Chemistry 321 Quiz 2
September 20, 2000 Name ______________________________
Determine and list the symmetry elements for the following molecules and then assign the molecule to the correct point group.
1. cis-dichloroethene (or cis-dichloroethylene)
Because of the double bond, the molecule is planar. Because the molecule is cis, consider the Cl's at the "top" of the trigonal arrangement around each C and the H's at the "bottom" of the arrangement.
There will be a C2 axis in the plane of the paper, through the C=C.
There are two planes: the plane of the paper and a plane perpendicular to the plane of the paper that intersect to form the C2 axis.
There are no C2 axes perpendicular to the C2 axis nor a plane perpendicular to the C2 axis - so the point group is C2v.
Benzene is a planar molecule with resonance making all the C-C bonds equivalent (no alternating single and double bonds.
Thus the principal axis is a C6 axis and there are 6 C2 axis perpendicular to it (Through opposite Carbons and the center of the bonds between the Carbons.
The plane of the paper is perpendicular to the C6 axis so that the point group is D6h.
Additional symmetry elements include a total of six vertical and dihedral planes that with the horizontal plane form the 6 C2 axes.
There is also an inversion center and an S6 axis that is coincident with the C6 axis.