Jahn-Teller-active ions

The Ti(CN)6- anions adopt nearly ideal octahedral geometry, not the distorted geometry expected from a d1 Jahn-Teller active ion. This behavior is common to many other Tim compounds. 

A particularly interesting case is when a set of hydrogens which are chemically equivalent in the unionized molecule become inequivalent in the positive ion. Obvious examples are Jahn-Teller active molecules, but the same phenomenon may be found also in Jahn-Teller inactive systems. Since deuteration fcr practical reasons must be done before ionization, it may happen that a single deuterated molecule may produce several inequivalent isomers of the radical cation, e.g., upon irradiation. This will obviously influence the recorded ESR spectrum. 

For instance Cr(CO)6+ is formed only during LI. The time-dependent behavior of the ion yields of Cr(CO)6+ is presented in Fig. 13. Deconvolution of the time-dependent ion yield with the instrument function derived from the Xe+ signal provides a measure of the time constant (ij) of 12.5 0.05 fs for the LI level (Table 2). This represents the time it takes for the excited Cr(CO)6 to cross to the repulsive surface through the conical intersection close to the Franck-Condon state. At the Franck-Condon point with Oh symmetry, the only coordinates with nonzero slope are the totally symmetric alg M-C stretch or the Jahn-Teller-active vibrations which have eg or t2g symmetry [32], The time taken for a wavepacket to travel from any... 

Figure 18 Representation of the potential energy surfaces involved in photoionization from a nondegenerate ground state of a molecule to a Jahn-Teller active state of a molecular ion. The distortion coordinate is the Jahn-Teller active vibration. The vertical arrows represent the most probable transitions...

In octahedral symmetry, the copper(ll) ion has a electronic ground state due to the d electron configuration with the unpaired electron in an Cg a anti-bonding orbital. An exact octahedral geometry of six-coordinate copper(II) complexes is never realized due to a strong Jahn-Teller effect. The symmetry of the Jahn-Teller active vibration is eg, the non-totally symmetric part of the symmetric square [Eg Eg]. For a Cu(Il)Lg complex, the two components of the degenerate eg vibration are shown in Fig. 1 a [2]. 

A different series of organic systems whose Raman spectra and R Ps have been studied in some detail (lijima et al., 1975 Muramatsu et al., 1977 M. Asselin and H. G. Bernstein, unpublished) are ions of the type C 0, where n = 4, 5, 6, belonging to the point groups D /, Dsi, and respectively. These ions show evidence of Jahn Teller effects. Thus in 404 , the Jahn-Teller active modes big and hj show strong Raman activity when approaching resonance with the allowed Ajg - E ( r7r ) transition. Similarly in CsOs", resonance with the A l i(7r7r ) transition leads to high activity of Jahn-Teller active e 2 modes, which have intensities comparable... 

Structurally [Pd (983)2] " consists of a centrosymmetric tetragonally elongated six-coordinate complex, as expected for this Jahn-Teller active (low-spin d ) ion (Fig. 6) [132]. Axial Pd-S distances exceed equatorial ones by 0.18 A (Table 1). Oxidation of [Pd(9S3)2] shortens the axial Pd-S distance by over 0.4 A, but equatorial ones by only 0.04 A. In its electrochemical behavior the... 

However, the e -electronic ground state of the benzene ion is degenerate and its w e function can be perturbed by certain vibrations of the molecule. This leads to Jahn-Teller active modes which are distinguished by angular momenta j = 1/2 and j = 3/2. This will allow further transitions from the vibrationless groundstate of to vibrational levels of j = 1/2 in the ion. 

Importantly, differences in the metal-Ugand bonding to the two tertiary amine donors have been shown to be of importance for the tuning of the spin state of the ferryl complexes (see Section 6.4) [13g] and the redox potentials and catalytic activities of Cu couples [21] the bispidine-derived geometry is particularly well suited for the Jahn-Teller active Cu" ion and has led to a rich Cu coordination chemistry [22] with interesting applications in bioinorganic model chemistry (hemocyanin [23], catecholase[24]), catalytic aziridination [21a,... 

A minor success is also seen in complexes of d and d" ions, in which the distorted octahedral geometries observed may be rationalized (and indeed predicted) in terms of the Jahn-Teller effect, and ultimately in terms of the steric activity of the open d shell. This is a common feature in copper(n) chemistry, and you will... 

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