Jahn–Teller Effects in Molecular Cations Studied by Photoelectron Spectroscopy and Group Theory

Abstract

The traditional “ball‐and‐stick” concept of molecular structure fails when the motion of the electrons is coupled to that of the nuclei. Such a situation arises in the Jahn–Teller (JT) effect which is very common in open‐shell molecular systems, such as radicals or ions. The JT effect is well known to chemists as a mechanism that causes the distortion of an otherwise symmetric system. Its implications on the dynamics of molecules still represent unsolved problems in many cases. Herein we review recent progress in understanding the dynamic structure of molecular cations that have a high permutational symmetry by using rotationally resolved photoelectron spectroscopy and group theory. Specifically, we show how the pseudo‐Jahn–Teller effect in the cyclopentadienyl cation causes electronic localization and nuclear delocalization. The fundamental physical mechanisms underlying the vaguely defined concept of “antiaromaticity” are thereby elucidated. Our investigation of the methane cation represents the first experimental characterization of the JT effect in a threefold degenerate electronic state. A special kind of isomerism resulting from the JT effect has been discovered and is predicted to exist in all JT systems in which the minima on the potential‐energy surface are separated by substantial barriers.

Bicircular High-Harmonic Spectroscopy Reveals Dynamical Symmetries of Atoms and Molecules

“…Symmetry breaking is the source of a myriad of important physical effects. Famous examples include parity violation through the weak nuclear force, which results in energy differences between enantiomers of a chiral molecule [4], and the Jahn-Teller effect, which is the key to understanding the structure and dynamics of many molecules, transition-metal complexes, and solids [5,6].…”

Rovibronic analysis of the Jahn–Teller effect in CH2D2+ at low energies

“…[4][5][6]12,13 Twelve such equivalent structures exist, which can be treated as two enantiomeric sets of six structures each. 4,6,13,21 Zero-point energy effects favor structures in which the D atoms are located on short bonds and, therefore, successive deuteration has a large effect on the structure and tunnel dynamics of the methane cation. Whereas the ground state of CH 4 + is delocalized over six equivalent potential wells and splits into two ͑T 2 and T 1 ͒ vibronic components separated by a tunneling splitting of about 16 cm −1 , 15 the ground state of CH 2 D 2 + is localized in a single potential well and can be approximately described as a rigid rotor.…”

“…In CH 3 D + and CD 3 H + , the Jahn-Teller effect gives rise to two energetically distinguishable isomeric forms in which either one or two of the three equivalent ligand atoms are located on a short bond. 13,17,21 High-resolution pulsed-field-ionization zero-kineticenergy ͑PFI-ZEKE͒ photoelectron spectra of jet-cooled CH 4 , CH 3 D, CH 2 D 2 , CD 3 H, and CD 4 only display a band structure in a narrow energy range above the adiabatic ionization threshold. Beyond 500 cm −1 of internal energy in the cation, this band structure evolves into an uninterrupted dense sequence of sharp lines.…”

Internal rotation in Jahn–Teller coupled systems: The ethene and allene cations

“…[1,2,3,4] and references therein). Most treatments focused on the E⊗e JT effect and on the rovibronic energy level structures of prototypical molecular systems including H 3 [5,6], Li 3 [7], Na 3 [8], CH 3 O [9,10], cyclopentadienyl [11,12], C 6 H + 6 [13,14,15] and C 5 H + 5 [16,17]. The vibronic coupling of a doubly-degenerate electronic state mediated by nondegenerate vibrational modes, i. e., the E⊗(b 1 ⊕b 2 ) or E⊗b JT effects apparent in molecules with a four-fold symmetry axis, has received less attention, maybe because it is considered to be the simplest [3].…”