C2 axis

The C2H2CI2 molecule has a ah plane of symmetry (plane of molecule), a C2 axis ( to plane), and inversion symmetry, this results in C2h symmetry. Using C2h symmetry labels... 

In Section 4.2.1 it will be pointed out that hydrogen peroxide (Figure 4.1 la) has only one symmetry element, a C2 axis, and is therefore a chiral molecule although the enantiomers have never been separated. The complex ion [Co(ethylenediamine)3], discussed in Section 4.2.4 and shown in Figure 4.11(f), is also chiral, having only a C3 axis and three C2 axes. 

The C3 axes, in the case of methane, are the directions of each of the C-FI bonds. The planes are the six planes of all the possible CFI2 fragments. The C2 axes are not quite so easy to see, but Figure 4.12(a) shows that the line of intersection of any two mutually perpendicular planes is a C2 axis. 

Flowever, for a non-planar 2 molecule, such as CFI2F2 (Figure 4.5), although it is natural to take the C2 axis to be the z axis, the choice of x and y axes is arbitrary. This example serves to demonstrate how important it is to indicate the axis notation being used. 

In a molecule such as the asymmetric rotor formaldehyde, shown in Figure 5.1(f), the a, b and c inertial axes, of lowest, medium and highest moments of inertia, respectively, are defined by symmetry, the a axis being the C2 axis, the b axis being in the yz plane and the c axis being perpendicular to the yz plane. Vibrational transition moments are confined to the a, b or c axis and the rotational selection mles are characteristic. We call them... 

Theoreticians did little to improve their case by proposing yet more complicated and obviously unreUable parameter schemes. For example, it is usual to call the C2 axis of the water molecule the z-axis. The molecule doesn t care, it must have the same energy, electric dipole moment and enthalpy of formation no matter how we label the axes. I have to tell you that some of the more esoteric versions of extended Hiickel theory did not satisfy this simple criterion. It proved possible to calculate different physical properties depending on the arbitrary choice of coordinate system. 

The aminalizalion of A.A -dimethyl hydrazone 5a derived from glyoxal 1 with enantiomerically pure 1,2-diamines 7a and 7b25 26 containing a C2 axis of symmetry is a straightforward process (in contrast to the acctalization with diols). 

In order to synthesize 1,3-diphenyl-l, 3-diamines 3 containing a C2 axis of symmetry, which can be employed as auxiliaries and controller groups in asymmetric syntheses, the diastereoselective addition of organometallic reagents to racemic pyrazolines 2, prepared from cinnamaldehyde (l)23, was investigated. 

Furan is the prototype of molecules that carry both non-bonding and aromatic tt bonding electron pairs. The usual model for the n-pair and tt electron density in this molecule is shown in Fig. 18. The oxygen atom is taken to have a non-bonding electron pair in an orbital whose symmetry axis coincides with the C2 axis of furan. 

Fig. 18 The n-pair/jt-pair model of furan together with the experimental geometries of furan- -HC1 and furan- -C1F. Furan- -HC1, which has a planar geometry of C2V symmetry with HC1 lying along the C2 axis, clearly obeys rule 3 but the observed face-on arrangement for furan- -C1F demonstrates that rule 3 is violated in this case. See Fig. 1 for key to the colour coding of atoms...

Interconversion of the two enantiomers is possible only if the molecule is removed from the surface and rotated by 180° around an axis parallel to the substrate surface. In the case of PVBA adsorbed on Ag l 11, hydrogen bonding leads to a preference for homochiral double chains based on head-to-tail N—H—O bonds and a C2 axis relating the two strands of the chains. The chirality of the chain can be recognized in the STM images by the stagger of one strand relative to the other that arises from C—H—O bonds, as shown in Figure 1.3 [6],... 

Sixfold barriers to internal rotation occur in molecules such as toluene andp-fluoro-toluene whose molecular frame has C2v symmetry about the rotor axis. The simplest spectroscopic model of internal methyl rotation assumes a rigid, threefold symmetric methyl rotor attached to a rigid molecular frame with the C2 axis coincident with the rotor top axis.25 The effective one-dimensional sixfold torsional potential takes the traditional form,... 

Finally, we return to 2-fluoro-6-chlorotoluene (Figure 10), for which it <— it excitation and n ionization cause only minor changes in the rotor potential. In this disubstituted case, the appropriate pseudo-C2 axis would coincide with the methyl rotor axis itself. We expect the Hiickel orbitals to align with the rotor axis, which would make 0a = Ob in all three electronic states, much as in sixfold cases. Accordingly, there is little effect of electronic excitation on the barrier height, which is presumably dominated by steric repulsion in all three electronic states. In terms of ic-electron effects, 2-fluoro-6-chlorotoluene behaves much like a sixfold case. 

Transannular cyclizations of nine-membered cyclic sulfides provided a novel route to thioanalogues of swainsonine derivatives. Treatment of the enantiopure polyhydroxylated thiacyclononane derivative 48a with trimethylsilyl iodide yielded the bicyclic sulfonium salt 49a as a single diastereomer, as shown in Equation (16) <2003JOC3311 >. The presence of a C2-axis and the complete regio- and stereoselectivity of the transannular substitution led to a single configurationally homogeneous product irrespective of the fact that the sulfur atom would attack C-5 or C-6. 

In addition to the C3 axis (the z-axis), there are also three vertical planes of symmetry which can be seen clearly in Figure 5.1. They are perpendicular to the plane of the molecule and bisect the molecule along each B-F bond. Because the molecule is planar, there is also a horizontal plane of symmetry (crj,) that bisects all four of the atoms. Each B-F bond is also a C2 axis because rotation around the bond would give the same orientation of the molecule except for changing the positions of the fluorine atoms. One of the C2 axes is shown above, and rotation by 180° around that axis would produce the orientation... 

Note that each of the C2 axes not only is coincident with a B-F bond but also is the line of intersection of the horizontal plane with one of the vertical planes. It is generally true that the intersection of a vertical plane of symmetry with a horizontal plane generates a C2 axis. The list of symmetry elements that we have found for the BF3 molecule includes one C3 axis, three vertical planes (horizontal plane (ah). A molecule possessing these symmetry elements, such as BF3, S03, C03 , and N03 , is said to have l)ih symmetry. In the cases of H20, C1F3, H2CO, and NH3, the symmetry elements included only a C axis and n vertical planes. These molecules belong to the general symmetry type known as C . Molecules that have a Cn axis and also have n C2 axes perpendicular to the C axis are known as Dn molecules. 

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