Structure and thermal properties associated with some hydrogen bonds in crystals. III. Further examples of the isotope effect

Abstract

The conclusion from previous experiments on the isotope effect in crystals (Robertson and Ubbelohde 1939) was that on substituting D for H in hydroxyl bonds (length 2.75-285 A) there was only a small change in the lattice spacings, whereas for the short hydrogen bonds (length ~2.55 A) there was a considerable expansion. From this conclusion it was suggested that resonance effects must contribute an appreciable proportion of the binding energy in short hydrogen bonds. One difficulty in this suggestion arose from the complicated crystal structures used, for which inferences about changes in bond lengths cannot be very direct. The aim of the present experiments was to substantiate the former results by investigating further examples of the isotope effect. In order to have a structurally simple example of the hydroxyl bond, pentaerythritol C(CH 2 OH) 4 was compared with C(CH 2 OD) 4 . The salt hydrates SrCl 2 6H 2 O and CuSO 4 5H 2 O were also compared with the corresponding deuterium compounds. For the short hydrogen bond a comparison was made between KH 2 PO 4 and KD 2 PO 4 . The (unknown) structures of NaHSO 4 and fumaric acid were also investigated. Finally, in order to obtain information on possible “hydrogen bonds” between atoms other than oxygen, urea was compared with CO(ND 2 ) 2 and KHF 2 with KDF 2 . In addition to confirming the previous conclusions, certain fresh facts have emerged about the behaviour of hydrogen bonds in crystals, as is recorded below.