The effect of pressure on the infrared spectra of the formates of the alkali and alkaline earth metals

Abstract

The infrared spectra of the solid metal formates HCO₂Li, HCO₂Li,H₂O, HCO₂Na, DCO₂Na, H¹³CO₂Na, HCO₂K, DCO₂K, HCO₂Rb, DCO₂Rb, HCO₂Cs, DCO₂Cs, (HCO₂)₂Ca, (HCO₂)₂Sr, (HCO₂)₂Ba and (HCO₂)₂Pb have been measured in the pressure range 0-42 kbar at 25°C. For the sodium salt two new modifications formed at high pressure have been observed, but the potassium, rubidium and caesium salts show only one phase transition. A phase change, though only at pressures far above 42 kbar, is indicated also for the calcium, the barium and probably the anhydrous lithium salt. The various spectral responses to changes in pressure strongly indicate that in the form prevailing at ordinary pressure and temperature, phase I, the formates of sodium and potassium have the same type of molecular packing in the crystal structure. Phases II of the sodium and potassium salt appear to belong to a new modification (?C?), whereas phase III of the sodium and phase II of the caesium salt belong to the modification ?B? previously observed in disc spectra. In a given phase, the effect of pressure on the formate vibration frequencies is sometimes quite different for the pure crystalline salt in bulk and for the same species when isolated by isotopic dilution. In these cases the main effect of pressure is that on the intermolecular coupling of vibrations. Several frequency changes accompanying phase transitions, by contrast, are essentially unchanged on isotopic dilution and arise mainly from changes in the effective intramolecular force constants.