Pseudorotation barrier

The CPF approach gives quantitative reement with the experimental spectroscopic constants (24-25) for the ground state of Cu2 when large one-particle basis sets are used, provided that relativistic effects are included and the 3d electrons are correlated. In addition, CPF calculations have given (26) a potential surface for Cus that confirms the Jahn-Teller stabilization energy and pseudorotational barrier deduced (27-28) from the Cus fluorescence spectra (29). The CPF method has been used (9) to study clusters of up to six aluminum atoms. 

Corriu and coworkers have found that the barrier to pseudorotation seems to depend on the number of electronegative groups present35. For species with one electronegative substituent, AG >20 kcalmol-1. With two electronegative groups, the barrier decreases to ca 9-12 kcalmol-1, and when three electronegative substituents are present, the pseudorotation barrier has A<7 kcalmol-1. 

There are perhaps only two research areas in which the quantitative estimation of steric factors is routinely done, namely for hindered biphenyls (Cooke and Harris, 1967) and rotational or pseudorotation barriers in ethanes and cyclic compounds (Allinger et al., 1967). There are indications, however, that this interest is widening. Simonetta and Favini (1966) performed conformational calculations on the Cope rearrangement via chair or boat transition states and indicated that the orbital preference emphasized in a previous section should be supplemented with a steric factor. 

Although quantum chemical calculations have not been carried out on 1,2,3-trithioIane (3) or its derivatives, these calculations on 1,2,4-trithiolane (4) have shown a gradation of stability (4g) > (4c) > (4b) > (4d) > (4a) (76JA2741). These calculations show that by analogy to 1,2,4-trioxolane (2) the symmetrical S—S half-chair conformation is the most stable, being favored over the envelope form (4d) by 13.0 kJ mol-1, which corresponds to the pseudorotational barrier between the symmetrical half-chair forms. The barrier for ring interconversion through the planar form (4a) was calculated to be 18.8 kJ mol-1. 

The pseudorotation barriers given in Table 8 are strongly dependent on the size of the CX2 group, whereas the barrier to ring inversion remains... 

Since oxygen is much smaller than a methylene group, the same kind of situation occurs in XVII as was discussed in the previous section. The barrier to methyl rotation in dimethyl ether is 2.7 kcal/mole >, only slightly lower than in propane, where the beirrier is 3.4 kcal/mole. Oxocane should therefore have the BC-1 conformation, as in methylenecyclooctaue rather than the BC-3 and BC-7 conformations. The presence of only a single process in the proton spectrum of XVII is immediately consistent with the BC-1 conformation, but requires rapid pseudorotation between the BC-3 and BC-7 forms at —170 °C if the latter two forms are the correct conformations. The pseudorotation barrier in XVII should be higher than in cyclooctane, and probably comparable to that in cyclooctanone (6.3 kcal/mole). Thus, pseudorotation of the BC-3 form should not be rapid at —170 °C, and further support for this h5q)othesis is provided by 1,3-dioxocane (see below). It is therefore probable that oxocane has the BC-1 conformation. 

The data on barriers to ring inversion in heterocyclic boat-chairs (Table 10) show a trend to lower barriers when the number of hetero-atoms is increased, and in the absence of gew-dimethyl substitution, pseudorotation barriers are also quite low. It is therefore reasonable to expect that the barriers to pseudorotation and inversion in the tetrathiocane boat-chair might be too low for nmr detection, and this would, of course, explain the temperature-independent spectrum. 

Ring inversion of crown family conformations generally proceeds by interconversion, over a relatively high barrier (10 to 12 kcal/mole), to boat-chairs, which invert relatively easily. Pseudorotation barriers in crown family conformations such as the twist-chair-chair are extremely low and not detectable by direct hne-shape measurements. 

The monomers most comprehensively studied in cationic polymerization are the non-planar tetrahydrofuran and 1,3-dioxolane. HowevCT, 5-membered rings do not assume such favorable conformations like the chair conformation of 6-membered rings. Consequently, the energetic barrier (pseudorotation barrier) between the con-... 

Benzene annelation significantly increases the barrier to ring inversion processes. Saturated seven-membered rings can invert by pseudorotation (pseudorotation barrier is 2 kcal/mole) which has not been measured by NMR techniques. The AG for the ring inversion process for the benzene-annellated derivatives of cycloheptane (which probably exists in a chair form) was determined to be 10.9 kcal/mole at Tj, = — 57°C (186, 298, 322). Equilibria in tricyclic organic derivatives have been studied more extensively. Several different inversion processes have been reported and depend on structural type. The following are examples of ring inversion processes. 

The value of the pseudorotational constant, 3.25 cm-1, was taken from the earlier far-infrared study6). Figure 4.28 depicts the potential function hindering pseudorotation. It is seen that the largest barrier encountered in one cycle is 55 cm-1, indicating that the approximation that the pseudorotation barrier be much less than the barrier to planarity has been met in this case. 

C5H10 1 1 CH2CH2CH2CH2CH2 Cyclopentane MIR, R Essentially pure pseudorotation. Barrier to planarity = 1824 cm-1. Isotopic species studied 7, 97, 188, 189)... 

What are the energy relations in the six-membered ring of pyranoid saccharides The energy difference of the axial and equatorial forms on the anomeric center is low, in the interval of 0-11 kJ.mol", depending on the medium (see Tables II, III, and VII). The interconformational (pseudorota-tional) barrier, AE, in six-membered rings is —42-46 kJ.moI". Some authors assumed an additional increase of the pseudorotational barrier due to the anomeric effect, but, in contrast, data for the ring inversion in... 

The alternative explanation of zwitterionic hexaco-ordinated intermediates (or transition states) to account for the faster pseudorotation rates with X=0, appears to be negated by studies of pseudorotational barriers in (16ab). In these molecules, no conformational transmission effect is possible but the presence of the tetrahydrofuran oxygen atoms would permit a zwitterionic T.S. to accelerate ligand reorganization. In fact the pseudorotational barriers for (16a) and (16b) at 55.2 and 71.9 kjmol respectively, are in excellent agreement with those found for (11a) and (11c) where no hexaco-ordinate species are possible along the isomerisation pathway. 

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