First-Order Jahn-Teller Distortion

It was shown by several workers that in this case the first-order Jahn-Teller distortion is due to an ej vibration, and that the second-order distortion vanishes. Therefore, in terms of simple Jahn-Teller theoi, the moat around the symmetric point should be a Mexican hat type, without secondary minima. This expectation was borne out by high-level quantum chemical calculations, which showed that the energy difference between the two expected C2v structures ( A2 and Bi) were indeed very small [73]. 

Wade expanded the 1971 hypothesis to incorporate metal hydrocarbon 7T complexes, electron-rich aromatic ring systems, and aspects of transition metal cluster compounds [a parallel that had previously been noted by Corbett 19) for cationic bismuth clusters]. Rudolph and Pretzer chose to emphasize the redox nature of the closo, nido, and arachno interconversions within a given size framework, and based the attendant opening of the deltahedron after reduction (diagonally downward from left to right in Fig. 1) on first- and second-order Jahn-Teller distortions 115, 123). Rudolph and Pretzer have also successfully utilized the author s approach to predict the most stable configuration of SB9H9 (1-25) 115) and other thiaboranes. 

FIGURE 23. First-order Jahn-Teller distortion of Dn symmetrical cyclopropyl radical cation. Geometries [UMP2/6-31G(d) calculations] and relative energy [CISD/6-31G(d)] from Reference 231... 

The absorption spectrum of the bare Cu Y zeolite was also measured and interpreted using a model of a CuII-0(Sl ) site. The 0-CuII-0 angle was determined to be 117° from optical spectra, in excellent agreement with 115.9° found by X-ray crystallography (20). Based upon the observed spectral splitting of 1800 cm- of the states of Cu -, first order vibronic theory predicted a Jahn-Teller distortion of 0.05-0.06 A along the Cu11- bonds. 

A general approach for predicting Jahn-Teller distortions of copper(II) hexa-amines has recently been published, and it has the potential to be applied to donor atoms other than nitrogen, metal centers other than copper(II), and various types of coordination polyhedra11641. The method is based on a harmonic first-order model11651 where the Jahn-Teller stabilization energy is the result of the Qg distortion mode (Fig. 11.1, Eqs. 11.3, 11.4). 

The cyclobutane radical cation was calculated by several groups, who evaluated Jahn-Teller distorted structures potentially arising upon ionization of cyclobutane [79, 83-87]. Four distorted local minima were considered (Figure 13). A rectangular structure with two weakened C-C bonds (type A) and a rhomboidal structure with four weakened C-C bonds (type B) result from first order Jahn-Teller distortion. Two further structures, a trapezoidal one (one weakened C-C bond type C), and an irregular structure shaped like a kite (two lengthened C-C bonds type D) can be envisaged as a consequence of second-order Jahn-Teller distortion [86]. A detailed recent study also considered puckered equivalents of the planar structure types, A-D... 

In 1991 we reported on the synthesis of P-silyl and -phosphanyl substituted silylidenephosphanes (phosphasilenes) 2, i.e., compounds of type IIA and III, which are accessible by thermally induced elimination of LiF from corresponding / -lithium-(fluorosilyl)phosphanides IV [5, 6] and possess a remarkable thermal stability (up to 110°C). In 1993 the first crystalline phosphasilene 5 [7], a compound of type IV, had been prepared and its structure was established by X-ray diffraction [8], whereas the Si=P compounds of type V were merely characterized by means of NMR spectroscopy [9]. Compound 5 possesses a relatively long Si=P bond length and a non-planar geometry around the A, , a -Si atom, which can be explained in terms of steric hindrance or a second-order Jahn-Teller distortion [10, 11]. In comparison, the Si=P bond in 6 [12] is significantly shorter and the Si atom is trigonal planar coordinated. 

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