Crystal conformation, pseudorotational

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. 

Relatively few such heterocyclic systems have been studied by microwave spectroscopy some data are included in Table 7. In 1,3-dioxolane the twist conformation (see Table 41) is more stable than the envelope conformation, and pseudorotation occurs. In 1,2,4-trioxolane the equilibrium conformation is the twist form, in which the peroxide bond straddles the plane of the other three atoms, and there is a barrier of 6.3 kj mol 1 opposing pseudorotation <1974PM H (6)53>. Preference for the twist conformation is also observed in the crystal structures of some 1,2,4-trioxolane derivatives (see Table 41, Section 2.4.4.4) . 

The pseudorotation concept was introduced to describe the continuous interconversion of puckered forms of the cyclopentane ring [104]. The same concept is applied to the furanose geometry where the Cl, 04, and C4 atoms lie in one plane and the C2 and C3 atoms lie above and below that plane. A statistical analysis of X-ray crystal structures of nucleosides and nucleotides has shown that North (N) 130 and South (S) 131 conformations are the most dominant forms, which has been the basis of the assumption of the two-state N — S pseudorotational equilibrium in solution (Fig. 2.44). 

A statistical survey of crystal structures of nucleic acid constituents has led to the proposal that a full description of nucleoside molecular geometry can be given in terms of four parameters the phase angle of pseudorotation, P, the puckering amplitude, the conformation of the side-chain, 5, and the glycosidic torsional angle, X these allow the variability of bond and torsion angles to be described. ... 

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