Infrared spectra (4000-200 cm-1; KBr discs) have been obtained for ten isotopic species of solid
sodium formate in bulk (isotopic purities ranging from 80% to 99%) and after isotopic dilution.
In probably the most systematic and comprehensive study of its kind to date on a polyatomic solid,
a large number of solid solutions, such as H12C16O2Na in D13C18O2Na and vice versa, D12C16O2Na
in D13C18O2Na and vice versa, etc., have been examined (solute concentration usually in the range
from 15% to 1 %). The CH and CD out-of-plane and in-plane bending frequencies are raised
somewhat on isotopic dilution, the CH and CD stretching, OCO scissoring and lower CO stretching
frequencies are lowered a little. For the latter two vibrations the frequency of the solid solute is
affected by that of the solid solvent only very slightly (by 0.5 cm-1 to 1.0 cm-1), but in a very
regular and systematic manner. For the higher carboxylate stretching frequency, by contrast, the
effect of isotopic dilution is much more varied. The solute frequency is raised (e.g. by 3.5 cm-1
for H12C16O2Na in D13C18O2Na), or lowered (e.g. by 6.5 cm-1 for D13C18O2Na in H12C16O2Na,
18 cm-1 for H13C16O2Na in H12C16O2Na, 22 cm-1 for H13C16O2Na in D13C18O2Na), depending
on whether the intrinsic solute frequency is higher or lower than that of the solvent, and by an
amount determined by the separation between the two. Thus, for this vibration strong coupling
between solute anion and solvent anions is still conspicuous after isotopic dilution, and the 'solute'
spectra observed are those of a cluster of solute anion surrounded by solvent anions; they do not
usually approximate to the spectra of individual isolated solute ions. Some combination frequencies
show much lower anharmonicity corrections after isotopic dilution than in bulk. With isotopically
mixed crystals transfer of absorption intensity is observed, to the bands at lower frequencies for
CH and CD in- and out-of-plane bending, and to those at higher frequencies for OCO scissoring
and both CO stretching vibrations. For larger crystals and solid films, an instance was found of a
large intensity transfer to higher frequencies, additional to the (small) one seen with KBr discs.
A satisfactory explanation in terms of the crystal structure is at present available only for the out-
of-plane bending results.
The puzzle of combination frequencies found in heat-driven pulsators