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.
K+[K(CO)8]-: an antipodal crystalline salt of alkalide [K(C222)]+K-
