Formate anion: The physical force field

Abstract

Unequivocal assignments for all of the vibrational modes of the formate ion have been made from isotopic shifts measured in the Fourier transform infrared and Raman spectra of polycrystalline samples of the sodium salts of HCO 2 −, H 13CO 2 −, DCO 2 −, and D 13CO 2 −. Literature frequencies for DCO 18O − and DC 18O 2 − have been added to the above results for the unique determination of the 10 force constants in the general harmonic force field. Product rule relations among the harmonic frequencies are shown to interfere seriously with the determination of the physical force field by the standard normal coordinate computation. The least-squares refinement is found to be particularly sensitive to identified types of anharmonic error in the frequencies. Refinements characterized as Φ aH and Φ aD improve reliability by elimination of the deuterium isotopic shift for νCH. The physical force field is interpreted in terms of the geometric and electronic structure.