Diffraction and Spectroscopic Studies of the Cobaltic Acid System HCoC2–DCoO2

Abstract

Cobaltic acid, HCoO₂, and deuterated cobaltic acid, DCoO₂, have been prepared and studied by x‐ray and neutron‐diffraction techniques, and by infrared spectroscopy and cold‐neutron scattering. The materials crystallize in apparent space group $D_{\text{3d}}^5\mathrm{–R<}\overline{3}m$ of the trigonal system with three molecules in a hexagonal cell of dimensions: HCoO₂, $\text{a\hspace{0.17em}=\hspace{0.17em}2.851(1), c\hspace{0.17em}=\hspace{0.17em}13.150(5) Å;}$ DCoO₂, $\text{a\hspace{0.17em}=\hspace{0.17em}2.854(1), c\hspace{0.17em}=\hspace{0.17em}13.354(5) Å}$ . The O–O distance, as determined by neutron‐powder studies, of HCoCO₂ is 2.50(2) Å. On this basis and the fact that there are three O–H–O bonds along the c direction, the O–D–O distance is 2.57(2) Å. Hence, the isotope effect here is as large as in the system HCrO₂–DCrO₂. Infrared studies at room temperature and at − 195°C over the region 70–4000 cm⁻¹ may be interpreted in terms of an effectively symmetric O–H–O bond and an effectively asymmetric O–D–O bond. Although the neutron inelastic scattering measurements are not inconsistent with this interpretation, there is a band centered at 264 cm⁻¹ in the neutron spectrum for HCoC₂, which is not observed in the infrared spectrum. Model calculations, based on a two‐dimensional potential function in which the asymmetric stretching frequency is coupled to the symmetric stretching frequency, were unsuccessful owing to the magnitude of the coupling term required to account for the very large isotope effect.