Pseudo Jahn-Teller distortions

Of the cata-condensed nonalternant hydrocarbons undergoing a pseudo Jahn-Teller distortion, pentalene (I) and heptalene (VII), having the largest value, are predicted to possess a strong bond alternation. This confirms the results of the previous theoretical investi-gations " and agrees with the available experimental facts ... 

One study (DeLeo et al., 1988 Fowler et al., 1989) has found that neutral H at the B site in Si has a tendency to preferentially bind to one of the two Si neighbors, leading to an asymmetric configuration, with Si—H distances of 1.48 A and 1.77 A respectively. This tendency was interpreted in terms of a pseudo-Jahn-Teller distortion. However, the potential barrier that leads to the asymmetric position is so low (< 0.2 eV) that it can readily be surmounted by zero-point motion of the proton. Experimental observation of such an asymmetry is therefore unlikely, except maybe through an isotope shift measurement in an infrared experiment (DeLeo et al., 1988). None of the other theoretical approaches has produced this type of asymmetry. 

It is well known that crystal and electronic structures are interdependent and define the reactivity of chemical substances. In Section 1.4.2, it was noted that copper-porphyrin complex gives cation-radicals with significant reactivity at the molecular periphery. This reactivity appears to be that of nucleophilic attack on this cation-radical, which belongs to n-type. The literature sources note, however, some differences in the reactivity of individual positions. A frequently observed feature in these n-cation derivatives is the appearance of an alternating bond distance pattern in the inner ring of porphyrin consistent with a localized structure rather than the delocalized structure usually ascribed to cation-radical. A pseudo Jahn-Teller distortion has been named as a possible cause of this alternation, and it was revealed by X-ray diffraction method (Scheidt 2001). 

The 3d ab initio simulations [4] for Na3 are based, in a similar way, on three ab initio potential-energy surfaces for Na3(X), Na3(B), and Na3(X), with 3d ab initio dipole coupling between Na3(X) and Na3(B) evaluated by V. Bonacic-Koutecky et al. [5] plus Condon-type coupling between Na3(B) and Na3(X). Additional potential-energy surfaces interfere at the conical intersections of the pseudo-Jahn-Teller distorted Na3(B) state (see Ref. 6), but we have tested carefully [4] that these interferences are negligible in the frequency domains of the experimental femtosecond/picosecond laser pulse experiments [7] as well as in the continuous-wave experiments [8]. 

Note that in fact cyclobutadiene does not have degenerate, singly-occupied molecular orbitals, as a Jahn-Teller type (actually a pseudo-Jahn-Teller) distortion lowers its symmetry from square to rectangular and leads to a closed-shell paired-electron molecule [4]. 

The spectroscopic properties that derive from these arguments for perfectly symmetrical antiaromatic systems are of limited practical interest since [4A/]annulenes are subject to Jahn-Teller and pseudo-Jahn-Teller distortions and do not have C symmetry in reality. What makes these results useful is that they provide a starting point for the introduction of perturbations that convert the parent perimeters into n systems of real interest. 

The planar geometry and equal N—S bond lengths suggest that the 10-u electron systems, such as (15), have substantial aromatic character. Substituents at the carbon atoms at positions three and seven with 7t-donor capacity can induce pseudo-Jahn-Teller distortion leading to the bent, bicyclic 8-7t system with the S—S transannular partial bond <84JA3437>. 

Cotton FA, Daniels LM, Murillo CA, Slaton JG (2002) A pseudo-Jahn-Teller distortion in an Mo2(P2-0)2 ring having the shortest Mo-IV-Mo-IV double bond. J Am Chem Soc 124 2878-2879... 

Molecular orbital (MO) diagrams show that when a [4n]annulene is reduced to a [4n- -2]dianion, the LUMO level becomes fully populated, resulting in a stable ground state and an aromatic system. Conversely, the reduction of a [4n- -2]annulene affords a [4n] dianion, that possesses a low-lying LUMO, which can undergo a pseudo-Jahn-Teller distortion. These [4n]annulenes are predicted to be anti-aromatic [lb, 56-61]. 

B2g. Thus, Fq = b g<8)0 g and Fq = b g< )b2g, and the molecule is susceptible to a pseudo Jahn-Teller distortion along a b g or 02g coordinate. As shown for the D4h A4 system in Appendix III, there are no normal modes of 02g symmetry but the b g motion takes the square H4 to a rectangle, a result anticipated by the orbital correlation diagram in Figure 5.5. This very same state of affairs occurs in the tt system of square cyclobutadiene. 

---