Skewed conformation

For 2,2,3-trimethylbutane (pentamethylethane 32), there is presumably a sixfold rotational potential with six equivalent conformations skewed on either side of perfect staggering. There is a barrier of 6.9kcalmol" to rotation about the central ethane bond . Pentaalkylethanes can also be analysed as trialkylmethanes, remembering... 

On the question of how cyclohexane accommodates a tert-huty group substituent, calculation suggests that torsion about the exocyclic bond relieves strain and the perfectly staggered arrangement is a transition state between conformations skewed by 6-10° in the equatorial and 15-27° in the axial conformation, where parallel 1,3-interactions are no doubt greater. How the ring distorts in these monoalkylcyclo-hexanes has also been studied ... 

The case of a,/3-unsaturated carbonyl compounds is analogous to that of 1,3-dienes, in that electronic considerations favor coplanarity of the atoms of the C=C—C=0 system. The rotamers that appear important are the s-trans and s-cis conformations. Skew forms, unlike the case of 1,3-dienes, do not appear to represent potential energy minima. The torsional and van der Waals interactions that destabilize the s-cis conformation of 1,3-dienes seem, therefore, to be less important in a,j8-unsaturated carbonyl compounds. Microwave data indicate that the s-trans form is the only conformation present in acrolein in detectable amounts. ... 

The various conformations of cyclohexane are m rapid equilibrium with one another but at any moment almost all of the molecules exist m the chair conformation Not more than one or two molecules per thousand are present m the skew boat confer matron Thus the discussion of cyclohexane conformational analysis that follows focuses exclusively on the chair conformation... 

A potential energy diagram for nng inversion m cyclohexane is shown m Figure 3 18 In the first step the chair conformation is converted to a skew boat which then proceeds to the inverted chair m the second step The skew boat conformation is an inter mediate in the process of ring inversion Unlike a transition state an intermediate is not a potential energy maximum but is a local minimum on the potential energy profile... 

Section 3 7 Three conformations of cyclohexane have approximately tetrahedral angles at carbon the chair the boat and the skew boat The chair is by far the most stable it is free of torsional strain but the boat and skew boat are not When a cyclohexane ring is present m a compound it almost always adopts a chair conformation... 

Skew boat (Section 3 7) An unstable conformation of cyclo hexane It is slightly more stable than the boat conforma tion... 

A modified boat conformation of cyclohexane, known as the twist boat (Fig. 1.8), or skew boat, has been suggested to minimize torsional and nonbounded interactions. This particular conformation is estimated to be about 1.5 kcal moE (6 kJ moE ) lower in energy than the boat form at room temperature. 

Dienes would be expected to adopt conformations in which the double bonds are coplanar, so as to permit effective orbital overlap and electron delocalization. The two alternative planar eonformations for 1,3-butadiene are referred to as s-trans and s-cis. In addition to the two planar conformations, there is a third conformation, referred to as the skew conformation, which is cisoid but not planar. Various types of studies have shown that the s-trans conformation is the most stable one for 1,3-butadiene. A small amount of one of the skew conformations is also present in equilibrium with the major conformer. The planar s-cis conformation incorporates a van der Waals repulsion between the hydrogens on C—1 and C—4. This is relieved in the skew conformation. 

The barrier for conversion of the skew conformation to the s-trans conformation is 3.9kcal/mol. This energy maximum presiunably refers to the conformation (transition state) in which the two n bonds are mutually perpendicular. Various MO calculations find the s-trans conformation to be 2-5 kcal/mol lower in energy than either the planar or skew cisoid conformations. Most high-level calculations favor the skew conformation over the planar s-cis, but the energy differences found are quite small. ... 

The two -OH groups in l,2 5,6-di-0-cyclohexylidene-wii/o-inositol and its di-O-isopropylidene analog are trans. The X-ray crystal structure of the latter compound suggests that the ring is in a skew conformation with the 0-3 and 0-4 -OH groups both in ax positions, but NMR studies and ab initio calculations indicate that a mixture of the skew and chair conformations, with 0-3 and 0-4 both in eq positions, is present. Formation of a dibutylstannylene acetal presumably locks these two compounds in the latter conformation. 

MacFadyen, 1. Andricioaei, I., A skewed-momenta method to efficiently generate conformational-transition trajectories, 7. Chem. Phys. 2005,123, 074107... 

Dienes in quasi-s-fraws conformation are found only in cyclic structures where perfect planarity is hindered. The DR also holds valid for this kind of conformation, as demonstrated by the considerations of Section II.D.l.a and also confirmed by all the reported calculations. Indeed, contrary to what is sometimes found for cisoid systems, the rotational strength evaluated by many types of calculation is invariably found to follow the diene rule for transoid systems. However, very small skew angles are usually found in real molecules and this implies that the main contribution to the observed optical activity cannot come from the weak intrinsic distortion, but is more likely to stem from the dissymmetric perturbations, notably of the allylic axial substituents. 

The planar conformations s-cis and s-trans cannot be appreciably populated owing to the repulsive steric interaction between the Cg—H and C5-methyl (in the s-cis) or between the same Cg—II and the methyl group on (Q in the s-trans). This repulsion is minimized by introducing a twist around the Q,—( 7 bond. Limiting conformations, with skew angles of about 40° and 140°, can be assumed, as shown in Scheme 8. In this way, an intrinsically dissymmetric chromophore is created. 

Butadiene can exist in two planar conformations, s-trans and s-cis. They belong to the C2h and C2v symmetry point group, respectively. Obviously, both forms have symmetry planes. A skewed conformation, instead, has only a C2 axis. We can choose the reference frames depicted in Figure 12, characterized by z as the symmetry axis and x directed along the single bond. 

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