Turnstile rotation

C. Pseudorotation.—An alternative intramolecular exchange process to Berry pseudorotation has been suggested, which also occurs with conservation of angular momentum. It has been called a turnstile-rotation process because it involves the rotation of an apical-radial pair of ligands... 

The suggestion that equivalent phosphine positions are attained by a turnstile rotation is but one possibility, which is deemed to be the... 

Ramirez, Ugi and coworkers28,84 have put forward the turnstile rotation (TR) mechanism, which can explain the fast stereomutations of compounds such as 41,42 or 43 without contradicting the conclusions of Whitesides and coworkers. Referring to Figure 17, the process can be defined with reference to three types of movements of the ligands of a TBP structure ... 

C. Comparison of the Turnstile Rotation and Berry Pseudorotation Processes... 

VI. Calculation of Binding Energies of Model Situations in Turnstile Rotation... 

Equatorial=e. Apical = a. 6 The identity operation causes no isomerization and does not represent a mechanism. c M=Muetterties see Ref. 78. d Alternating cyclic permutations of four elements with one fixed point. Mg=BPR (see Ref. 53). 1 Turnstile rotation (see Ref. 65) p=pair t=trio. 

Among the formal mechanistic classes C2, Cs, C2+2, and C4, only the latter in the form of BPR or M is physically reasonable in terms of the symmetry of the skeleton in the intermediate state. However, there is a second formal mechanism that is reasonable by these criteria, namely, a subclass of the class C2+8. This mechanism, called the turnstile rotation was first recognized in 1970 it is represented by those permutations containing one apical and one equatorial ligand in a cycle of two, and the remaining ligands in a cycle of three elements.55... 

Figure 5 justifies the name turnstile rotation given to this mechanism. In the formal treatment presented in Section III, the single TR corresponds to a subclass of the class C +s. in which the permutation is of the type (a e )(ee"a), where a =apical and e =equatorial. As a consequence of the skeletal symmetry of the TBP, the following four... 

Fig. 5. Convenient representation of the turnstile rotation. The actual phenomenon is the 60° internal relative rotation of pair vs. trio. The hexagons are sterio frame of reference.

Fig. 6. Binding energies of PF in turnstile rotation model situations, from CNDO/2 calculations. Z-axis as TR-axis reaction coordinates relative change, A, in the azimuth coordinates of the pair and trio ligands.

Fig. 7. Itinerary for permutational isomerizations by single turnstile rotation, or by Berry pseudorotation, when the five ligands in trigonal bipyramidal phosphorus are different. Isomers are denoted by Permutational Notation.

The itinerary of isomerizations by the double turnstile rotation, or (TR)2 is shown in Fig. 9. Figure 9 corresponds to isomerizations when the five ligands are different. The lines connect isomers which fall on the same hemispheres. There are no connections between isomers which fall on different hemispheres i.e., there can be no inversions. Each isomer is connected to six others by (TR)2. The isomers that were connected by... 

---