Question

(a) Identify the symmetry elements in ethene and in allene, and assign each molecule to a point group. (b) Consider the biphenyl molecule, Ph-Ph, in which different conformations are possible according to the value of the dihedral angle between the planes of the two benzene rings: if this angle is $0^{\circ}$ the molecule is planar, if it is $90^{\circ},$ the two rings are perpendicular to one another. For each of the following dihedral angles, identify the symmetry elements (i) $0^{\circ}$ (iii) $45^{\circ}$ present and hence assign the point group: (ii) $90^{\circ}$ (iv) $60^{\circ}$

          (a) Identify the symmetry elements in ethene and in allene, and assign each molecule to a point group.
(b) Consider the biphenyl molecule, Ph-Ph, in which different conformations are possible according to the value of the dihedral angle between the planes of the two benzene rings: if this angle is $0^{\circ}$ the molecule is planar, if it is $90^{\circ},$ the two rings are perpendicular to one another. For each of the following dihedral angles, identify the symmetry elements
(i) $0^{\circ}$
(iii) $45^{\circ}$ present and hence assign the point group:
(ii) $90^{\circ}$
(iv) $60^{\circ}$

Added by Allison B.

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