Fine tuning of hydrogen bond strength in crystals: a case study of O-H-O hydrogen bond in ammonium substituted potassium dihydrogen phosphate

Hydrogen bonds are one of the most enigmatic intermolecular interactions where the strength of interaction depends very intimately on the local environment of the bonded atoms. One can change the entire nature of the interactions by changing its local environment. Many of the enzymatic reactions work by this above-mentioned mechanism. Hence, it is important to understand how changes in the local environment can modify hydrogen bonds. In line with this, complete structural and vibrational investigations on ammonium ion substituted potassium dihydrogen phosphate mixed crystals K-0.988(NH4)(0.012)H2PO4 and (K-0.956(NH4)(0.044)H2PO4) are performed. The behaviour of O-H-O hydrogen bond in these crystals is very sensitive to changes in the local environment. A small change in the concentration of K+ ion has an observable effect on hydrogen bond geometry and hence on the double-well potential energy contour for these bonds. The dynamics of H atom depends on the potential barrier height. It changes gradually with decreasing temperature from predominantly inter-well to completely intra-well dynamics with an intermediate precursor region, where inter-well and intra-well dynamics are important.