Potassium hydrogen malonate

If a hydrogen bond is found by neutron diffraction to be centred, then we can reasonably conclude that it is indeed strong and the proton is above the central energy barrier. The longest OHO bond for which a centred proton has been observed appears to be potassium hydrogen malonate with 0 = 246.8 pm, which has the proton located exactly midway between the oxygens (Schuster, 1976). It thus seems likely that in other OHO bonds shorter than this, the proton will be above the energy barrier, yet it can still be unsymmetrically placed, as some of the compounds in Table 2 with shorter / o o values show. 

Recently, solid state nmr spectroscopy has been used to probe hydrogen-bonded materials and first reports showed that there was a direct relationship between R0...0 and cr, the anisotropic chemical shift as bonds become shorter the chemical shift moves downfield. Twenty-four compounds were examined and some, such as potassium hydrogen malonate, had a o-value below 20 ppm (Berglund and Vaughan 1980). 

Currie M, Speakman JC (1970) The crystal structures of the acid salts of some dibasic acids. Part III. Potassium hydrogen malonate a neutron diffraction study. J Chem Soc A 1923-1926... 

Isotropic H chemical shifts for weakly hydrogen bonded hydrates have recently been compared [81] with previous data on carboxylic acids with O-H 0 hydrogen bonds of strong and medium strengths. The values of SH for the hydrogen bonded protons in this work varied from 4.8 ppm in NaCl04-H20 to 20.5 ppm in potassium hydrogen malonate. 

T. Brun, J. Howard J. Tomkinson (1986). Spectrochim. Acta, 42A, 1209-1216. A single-crystal and polycrystalline study of potassium hydrogen malonate using inelastic neutron-scattering. 

Amongst the acid salts of the simpler dicarboxylic acids, at any rate, we find much more frequently structures corresponding to Type A. For instance, in potassium hydrogen malonate the two carboxyl groups of the mono-ionized anion are crystallographically equivalent. Such structures we classify as Type A j. 

Table 11. Some dimensions in potassium hydrogen malonate (X—X-ray, N = neutron diffraction results)...

Fig. 12. Crystal structure of potassium hydrogen malonate (a) e-axis projection (b) 6-axis projection, with inset showing relationship between habit and structure...

The infrared spectrum (Fig. 20 (a)) of potassium hydrogen malonate is anomalous . 

Figs. 19 and 20 show the infrared spectra of some acid salts and related crystalline compounds. The spectrum in Fig. 19(a) is from an acid salt of Type B, and it approximates to a superposition of the spectra of free acid and neutral salt. The other spectra are all of Hadzi s T5q>e (ii), which is shown in its starkest form in Fig. 19(c) (sodium hydrogen diacetate) with a window near 950 cm h Potassium hydrogen malonate, a Type A structure, has a similar spectrum (20 (a)). The picoline-N-oxide hemi-hydrobromide (19(d)) is a T5q>e A basic salt Cook s basic salt (Sect. XI) is of pseudo-Type A and gives the spectrum (20(b)). Sodium bicarbonate (20(c)) and potassium hydrogen oxalate (20(d)) are acid sdts of intermediate character (see Sect. XV and XVI A). 

Fig. 20 Further infrared spectra (a) potassium hydrogen malonate (b) pseudo-Type A basic salt described in section XI (c) sodium bicarbonate (d) potassium hydrogen oxalate...

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