THE EFFECT OF HYDROGEN-BONDING ON THE BOND LENGTHS AND ANGLES IN THE CARBOXYL GROUP

The effects of hydrogen bonding on the geometry of the carboxyl group have been studied systematically based on accurate X-ray crystallographic data. In general, the C-O(H) bond length increases with increasing O ⋯ O hydrogen-bond length, while the C=O bond length decreases. These variations become less pronounced for longer O ⋯ O distances. The O=C-O(H) and C-C-O(H) angles decrease with increasing O ⋯ O separation, while the C-C=O angle increases. The sum of the three angles remains close to 360 °, testifying to the planarity of the carboxyl group. These correlations are not observed to hold for cyclic hydrogen-bonded dimers, indicating that a continuous variation in the degree of disorder of the protons may be present, ranging from the ordered case with easily distinguishable C=O and C-O(H), to the 50%-50% disordered case, where the two C-O distances become equal. © 1980 Plenum Publishing Corporation.