Low-Frequency Motions of H2O Molecules in Crystals. II

Abstract

Translational and librational motions of H₂O molecules in a frequency region lying below 1000 cm⁻¹ have been previously examined for various configurations using the inelastic scattering of low‐energy neutrons (0.005 eV). In the present work, H₂O motions are studied in cases where neither of the lone electron pairs on the oxygen of the H₂O molecule is specifically directed (K₂C₂O₄·H₂O), where the lone pairs are directed toward two monovalent metal ions (KF·2 H₂O), where the H₂O molecule has a single orientation [Ba(ClO₃)₂·H₂O] or two distinct, equally probable orientations (Li₂SO₄·H₂O). The neutron spectrum of K₂C₂O₄·H₂O typically exhibits torsional peaks at 500, 605, and 700 cm⁻¹ which shift by 1.35 in frequency upon deuteration. They are well separated from a sharp peak at 195 cm⁻¹ which is assigned to the stretching motion of an H₂O molecule H bonded to the oxalate ion and does not shift upon deuteration. The neutron spectra are discussed and compared with the results of proton magnetic resonance, Raman, and infrared measurements.