Pseudorotating ring molecules

We shall also not deal in our paper with the large amplitude ring puckering and pseudorotation in small ring molecules. Molecular dynamics of these motions and the determination of the barriers of pseudorotation from microwave and infrared spectra have been the subject of a great number of papers this problem has been recently reviewed in the monography ... 

Ring inversion (when strictly defined) of achiral conformations is nothing more than a pseudorotation. Ring inversion in the cyclohexane chair, for example, leaves the molecule apparently rotated by 60° along the C3 axis. Nevertheless, in order to conform with common usage, we will exclude ring inversion from the definition of ring pseudorotation. 

An analysis of multipath transmission of spin-spin coupling in cyclic compounds has been made by Wu and Cremer with the help of partially spin-polarized orbital contributions. The calculations have been performed for cyclopropane, cyclopentane, bicydo[1.1.1]pentane and tetrahydrofuran. The authors have shown that the measured and calculated Vcc couplings of cyclopentane and tetrahydrofuran are averages over the pseudorotational motion of these ring molecules where each individual coupling of a conformation passed in the pseudorotation is the sum of different path contributions. 

NMR spectroscopy ( Se, /= 1/2, 7%) is a powerful technique for identifying cyclic selenium molecules, especially the heteroatomic ring systems that contain sulfur or tellurium in addition to selenium, for which several isomers are possible for most compositions (Section 12.1.2). Solutions of monoclinic selenium in CS2 have been shown by high-performance liquid chromatography to form an equilibrium mixture of cyclo-Seg, cyclo-Sey and cyclo-Se6. The Se NMR spectra of such solutions show two singlets that are attributable to cyclo-Seg and cyclo-Se with relative intensities that correspond to a molar ratio of ca 6 No resonance is observed for cyclo-Sey presumably as a result of the fluxional behaviour (pseudorotation) of the seven-membered ring (Section 12.1.2). 

Cyclopentane was studied as early as in 1931 by Wierl258 who found that the molecule was planar. In 1946 Hassel and Viervoll259 found that the carbon ring deviates slightly from planarity. This has later been confirmed by repeated investigations241, 26°. Spectroscopic studies239 have shown that this molecule undergoes nearly free pseudorotation. 

J The Berry mechanism in the case of PF5 gives the impression that the whole molecule has been rotated, but since in Berry s definition there is no rotational motion, the process was named pseudorotation. The term pseudorotation was originally applied to the rapid up-and-down motion of the carbon atoms in cyclopentane, and later extended to the puckering of rings in general see Ref. 54. Objections can be raised to this duplication in terminology, and in this review we denote the Berry mechanism simply as BPR. 

The question might be asked Are there similar mechanisms for changing the configuration of molecules without breaking bonds in molecules with coordination numbers other than 3 and 5 The answer is yes. One of the most important series of inorganic compounds consists of six-coordinate chelate compounds exemplified by the tris(ethylenediamine)cobalt(III) ion. Because of the presence of the three chelate rings, the ion is chiral and racemization can take place by a mechanism that is closely related to atomic inversion or Berry pseudorotation (the mechanism for six-coordination is termed the Bailar twist see Chapter 13). 

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