Symmetry planes, carbonyl

The CD of the ketone will not be affected by substituents having counterparts symmetrically placed across the carbonyl symmetry planes or by atoms lying in the symmetry planes. 

The shape of the third octant surface, XY, is of importance only in cases where perturbers lie in front of the carbonyl oxygen. The substituents lying in symmetry planes XZ and YZ may in fact give very small, but nonzero, contributions, as in chiral molecules the C = 0 symmetry-derived octant surfaces are distorted from planarity11. 

The point here is that the entire symmetry classification of reaction (69) is based on the presumption that a plane of symmetry is conserved thoughout the hydrogen abstraction process (the plane incorporating the C—H bond and the carbonyl group). If one or more atoms are displaced slightly so as to destroy the symmetry plane the crossing becomes avoided. A comparison of the in-plane and slightly out-of-plane cases is illustrated in Fig. 16a and b (Salem etal., 1975). 

Fig. 16 Energy diagram for the hydrogen abstraction reaction of ketones showing the interaction of the ground state and n,rc excited state curves, (a) In-plane approach. A real crossing occurs when the carbonyl group and the C—H bond lie in a plane since the two state curves are of opposite symmetry, (b) Out-of-plane approach. An avoided crossing (dotted lines) occurs when the symmetry plane is destroyed and the two states can mix. (Adapted from Salem et al., 1975)...

FIGURE 4. (a) Coordinate system for C=0 group, (b) Relevant n-orbital of ketone carbonyl n - tt transition. The vertical XZ plane is a nodal plane for the n-orbital that bisects the R—C—R angle and lies perpendicular to the R(R )C=0 local symmetry plane, (c) Relevant w -orbital of ketone n - tt transition. The horizontal YZ plane is a nodal plane for the tt orbital and lies on the R(R )C=0 local symmetry plane, (d) Carbonyl oxygen, as viewed looking down the Z-axis from O to C, showing circular movement of electron from the n to the tt orbital... 

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). 

Silylcarbynes 2a and 2b are characterized by two strong stretching frequencies in their infrared spectra (at 1997 and 1911 cm-l for 2a 1982 and 1889 cm l for 2b). NMR spectroscopy indicates Cg molecular symmetry, as a 2 1 out-of-plane in-plane pattern is observed for those signals associated with the three pyrazole rings of the Tp ligand. The symmetry plane contains the M=C bond and one Tp pyrazole ring and bisects the two terminal carbonyls. Additional resonances are easily assigned to the silicon substituents. 

The first type comprises the reactions in which intersection of the surfaces occurs with retention of a certain symmetry element thus garanteeing the validity of the second equality of Eq. (1.43). In this connection, one may point to the important case of retention of the symmetry plane which provides for separation and orthogonality of the a and n orbitals and for the absence of interaction between the electrons in these orbitals. An example is given by the photochemical reaction of splitting off of hydrogen by carbonyl compounds leading to the biradical state and the transition of one of the electrons from the a to the n orbital ... 

Figure 10 Octant projection of a compound with a carbonyl group showing a CE induced by the second sphere. Each atom outside the symmetry planes contributes to the CD. The contributions of atoms which are in mirror image positions compensate.

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