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 mang anese trifluoride, combined with high-level quantum chemical calculati ons, provides direct geometrical evidence of a Jahn-Teller distortion in a free molecule. The potential energy surface of the molecule was s canned extensively by computation at the SCF level. CASSCF and CASPT2 calculations established that of the many C-2v symmetry stationary poi nts on the potential energy surface the lowest energy ones are quintet s. 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 s horter Mn-F bonds, with two bond angles close to 106 degrees and one b ond 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 ima ginary b(2) (in plane) bending frequency. A constrained planar structu re of D-3h molecular symmetry has appreciably higher energy than the B -5(2) symmetry structure. The experimental data are in complete agreem ent 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 An gstrom (once), r(g)(Mn-F) = 1.754 +/- 0.008 Angstrom (twice), angle(al pha)(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 spe ctra of MnF3.