Octant rule perturbers

The validity of the octant rule is supported largely and convincingly by ORD and CD spectra from numerous examples where the dissymmetric perturbers (the groups perturbing the carbonyl chromophore in a nonsymmetric way), such as the methyl group of... 

Figure 7. (Left) Classical octant rule diagram (ref 1) for the ketone carbonyl n - x transition. Local symmetry-derived, orthogonal octant planes XZ and YZ divide all space into quadrants, and a non-symmetry-derived third nodal surface (A) is approximated by an orthogonal plane bisecting the C=0 bond. "Front" octants are those nearer an observer along the +Z axis, while "back" octants lie towards -Z. (Middle) Octant contribution signs that perturbers make in back and front octants. (Right) Revised octant rule [6] with octant planes XZ and YZ unchanged and the third nodal surface defined theoretically as a concave surface (B). [Reprinted with permission from ref. 7. Copyright ° 1986 American Chemical Society.]...

One of the more surprising octant contributions was discovered for a /3-axial methyl perturber, which was weak and anti-octant. [13] This finding led to a reinvestigation of the octant rule that resulted in a clarification of the third nodal surface (Figure 7). [6]... 

The influence of a - and )S -substituents on the Cotton effects of saturated ketones is well known, and is expressed in the familiar octant rule . Considerable effects due to suitably oriented y - and -substituents have now been demonstrated. Although no convenient series of steroidal ketones with y -substituents was available, a wide range of anti-Ti-substituted camphor derivatives (9) reveals effective perturbation of the carbonyl n—> n transition by... 

On the lowest level, using symmetry and nodal properties of the MOs of H2C=0 as a representative of the inherently symmetric carbonyl chromophore, an octant rule can be derived for the influence of a (static) perturber of the (n, n ) excitation The geometrical symmetry (C2v) of the H2C=0 unit with the two symmetry planes, yz and xz, leads to a quadrant rule. If the orbitals involved in the (n, n ) excitation of ketones are described as " = 2p and n = N 2p — 2pJ) (N being a normalization constant), the nodal plane (xy) of the virtual orbital generates the octant diagram for the contributions of substituents of chirally perturbed compounds (Figure 10). 

For the discussion of the relationship between the sign of the (n, tc ) Cotton effect and molecular structure of spiran-type cyclopropyl ketones, such as 3a,5-cyclo-5a-cholestan-6 one (142), one must consider that the perturbing group comes into the near lower left (nil) octant. Then, the reversed octant rule predicts a negative sign for the (n, tc ) CE which is... 

For 152-154 octant projections predict that static chiral perturbers will contribute (according to the octant rule) only little to the overall Therefore, it was... 

If viewed from the oxygen, most optically active carbonyl compounds have their substituents only in the rear octants. The appearance of the plane that separates the rear octants from the front octants is not determined by the symmetry of the isolated chromophore. Calculations have shown that it has approximately the shape depicted in Figure 3.4b. Many examples have verified the validity of the octant rule, but there are also cases where it is not applicable, at least not in its original, simple form. This is true for ketones with a cyclopropane ring in the a, jS-position and for fluorosubstituted ketones, for which the experimentally observed sign can be reproduced only if the perturbation due to the fluorine atom is assumed to be smaller than that due to the hydrogen atom. More recent detailed calculations solved some of these problems. (Cf. Charney, 1979.)... 

Optically active N-(a-phenethyl) derivatives of nortropinone (14) and nor-pseudopelletierine have been prepared for a c.d. study. The octant rule can be applied to these tropanes since the carbonyl chromophore, although four Cotton effect. Thus,... 

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