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

Abstract

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 5A1 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 5B2 symmetry, which turned out to be a transition state with an imaginary b2 (in plane) bending frequency. A constrained planar structure of D 3 h molecular symmetry has appreciably higher energy than the 5B2 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 MnF3.

Jahn–Teller and spin–orbit coupling effects in transition-metal trifluorides

“…31,32 In the gas phase, transition-metal trihalides are mostly planar systems of D 3h symmetry with highspin ground states. 33,34 The structures of a few transition-metal trifluorides have been determined by electron diffraction in the gas phase. [34][35][36] Spectroscopic information on transition-metal trihalogenides is scarce and limited to vibrational spectra in rare-gas matrices.…”

“…33,34 The structures of a few transition-metal trifluorides have been determined by electron diffraction in the gas phase. [34][35][36] Spectroscopic information on transition-metal trihalogenides is scarce and limited to vibrational spectra in rare-gas matrices. [37][38][39] A systematic study of the electronic structure of the trihalides of the firstrow transition metals has been performed by Yates and Pitzer with the spin-restricted Hartree-Fock (HF) method.…”

“…The E ×e JT coupling has been discussed by Yates and Pitzer for CoF 3 , by Solomonik et al for VF 3 47 and by Hargittai et al for MnF 3 . 34 In none of these studies, SO coupling effects were included.…”

“…33 MnF 3 has a 5 E ′ electronic ground state 33 and should thus be JT distorted in the ground state. 34 CoF 3 has a 5 A 1 ′ ground state and a low-lying 5 E ′′ excited electronic state. 33 The ab initio studies for MnF 3 and CoF 3 predicted strong JT activity of the degenerate bending mode, while the JT activity of the degenerate stretching mode is weak.…”

“…The JT stabilization energy (E JT ) of the 5 E ′ state in the quadratic approximation is 2250 cm −1 , which is in qualitative agreement with previous results. 34,87 For MnF 3 , we consider the fluorescence emission spectrum from a nondegenerate excited electronic state to the 5 E ′ electronic ground state. For clarity, the external vibronic spectra of the primarily JT-active bending mode are considered first.…”