Electron nuclear double resonance evidence for bifurcated hydrogen bonds in KH 2PO 4: Possible role in the ferroelectric transition

Abstract

Electron nuclear double resonance measurements on OH⋯O protons in KH 2PO 4, using SeO 3− 4 as a probe, demonstrate that the superhyperfine tensor for the “far” (O⋯H) proton is dominated by dipolar interaction with the unpaired electron in the 2 p z orbital of an oxygen to which a “close”, i.e., O-H, proton is attached. This result implies that each hydrogen is involved in a bifurcated hydrogen bond. This conclusion was supported by a reanalysis of the earlier reported ENDOR data that was obtained using the AsO 4− 4 radical as a probe in KH 2AsO 4, in particular, by the fact the isotropic part of the tensor for the “far” protons is sizable (2.8 MHz) and has same sign (negative) as that of the “close” proton. Moreover, the various O⋯H distances and bond angles for the proposed bifurcated hydrogen bonding are comparable to the literature values for these parameters for carbohydrates. These findings provide the basis for a new contribution toward the role of hydrogens in the mechanism of phase transition in the KDP-type ferroelectrics.