Infrared Spectra of the dn-Borohydride Ions

Solid solutions of the d_n‐borohydride ions in potassium bromide were prepared by diffusing the borohydride ions into optical grade single crystals. Their infrared spectra at 90°K yielded frequencies for each of the 24 infrared‐active fundamental vibrations. Four fundamentals are significantly perturbed by Fermi resonance. The frequencies of the remaining 20 are reproduced to 10 cm⁻¹ or better by the symmetry force constants $F_1{\text{\hspace{0.17em}=\hspace{0.17em}2.99, F}}_2{\text{\hspace{0.17em}=\hspace{0.17em}0.316, F}}_3{\text{\hspace{0.17em}=\hspace{0.17em}2.79, F}}_4\text{\hspace{0.17em}=\hspace{0.17em}0.317,}\mathrm{and}{F}_5\text{\hspace{0.17em}=\hspace{0.17em}0.018}\mathrm{mdyn}\mathrm{/\AA }$ .