The Effects of Isotopic Dilution on the Infrared Spectrum of Solid Sodium Formate

Abstract

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.