Crystal Growth of Urea and Its Modulation by Additives as Analyzed by All-Atom MD Simulation and Solution Theory.

The crystal growth of urea was analyzed with all-atom molecular dynamics (MD) simulation for the (001) and (110) faces in contact with aqueous solutions. The local environment of a crystallizing molecule was treated in terms of the numbers of crystalline neighbors and the orientation relative to the crystal surface, and the molecular-level inhomogeneity of a growing surface was addressed by decomposing the overall rate of growth into a sum of the contributions conditioned by the local structure and orientation mode. The contrast of the growth mechanism between the (001) and (110) faces was then evidenced by the local contributions, and the roles of the outer layers of the crystal toward the liquid region were pointed out for (001). The effect of the additive species in the liquid on the crystal growth of urea was investigated with biuret, N,N-dimethylformamide (DMF), and acetone. The growth was observed to be suppressed more strongly in the order of biuret > DMF > acetone, and it was found that the ordering of suppression by the additive is common irrespective of the local environment of a crystallizing urea. This finding implies that the additive's effect on the crystal growth can be predicted by treating the flat surface, which is a convenient system for detailed analyses at atomic resolution. The correspondence to the free energy of adsorption of the additive was then examined for the additive-induced modulation of the growth rate. It was seen that the adsorption free energy correlates to the extent of modulation of the growth rate, and the interaction components that govern the adsorption propensity were identified.

Correction: Construction of isostructural hydrogen-bonded organic frameworks: limitations and possibilities of pore expansion.

[This corrects the article DOI: 10.1039/D1SC02690A.].

Construction of isostructural hydrogen-bonded organic frameworks: limitations and possibilities of pore expansion.

The library of isostructural porous frameworks enables a systematic survey to optimize the structure and functionality of porous materials. In contrary to metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), a handful of isostructural frameworks have been reported for hydrogen-bonded organic frameworks (HOFs) due to the weakness of the bonds. Herein, we provide a rule-of-thumb to develop isostructural HOFs, where we demonstrate the construction of the third and fourth generation of isostructural HAT-based HOFs (TolHAT-1 and ThiaHAT-1) by considering three important structural factors, that are (1) directional H-bonding, (2) shape-fitted docking of the HAT core, and (3) modulation of peripheral moieties. Their structural and photo-physical properties including HCl vapor detection are presented. Moreover, TolHAT-1, ThiaHAT-1, and other isostructural HOFs (CPHAT-1 and CBPHAT-1) were thoroughly compared from the viewpoints of structures and properties. Importantly, molecular dynamics (MD) simulation proves to be rationally capable of evaluating the stability of isostructural HOFs. These results can accelerate the development of various isostructural molecular porous materials.