First-Order Jahn-Teller Effect

If the states are degenerate rather than of different symmetry, the model Hamiltonian becomes the Jahn-Teller model Hamiltonian. For example, in many point groups D and so a doubly degenerate electronic state can interact with a doubly degenerate vibrational mode. In this, the x e Jahn-Teller effect the first-order Hamiltonian is then [65]... 

Vibrational stmcture is, at best, only partially resolved. The stmcture in the first and second systems is complex which could be due, in part, to there being a Jahn-Teller effect in each of them. Such an effect may arise when a molecule is in an orbitally degenerate E (or T) state. The Jahn-Teller effect involves a distortion of the molecule in order to destroy the... 

This general feature of both class I and class II behaviour of the copper(II) ion, i.e. the ability to exist in a high symmetry environment against the prediction of both the first- and second-order Jahn-Teller effects, is the best single piece of evidence for the cooperative Jahn-Teller effect and has recently been reviewed.432 It is generally responsible for the whole range of fluxional copper(II) stereochemistries and of the temperature variable ESR spectra of... 

Richard Bader was among the earliest of workers to realize the importance of electron density in providing an understanding of chemistry. Early on he was led to formulate the first symmetry rule governing a chemical reaction in answer to the question of how the electron density changes in response to a motion of the nuclei. This rule, termed the pseudo- or second-order Jahn-Teller effect, provides the theoretical underpinnings of frontier molecular orbital theory and is still widely used in discussions of reaction mechanisms and molecular geometries. 

First-order Phase Transition in U02 The Interplay of the 5f2-5f2 Superexchange Interaction and Jahn-Teller Effect... 

The essence of the first or second order Jahn-Teller effect is that a high-symmetry geometry generates a real or near degeneracy, which can be broken with stabilization by a symmetry-lowering deformation. Note a... 

Investigation of 1 upon removal of an electron has intrigued both theoreticians and experimentalists because the properties of the formed cyclopropyl radical cation provide a basis to test MO models of three-membered rings (the Walsh MO model cannot explain the properties of the cyclopropyl radical cation ). Ejection of an electron from the 3e MOs of 1 (MOs 11,12 in Figure 6) leads to the Jahn-Teller unstable electron configuration (see Section X), which can be stabilized by distortion to the C2v-symmet-rical states and 82 (first-order Jahn-Teller effect, see Figure 23). 

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