INFRARED STUDY OF NU(OD) MODES IN ISOTOPICALLY DILUTE (HDO) KIESERITE-TYPE COMPOUNDS MXO4.H2O (M=MN, CO, NI, ZN, AND X=S, SE) WITH MATRIX-ISOLATED M'(2-4(2-) GUEST IONS() AND X'O)

The hydrogen bond strength in kieserite-type sulfate and selenate mono hydrates has been studied by the method of double-matrix spectroscopy. The infrared spectra of isotopically dilute (matrix-isolated HDO mole cules) kieserite-type compounds MXO4H2O (M = Mn, Co, Ni, Zn, and X = S , Se) with matrix-isolated X'O-4(2-) and M'(2+) guest ions are present ed and discussed in the region of the OD stretching modes. The OD freq uencies indicate that the compounds under investigation form comparati vely strong hydrogen bonds. The frequency shifts of the uncoupled OD s tretching modes of the HDO molecules within the isostructural series a nd those influenced by the guest ions, and the strength of the hydroge n bonds formed, are discussed in terms of the respective O-w...O dista nces, which hint at stronger hydrogen bonds for the sulfate series tha n for the selenate one by mistake, the larger hydrogen bond acceptor c apability of SeO42- ions compared to SO42- ones, the different metal-w ater interactions and repulsion potentials of the lattice, and the reo rientation of the water molecules caused by the guest ions. The shifts of the OD stretches of the OD ... OSe'O-3 bonds (Se'O-4(2-) matrix is olated) to the lower wavenumbers as compared to the parent selenates a re caused by the reorientation of the hydrate water molecules and stre ngthening the hydrogen bond to the stronger acceptor and vice versa. W hen smaller metal ions having smaller M-OH2 bond lengths and, hence, s tronger synergetic effect replace larger ones, the OD stretches are sh ifted to lower wavenumbers as compared to those due to the host M-O-w. .. O bonds and vice versa. (C) 1998 Elsevier Science B.V. All rights r eserved.