Investigation of nuclear quantum effect on the hydrogen bonds of ammonium dihydrogen phosphate using single-crystal neutron diffraction and theoretical modelling

Abstract

Potassium dihydrogen phosphate (KDP) family of hydrogen-bonded crystal constitutes an important family of crystals not only because of their immense importance in the field of nonlinear optics (NLOs) but also due to the fact that hydrogen bonds of these crystals provide us with a rare opportunity to understand the fundamental nature of hydrogen bonds, such as the effect of local chemical environment on the strength of hydrogen bonds and nuclear quantum effect on strong, moderate and weak hydrogen bonds. Keeping this aim in mind, we have undertaken detailed single-crystal neutron diffraction (SCND) investigations on ammonium dihydrogen phosphate (ADP) and KDP along with their deuterated analogue crystals under ambient conditions. Fine differences in the hydrogen bonds of the above-mentioned crystals are analysed in the light of a simple diabatic two-state theoretical model for hydrogen bonds. It is proposed that the presence of a partially covalent N–H $$_\mathrm{N}$$ –O bond in ADP has a very significant effect on its O–H bond making it highly anharmonic. It is this higher bond anharmonicity in ADP that is most likely responsible for its larger NLO coefficient compared to KDP.