the series arsenic, phosphoric, phosphorous, and hypophosphorous acid.