Pseudorotation, energy barrier

It is interesting to note that many of the techniques developed in phosphorus chemistry are npw being routinely applied to hypervalent molecules of other elements. For instance, Martin et al. have studied the pseudorotational (Berry) mechanism for the inversion of 10-Si-5-siliconates (1) by 19F n.m.r. and demonstrated a linear correlation between AG for inversion at silicon and the a values of the variable ligand, Y The energy barriers for... 

Fig. 27. Energy profile for the Berry pseudorotation in PH5 (schematic). The reaction progress abscissa is made of an appropriate combination of two HPH angles. The energy barrier is in Kcal/mole.

However, the ESR spectra for coronene monoanion did not exhibit such complicated peaks as corannulene monoanion. This suggests that AE in the coronene ion is smaller than that of the corannulene ion. In fact, the calculated energy barrier AE of one of the Dlh saddle points between the stable Clh structures is 0.2 meV, which is smaller than that of corannulene as mentioned previously. Therefore, the pseudorotation about the D6h JT crossing makes the HFC constant averaged and all the peripheral hydrogen atoms equivalent even at low temperatures. 

From the previous sections on theoretical and spectral aspects of 1,3-dioxolanes, it is clear that the ring is conformationally labile and the data are best explained in terms of a freely pseudorotating system with a small energy barrier between conformers (B-80MI43001). The half-chair (39) and envelope (40) forms are the two lowest-energy conformers and it is the interconversion of these structures which best describes the pseudorotation (75JA1358). The X-ray structure of (1) shows the dioxolane ring has a conformation mid-way between these forms. 

An especially interesting situation is found in the field of heterocycles containing more than one sulfur atom. Here two different relatively stable conformations (f.i. chair and twist-boat form) have often been detected in the NMR-spectra they are intercon-verted by two processes (version and pseudorotation) which differ significantly in the height of their energy barrier. In a few cases the mobile heterocycles crystallize in only one of these conformations. 

There is no potential energy barrier between the sqnare pyramid at a = b 100° and the two slightly more stable trigonal bipyramids at (pp, = 90, b = 120° and ipp, = 120, < B = 90°. Movement along this reaction coordinate connecting the two trigonal bipyramids is usually described as Berry Pseudorotation Any one of the three eqnatorial site of the trigonal bipyramid can become the apical site of a sqnare pyramid and so repetition of this process scrambles all atom sites. 

Table 9 gives the free energy barriers for conformational interconversions in cyclooctanone. This table also gives the barrier to pseudorotation in... 

Pseudorotation The progression of one conformer of a five-membered ring to another conformer. In the case of cyclopentane there is no planar intermediate all conformers have at least one carbon atom out of the plane of the other carbon atoms. The maximum pucker can, in this case, rotate with almost no potential energy barrier between conformers. Each of the multitude of possible conformers can be described in terms of the maximum pucker and the pseudorotation phase angle, that is, where the conformer lies on a pseudorotation cycle (with an arbitrarily chosen origin). 

All MO calculations, both semiempirical and ab initio, have demonstrated that the barrier to aZ>3h-C4v-D3h interconversion, the Berry pseudorotation process, is small for acyclic phosphoranes (see also Section 2.1.). Estimates of 1.4 (extended Hiickel MO34 ), 4.8 (ab initio35 ) and 3.5 kcal/mol (CNDO/231 ) have been obtained for PFS. The barrier computed from the ab initio calculation increases to 8.5 kcal/mol if the 3c -orbitals are not included. The turnstile rotation mechanism traverses a much higher energy barrier 10.0 (extended Hiickel MO), 18.1 (ab initio) and 9.1 kcal/mol (CNDO/2). On the basis of these calculations, the Berry pseudorotation mechanism must be the preferred explanation of pentatopal isomerisation in acyclic phosphoranes (see Section 2.1.). 

The great flexibility of the trigonal bipyramidal skeleton displayed in the structures of many cyclic oxyphosphoranes suggests that the Berry pseudorotation mechanism may be an oversimplified representation for the permutational isomerisation of such species. Theoretical calculations and statistical considerations combined with these studies lead to the conclusion that this isomerisation may take place by a continuum of different routes (including turnstile rotation pathways) over a broad relatively flat potential surface, which have similar energy barriers. The Berry pseudorotation will, however, represent the energetically most favourable pathway for acyclic derivatives. 

A detailed dynamic NMR study of phosphoniummolybdacyclopropane (16) reveals a low energy barrier for cyclopropane rotation (AGf = 12.9 kcal mol-1) with a coalescence temperature of 263 K <84CB127>. Similar studies of (12) show a coalescence at 253 K which is attributed to a phosphine pseudorotation which includes a 90° cyclopropane rotation <85JCS(D)2025>. 

Pseudorotation about pivot ligand 1 is illustrated by Eq. (9). To be operationally detectable, it is necessary that the energy barriers for pseudorotation are accessible, and that the phosphorane has a sufficient lifetime, relative to its tetracoordinate reaction partners. 

The very fast Berry and tumstile pseudorotation processes can lead to the presence of a number of stereoisomers. The Berry process occurs through a transition state possessing the C4V square-pyramidal geometry with an energy barrier of ca. 2-3 kcal/mol in the case of phosphorus derivatives.48,49 (Scheme 2.5)... 

Table 2.1 Energy barriers (in kcal/mol) to Berry pseudorotation for...

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