Spectroscopic properties of SO₄ (and OH) in different molecular and crystalline environments. I. Infrared spectra of Cu₄(OH)₆SO₄, Cu₄(OH)₄OSO₄, and Cu₃(OH)₄SO₄

Abstract

The infrared spectra over the range 4000–50 cm −1 of three copper basic sulfates, viz. Cu 4 (OH) 6 SO 4 , Cu 4 (OH) 4 OSO 4 , Cu 3 (OH) 4 SO 4 along with site group and factor group analysis for Cu 4 (OH) 6 SO 4 are reported for the first time. The observed absorption frequencies associated with OH and SO 4 activities show distinct individual structural features.Three distinct types of OH groups are present in Cu 4 (OH) 6 SO 4 and Cu 4 (OH) 4 OSO 4 but only two types are evident in Cu 3 (OH) 4 SO 4 . The two OH groups involved in the dehydroxylation reaction to give the oxycompound Cu 4 (OH) 4 OSO 4 are tentatively identified. The multiple frequencies involving Cu—O—H bending modes further reflect the varying degrees of H-bonding and OH coupling in Cu(OH) 4 O 2 and Cu(OH) 5 O entities.The SO 4 internal optic modes' multiplet structure and frequencies in the basic sulfates illustrate the different extent of dipolar coupling effects (OH and Cu) with respect to specific modes relative to Li 2 SO 4 with same site symmetry. The SO 4 external optic modes relative to Li 2 SO 4 are barely affected by neighboring OH groups.The Cu—O mode frequencies show parallel behavior with minor interaction effects in all compounds.