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Download Principles of Organic Chemistry POINT GROUP SYMMETRY
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Principles of Organic Chemistry lecture 3, page 1 POINT GROUP SYMMETRY • Assigning some point groups to molecules. 1,2-diaminocyclohexane, C2 2,3-pentadiene, C2 • Do you notice anything peculiar about the objects/molecules above? allene, C3H4, D2d ethylene twisting, flat D2h; n°, C2 ; 90°, D2d acetylene, a rod, D∞h • Infinite number of operations a spike, a party ballon, HCl, C∞v • Infinite number of operations diborane B2H6, D2h ferrocene H H H H H Fe H H • • • H H H D5d Now let’s systematically break the symmetry in ferrocene with some ethyl substituents. o The ethyl group is in the plane of the cyclopentadienyl ligand Fe • o We destroyed all σ (internal reflection) planes. o The point group is now S10. o This ferrocene is achiral despite the reduction in symmetry due to the presence of the improper rotation coincident on the major rotational axis. σ planes or S rotations (improper symmetry) Principles of Organic Chemistry lecture 3, page 2 specifies achiral point groups. methane, Td • This is a high symmetry point group that will easily yield to inspection. • 24 symmetry operations o You will not be asked to repeat them in a test. o You don’t have to remember them all. o There are 24 symmetry operations generated from the following elements o E, 4C3, 3C2, 3S4 (collinear with C2), 3σd. For each C3, we have a C32. For each S4 we have an S42 = S2 = i and we have an S43. So, 1 + 8 +3 + 9 + 3 = 24 a cube, cubane, UF6, Oh. • This is a high symmetry point group that will easily yield to inspection. • 48 symmetry operations. o E, 3C4, 4C3, 4C2, S6, S4, 3σh, 3σd. C60, Ih. • 120 symmetry operations. o C6, C5, C3, C2, etc. o Soccer ball The light cast by a distant star in the void of space • Infinite number of operations • Kh. • Which one is more symmetric out of the last four mentioned? Which one is next? Which one next and then least? o Show how td and oh differ by inscribing a tetrahedron in a cube on the board and asking the students to name the symmetry ops that are missing in the td group. we should find 8S6, 3σh in the cube • The more symmetry elements an object has the more symmetric it is. • The sphere as an infinite amount of all symmetry elements. • All point groups in terms of symmetry are measured against the sphere, Kh, which has infinite symmetry ops. Principles of Organic Chemistry lecture 3, page 3 o the sphere is isotropic all else is anisotropic.
