SHORT HYDROGEN-BONDS IN SALTS OF DICARBOXYLIC-ACIDS - STRUCTURAL CORRELATIONS FROM SOLID-STATE C-13 AND H-2 NMR-SPECTROSCOPY

Solid-state C-13 and H-2 NMR spectra are found to be very suitable for characterizing the short O-H ... O hydrogen bonds observed in acid sa lts of dicarboxylic acids. The majority of the investigated compounds are acid salts of malonic, succinic and tartaric acid with monovalent cations derived from alkali metals and small aliphatic amines. They in clude systems with symmetric and asymmetric hydrogen bonds. Accurate s tructural information about their geometry is available from low-tempe rature X-ray diffraction data. The C-13 chemical shifts of the C atoms in the different carboxy groups display a linear variation with the a bsolute difference between the two C-O bond lengths. Theoretical ab in itio calculations for model systems showed that the nuclear quadrupole coupling constant NQCC for H-2 increases with increasing asymmetry of the hydrogen-bonded system. NQCC values for H-2 in the short symmetri c hydrogen-bonded systems are in the range 53-59 kHz compared with the larger values of up to 166 kHz found in systems with longer asymmetri c hydrogen bonds. The H-2 NQCC values display a perfect linear depende nce on the asymmetry of the hydrogen bond. H-2 NQCC decreases with dec reasing temperature in the symmetric hydrogen bonds showing that the c orresponding potential has a single well.