In order to get the exact hydrogen-bonding scheme in triglycine sulphate (TGS), which is an important hydrogen bonded ferroelectric, a single crystal neutron diffraction study was undertaken. The structure was refined to an 𝑅-factor of $R[F^{2}] = 0.034$. Earlier neutron structure of TGS was reported with a very limited data set and large standard deviations. The differences between the present and the earlier reported neutron structure of TGS are discussed.

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 thehydrogen bonds of the above-mentioned crystals are analysed in the light of a simple diabatic two-state theoreticalmodel for hydrogen bonds. It is proposed that the presence of a partially covalent $\rm{N–H_{N}–O}$ bond in ADP has a very significant effect on its O–H bond making it highly anharmonic. It is this higher bond anharmonicity in ADPthat is most likely responsible for its larger NLO coefficient compared to KDP.

4-Hydroxybenzaldehyde (4-HOBAL) is an important organic compound widely used in the synthesis of various polymers and pharmaceuticals. It is crystallised in two polymorphic forms, namely Form-I and Form-II. Subtle differences in intermolecular interactions between 4-HOBAL molecules in Form-I and Form-II are analysed to understand the difference in the relative stability of the two polymorphs. Packing energy at room temperature is found to be slightly more stabilising for the stable polymorphic Form-I than for the metastable polymorphic Form-II. It is demonstrated that fine-tuning of the intermolecular interactions by changing the solvent polarity can regulate the outcome of the crystallisation procedure for 4-HOBAL. Crystallites obtained from solvents with a relative polarityof more than 0.654 have Form-I whereas those obtained from solvents with polarity less than 0.654 have Form-II. Colourisation of Form-I crystallites obtained from solutions with water–acid mixtures as solvent is explained basedon the presence of hydrated 4-HOBAL molecules within these crystallites. These modified molecules act as local defects that absorb visible photons making the crystallites coloured.