Conformational pseudorotation

Further experimental confirmation of the preferred geometry of oxepane (1) has been obtained from its IR and Raman spectra. These vibrational and rotational spectra have been rationalized in terms of the lowest energy twist-chair conformations pseudorotating via a chair form (771A2866,2876). 

The furanose rings of the deoxyribose units of DNA are conformationally labile. All flexible forms of cyclopentane and related rings are of nearly constant strain and pseudorotations take place by a fast wave-like motion around the ring The flexibility of the furanose rings (M, Levitt, 1978) is presumably responsible for the partial unraveling of the DNA double helix in biological processes. 

Dioxolane also pseudorotates essentially freely in the vapor phase. 2,2 -Bi-l,3-dioxolane (128) has been shown by X-ray crystallography to have a conformation midway between the half-chair and envelope forms. The related compound 2-oxo-l 3-dioxolane (129) shows a half-chair conformation. This result is confirmed by microwave spectroscopy and by NMR data. Analysis of the AA BB NMR spectra of the ring hydrogen atoms in some 1,3-dioxolane lerivatives is in agreement with a puckered ring. Some 2-alkoxy-l,3-dioxolanes (130) display anti and gauche forms about the exocyclic C(2)—O bond. 

Cyclopentane is nonplanar, and the two minimum-energy geometries are the envelope and half-chair. In the envelope conformation, one carbon atom is displaced from the plane of the other four. In the half-chair conformation, three carbons are coplanar, vdth one of the remaining two being above the plane and the other below. The energy differences between the conformers are very small, and interconversion is rapid. All of the carbon atoms r idly move through planar and nonplanar positions. The process is called pseudorotation. 

The total spread in energies calculated for the four conformations is only 2.7 kcal/mol. The individual twist-chair conformations interconvert rapidly by pseudorotation. 

There are two possible Cs conformations of the Sy homocycle exo and endo. The exo global minimum (Fig. 2) lies 15 kJ mol below the endo-form. Both conformers undergo facile pseudorotation through C2 transition states, with barriers of less than 4 kJ mol [54]. The exo-conformer possesses the geometry found in the sulfur allotropes y-Sy and 5-Sy [72]. This Cs structure has four bonds near the length of a normal S-S bond and one rather long bond of 215 pm with a dihedral angle of 0°. 

UB3LYP theory predicts four minima of Sg which possess Cj, Cjhy O2 d C2 symmetries. At the UMP2 level of theory, no stationary point corresponding to the C2 minimum can be located and two new local minima with and D2 symmetries appear. The Sg conformers are found to be very prone to pseudorotation and are predicted to interconvert readily. For this reason, Cioslowski et al. refer to Sg as a fluxional species [93]. Interestingly, they found that the structures corresponding to local minima are not directly interconvertible. 

The equilibrium between the conformers (42a) and (42b) of the l l-adduct of dimethyl t-butylphosphonite (41) and benzylideneacetylacetone is so far to one side that the n.m.r. spectrum of the adduct does not vary with temperature. The phosphoranes (43) have been obtained from (41), and the corresponding phenylphosphonite, and methylenedeoxy-benzoin. In both adducts pseudorotation between (43a) and (43b) became slow on the n.m.r. time scale below — 10 °C. 

The tetraoxahydrospirophosphorane (57) has been isolated in 66% yield from the reaction of (55) with triethylammonium perfluor-opinacolate (56). Hexafluoroacetone inserts into the P-H bond of (57) to form (58) which may also be obtained from (59) as shown1 1. The 1H and 19F n.m.r. spectra of the phosphoranes reveal rapid pseudorotational processes and a time-averaged conformation of a flattened chair for the six-membered rings. 

These conclusions can be drawn by simple inspection of the molecular orbitals of PHS. The next question is the energetics of the interconversions of the various conformations. The result sketched in Fig. 27 refers to the Berry pseudorotation (J5) process of PHS. The square pyramid is a transition state for isomerization of the trigonal bipyramid, but the barrier is extremely small, in agreement with available experimental evidence. The more complex process ... 

A potential energy surface was computed for Berry pseudorotation in IrCl4N02- and Mn(CO)4NO the results confirm the qualitative conformational trends obtained from orbital interaction arguments. 

From the pseudorotating transition state the inversion process proceeds via an intermediate minimum of D2-symmetry (twist-conformation) and across a symmetry-equivalent second pseudorotational transition state to the inverted chair-conformation. The symmetric boat-form of cyclohexane (symmetry C2v) corresponds to a one dimensional partial maximum, i.e. a transition state (imaginary frequency 101.6 cm-1). It links sym-... 

Fig. 18. Top transition coordinates (with symmetry species) of conformational transition states of cyclohexane (top and side views). Hydrogen displacements are omitted. The displacement amplitudes given are towards the C2v-symmetric boat form, and towards >2-symmetric twist forms (from left), respectively. Inversion of these displacements leads to the chair and an equivalent T>2-form, respectively. Displacements of obscured atoms are given as open arrows, obscured displacements as an additional top. See Fig. 17 for perspective conformational drawings. Bottom pseudorotational normal coordinates (with symmetry species) of the Cs- and C2-symmetric transition states. The phases of the displacement amplitudes are chosen such that a mutual interconversion of both forms results. The two conformations are viewed down the CC-bonds around which the ring torsion angles - 7.3 and - 13.1° are calculated (Fig. 17). The displacement components perpendicular to the drawing plane are comparatively small. - See text for further details.

The conformational properties of a glyconucleoside containing an. S -glycosidic linkage have been also studied and compared to those of the corresponding O-disaccharide. A / and NOE data examination allowed a full pseudorotational analysis, showing a switch in the conformational preferences between the two molecules, the natural and the mimetic one.59... 

The difluorophosphoranes (18),17 (19),18 and (20)18 have been obtained using xenon difluoride as shown. The most stable conformations of the trifluorophosphoranes (21)19 and (22)20 have apical fluorines pseudorotation was slow on the n.m.r. time-scale below — 60 °C (21) and — 40 °C (22). The intermolecular exchanges of fluorines... 

Electron diffraction data on gaseous cyclopentasilane (61) fit both the C2 and C, models. Rustad s simple MM method calculates virtually equal energies for these conformers, and 61 is likely to undergo pseudorotation, as does cyclopentane (200). 

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