The structure of the free MnF3 molecule - A 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-2v 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-2v symmetry structure there are two longer and one somewhat shorter Mn-F bonds, with two bond angles close to 106 degrees and one bond angle of about 148 degrees. The second lowest energy state was of B-5(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-3h molecular symmetry has appreciably higher energy than the B-5(2) symmetry structure. The experimental data are in complete agreement with the results of the computations in giving the best agreement with a C-2v structure characterized by r(g)(Mn-F) = 1.728 +/- 0.014 Angstrom (once), r(g)(Mn-F) = 1.754 +/- 0.008 Angstrom (twice), angle(alpha)(F-Mn-F) = 106.4 +/- 0.9 degrees (twice), and angle(alpha)(F-Mn-F) = 143.3 +/- 2.0 degrees (once). Our computed infrared wavenumbers and intensities make an assignment possible for published vibrational spectra of MnF3.