The Structure of the Free MnF3 MoleculeA Beautiful Example of the Jahn−Teller Effect

A new high-temperature gas-phase electron diffraction analysis of manganese trifluoride, combined with high-level quantum chemical calculations, provides direct geometrical evidence of a Jahn−Teller distortion in a free molecule. The potential energy surface of the molecule was scanned extensively by computation at the SCF level. CASSCF and CASPT2 calculations established that of the many C 2 v symmetry stationary points on the potential energy surface the lowest energy ones are quintets. The global minimum is a quintet state of 5 A 1 symmetry. In this planar C 2 v symmetry structure there are two longer and one somewhat shorter Mn−F bonds, with two bond angles close to 106° and one bond angle of about 148°. The second lowest energy state was of 5 B 2 symmetry, which turned out to be a transition state with an imaginary b 2 (in plane) bending frequency. A constrained planar structure of D 3 h molecular symmetry has appreciably higher energy than the 5 B 2 symmetry structure. The experimental data are in complete agreement with the results of the computations in giving the best agreement with a C 2 v structure characterized by r g (Mn−F) = 1.728 ± 0.014 Å (once), r g (Mn−F) = 1.754 ± 0.008 Å (twice), ∠ α (F−Mn−F) = 106.4 ± 0.9° (twice), and ∠ α (F−Mn−F) = 143.3 ± 2.0° (once). Our computed infrared wavenumbers and intensities make an assignment possible for published vibrational spectra of MnF 3 .