Infrared Spectra of Matrix‐Isolated ClF3, BrF3, and BrF5; Fluorine Exchange Mechanism of Liquid ClF3, BrF3, and SF4

The infrared spectra of matrix‐isolated ClF3, BrF3, and BrF5 have been investigated in the region from 800 to 33 cm−1. All six fundamentals have been observed in Ar and N2 matrices for ClF3 and BrF3, which are planar T‐shaped molecules, and most of them in a Ne matrix. The two lowest‐frequency fundamentals, which could not be separated in the gas‐phase spectra [H. Selig, H. H. Claassen, and J. H. Holloway, J. Chem. Phys. 52, 3517 (1970)] are clearly visible in the matrix‐isolation spectra. Both molecules show only small gas‐matrix frequency shifts, and multiplet structure for both molecules suspended in Ar matrices may be understood in terms of isotope and matrix site effects. Several absorption bands found at low matrix‐isolation ratios are attributed to dimers and a tentative partial vibrational assignment is presented. Similar dimer structures are assumed for both substances. The structure seems best represented with two F bridges formed using the long‐bond F atoms. The structure is favored by the x‐ray and NMR data. Re‐evaluation of the spectral data for SF4, and comparison with ClF3 suggests reversing the literature assignments of the antisymmetric axial and equatorial S–F stretching vibrations for the SF4 monomer. Then consistent experimental agreement between the closely related ClF3 and SF4 molecules ensues and analogous structures for the matrix‐isolated dimers follow. Considered as transient liquid‐state species, the dimers demonstrate a highly likely fluorine exchange mechanism involving an intermolecular transfer of the bridging F atoms that can account for the exchange of nonequivalent F atoms which is observed in the liquid state for these systems. The vibrational frequencies of ClF3, SF4, BrF3, and BrF5 are compared and force constants are calculated for ClF3 and BrF3.