Jahn-Teller static

The stoi7 begins with studies of the molecular Jahn-Teller effect in the late 1950s [1-3]. The Jahn-Teller theorems themselves [4,5] are 20 years older and static Jahn-Teller distortions of elecbonically degenerate species were well known and understood. Geomebic phase is, however, a dynamic phenomenon, associated with nuclear motions in the vicinity of a so-called conical intersection between potential energy surfaces. 

Now, we examine the effect of vibronic interactions on the two adiabatic potential energy surfaces of nonlinear molecules that belong to a degenerate electronic state, so-called static Jahn-Teller effect. 

V(CO)e generated by cocondensing presynthesized V(CO)6 with N2 at 10 K has been observed (44, 45). As suggested in a metal-atom study (125), the results indicated that a static, Jahn-Teller distortion is present. Matrix MCD also proved useful in confirming the predicted paramagnetism of Fe(CO)4 (45) (produced by photolysis of Fe(CO)5). In addition, matrix MCD was used to detect such paramagnetic species as MnOaCU in the presence of MnOsCl (45). 

In the limit of strong vibronic coupling, F4S = 0, c = 5 /3, s=, c2 - s2 =, and the dynamic Jahn-Teller effect thus renders nugatory the orbital contributions to the angular momentum, and reduces the splitting, A, by a factor of two. Note in addition that the c and s quantities used in the vibronic treatment do not correspond to those of the adiabatic case, although the expressions are formally similar, so that the static distortion, A, cannot accurately be calculated from the c and s values deduced from the and 4 data. 

Static Jahn-Teller distortions and lower-symmetry structures... 

Due to this mixing a Jahn-Teller molecule is expected to be rather fluxional and this often makes it difficult to detect the effect. It is usually observed in crystals where due to the so-called Jahn-Teller cooperativity a static distortion may occur in the crystal. The most typical examples are the tetragonally distorted octahedral structures, such as MnFs or KCuFs. 

The cyclopropane radical cation can be prepared by y-radiolysis in rigid matrices. At temperatures as low as 4.2 K, its ESR spectrum shows evidence for static Jahn-Teller distortion, resulting in a structure of the (ring-closed) trimethylene ( Ai) type, r " ... 

Ballhausen, C. and H. Johansen Molecular-orbital calculation of static Jahn-Teller effect in CuFg. Mol. Phys. 10, 183 (1966). 

An interesting aspect of many structural phase transitions is the coupling of the primary order parameter to a secondary order parameter. In transitions of molecular crystals, the order parameter is coupled with reorientational or libration modes. In Jahn-Teller as well as ferroelastic transitions, an optical phonon or an electronic excitation is coupled with strain (acoustic phonon). In antiferrodistortive transitions, a zone-boundary phonon (primary order parameter) can induce spontaneous polarization (secondary order parameter). Magnetic resonance and vibrational spectroscopic methods provide valuable information on static as well as dynamic processes occurring during a transition (Owens et ai, 1979 Iqbal Owens, 1984 Rao, 1993). Complementary information is provided by diffraction methods. 

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