Knowledge Bank

The infrared spectra of matrix-isolated $CIF3$ and $BrF3$ have been investigated in the region from $800cm-1$ to $33cm-1$. All six fundamentals have been observed in Ar and $N2$ matrices for both molecules, which are of similar planar T-shape, and most of them in a Ne matrix. The two lowest frequency fundamentals, which could not be separated in the gas phase $spectra1$ were clearly visible in the matrix-isolation spectra. Both molecules show only small gas-matrix frequency shifts. Multiplet structure for both molecules suspended in Ar matrices may be understood in terms of isotope and matrix site effects. Force constants were calculated for $CIF3$ and $BrF3$ monomers. Several absorption bands found at lower matrix isolation ratios are attributed to dimers and a tentative vibrational assignment is presented. A similar dimer structure is assumed for both substances. It seems best represented by associations through the long $X-F$ bonds: a structure also favored by the x-ray and NMR data. Comparison of the overall data for $CIF3$ with that for $SF1$ suggests reassignment of the axial and equatorial $S-F$ stretching vibrations for $SF1$ monomer. In this way, consistent experimental agreement between these closely related molecules is achieved and a highly likely intermolecular mechanism is favored for the exchange of nonequivalent F atoms that is observed in the liquid by NMR spectroscopy.