Crystal structures of some acid salts of monobasic acids. Part XIX. Potassium hydrogen dicrotonate, X-ray and neutron diffraction studies

Abstract

The crystal structure of the title compound KH(C₄H₅O₂)₂, has been determined, and refined to R 0.069 for 2199 X-ray, and 0.037 for 1946 neutron, reflexions. This acid salt, which is of pseudo-Type A, contains a hydrogen dicrotonate anion, CH₃·CH:CH·CO₂⋯ H ⋯ O₂C·CH:CH·CH₃, i.e. nearly, but not quite, symmetrical. The hydrogen bond is ‘very short’[O ⋯ O 2.488(2), O ⋯ H1.141(2),1.348(2)Å]. This asymmetry and various minor differences between the crotonate residues are attributed to differences in O: ⋯ K⁺ contacts: in the crotonate with the shorter O ⋯ H, each oxygen atom makes ionic contact with a single cation, whilst in the other, each oxygen makes contact with a pair of cations. As is usual in such short OHO bonds, the relative vibrational motion of the proton appears to be a maximum along the bond direction. The geometries of the carboxy groups in this salt are compared with those of a number of Type A acid salts.

Ammonium and rubidium hydrogen dicrotonates are isomorphous with each other, but not with the potassium salt; and a note on these compounds is appended.