Coupling of molecular orientation with the hydrogen-bond dimensions and H-sites in carboxylic acids

The location of H-atoms sites in carboxylic-acid dimers has been related to the crystal environment. It has been shown that the H-sites are coupled to the orientation of carboxylic-acid groups. Consequently, the H-bonds can be strained and the H-atoms destabilized and disordered by the symmetric crystal environment of symmetric molecules. The coupling results in the O⋯O distance contraction by about 0.015 Å in carboxylic acids with disordered H-atoms, this magnitude calculated from H-bonds strain being consistent with that observed in crystal structures. This strain analysis provides an easy method of locating the H-atoms in hydrogen bonds between carboxyl groups, verifying experimental data determined by X-ray diffraction, calculating the magnitudes of O⋯O distance contraction and molecular reorientations induced by the H-atoms disordering structural effects, and predicting properties related to the H-atom dynamics. The survey of carboxylic-acid structures corroborates the effects of the molecular environment on the H-sites and hydrogen-bond dimensions.

► Interdependence between H-sites and orientation of OH···O bonded carboxyl groups and molecules.
► A new method for predicting details of molecular arrangement in crystals.
► Understanding the dynamics of disordered H-atoms in hydrogen bonds.
► Validation of crystal structures of carboxylic acids.
► Systematic differences between hydrogen bonds in carboxylic acids with H-atoms ordered and disordered.