The Jahn–Teller effect as a general tool for solving molecular and solid state problems: Novel findings

Abstract

In this (partly review) paper several important situations are analyzed in which molecular systems are distorted, but there are no apparent degeneracies or close in energy states to justify their origin as due to the Jahn–Teller effect (JTE) or pseudo JTE (PJTE). It is shown that in all these cases above the JTE are “hidden” in the excited states of the undistorted configuration even when the energy gap to these states is very large, and the involvement of these states in the vibronic coupling explains the JT origin of the distortions. This confirms the general validity of the JTE as the only source of instabilities and distortions of high-symmetry configurations of polyatomic systems. For the example of the ozone molecule, which has no degenerate ground state, neither in the distorted (obtuse-triangular), nor in the undistorted (regular triangular) configurations, and there are no low-lying excited states, the distortion is shown to emerge from the JTE in the excited E state situated at ∼8.3 eV above the ground state. The JT stabilization energy is more than ∼9 eV which makes the distorted configuration the lowest in energy. For molecular systems with half-closed-shell electronic configurations e 2 and t 3 (with degenerate orbitals), which produce totally symmetric charge distribution and are not subject to the JTE (neither in the ground high-spin (HS) state, nor in the excited low-spin (LS) states formed from these configurations by interelectron interaction), distortions were shown to occur due to a strong PJT mixing of two excited states. Again, the PJT stabilization energy in these cases is larger than the energy gap to the ground state, so the low-spin excited state of the undistorted geometry becomes the lowest in energy in the distorted configuration, and it is shown that this distortion is accompanied by orbital disproportionation that makes the charge distribution lower in symmetry. Since the two spin-states are stable in two different nuclear configurations, distorted (LS) and undistorted (HS), a novel phenomenon emerges in these systems: JT-induced spin-crossover. Distinguished from the known spin-crossover in transition metal compounds, the JT-induced spin-crossover takes place in a large variety of molecular systems from different classes including organic compounds, and it has a much lower rate of relaxation between the two states; this makes them candidates for single-molecule materials for electronic devices.