Fig. 2.

Distributions of (a) $\mathrm{\text{O–H}}\cdots \mathrm{\text{O}}$ and $\mathrm{\text{N–H}}\cdots \mathrm{\text{O}}$ bond-length and (b) corresponding directionality, based on analyses of crystal structures from the Cambridge Structural Database with updates by August 2018. Structures obtained from the database were applied with a bonding cutoff of 3.04 (for $\mathrm{\text{O–H}}\cdots \mathrm{\text{O}}$) and 3.07 Å (for $\mathrm{\text{N–H}}\cdots \mathrm{\text{O}}$), i.e. the sum of van der Waals radii. The bond angle cutoff is 4120°.