On symmetrical O-H-O hydrogen bonds

Several hydrogen bonds with O-H-O bond lengths of about 2.4 A are now known. Some of these have been examined well enough by neutron diffraction and other means to know with reasonable certainty that they are symmetrical hydrogen bonds, with either a single or a symmetrical double distribution peak. Even neutron diffraction results remain ambiguous as to whether the distribution has a single or double maximum. Several investigators have reported crystallographically symmetrical O-H-O hydrogen bonds with distances of about 2.5 — 2.6 A. Thermal amplitudes of some of these have been given careful study, and most of these longer bonds appear to be truly, not statistically, symmetrical. It seems likely that the bonds of about 2.4 A are distributions with single maxima, while the longer, apparently symmetrical hydrogen bonds are cases of double maxima. IR and NMR evidence that there are O-H-O bonds as long as 2.6 A with single maxima should be carefully checked before they are accepted. From some theoretical calculation of hydrogen distributions in double and single wells it can be shown that cases with double maxima have a strong isotope effect with a shorter O-H-O distance than O-D-O distance, while cases with single maxima should show almost no isotope effect. With present accuracy in structure determinations a distinction between the two cases seems possible with or without the use of neutron diffraction. As an example that bond distances reliable to ~ 0.01 A are now obtainable in nearly routine fashion, the structure of 3, 4-furandicarboxylic acid was determined to about this accuracy with data obtained in ten days.