An ab initio Study of the Vibrational Properties of Alkaline-Earth Metal Nitrates and Their Crystallohydrates

The vibrational frequencies and corresponding intensities have been calculated ab initio for the center of the Brillouin zone of crystalline magnesium, calcium, strontium, and barium nitrates; magnesium nitrate hexahydrate; and calcium nitrate tetrahydrate. The calculation has been performed within the electron- density functional theory using the PBE exchange-correlation functional in the basis of localized atomic orbitals with the aid of the CRYSTAL14 software. The calculated values and the experimental IR and Raman spectral data on strontium and barium nitrates are shown to be in satisfactory agreement. The frequencies of normal long-wavelength vibrations in the nitrates become red-shifted with an increase in the cation atomic mass. The occurrence of several peaks due to the vibrations of hydrogen atoms in water molecules with different dynamic charges is predicted in the IR spectrum of hexahydrate magnesium in the frequency range above 3430 cm(-1).