Neutron diffraction studies of potassium hydrogen diformate, KH(HCOO)2, at 120 and 295 K

Abstract

Mr= 130.144, orthorhombic, Pbca, Z = 8; at 120 K a = 17.705 (8), b = 7.349 (4), c = 7.302 (3)/k, V = 950.1 (8) ~3, D~, = 1.820 (1) g cm-3; at 295K a=17.7079(9), b=7.5096(4), c= 7.3774 (4) ,~, V= 981.05 (9) /k 3, Dx=l.7623(2)g cm -3. The crystal structure has been investigated by neutron diffraction at 120 K (2 = 0.7228/k) and 295 K (2= 1.210/~); final R(F 2) values are 0.041 (1418 reflections) and 0.065 (1365 reflections), respectively. The short intermolecular O...O hydrogen bond (2.437 (1),~ at 120 K) is found to be asymmetrical (O-H 1.167 (1), H...O 1.270 (1) A) even though the bonding situations around the two formate groups are quite similar. Introduction. The main purpose of this investigation is to study the geometry and electron density in the very short intermolecular hydrogen bond (O..-O: 2.437 (1)/k at 120 K) between the two formate groups in KH(HCOO) 2. The crystal structure was determined previously from room-temperature single-crystal X-ray data by Larsson & Nahringbauer (1968). Most of the very short hydrogen bonds investigated by diffraction methods are either of an intramolecular type or they have a symmetry restriction on the bond, which makes it difficult to distinguish between a disordered non- centred bond and a centred bond with a large proton thermal vibration. This is not the case in potassium hydrogen diformate, where the two formate ions, hydrogen-bonded to form a dimer, are crystal- lographically non-equivalent. In the present paper neutron diffraction studies at 120 and 295 K will be presented. The deformation electron density distribution at 120 K will be described in a later publication. Experimental. Crystal growth: KH(HCOO) 2 was pre- pared from formic acid and potassium hydroxide and recrystallized several times from an aqueous solution containing a small amount of formic acid. A large crystal was grown in a quartz-glass tube (Q ~3-5 mm) with the zone-melting technique. A cylinder-shaped 0108-2701/83/111507-04501.50