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Group Theory and Symmetry .

Group Theory and Symmetry. two-fold rotational axis. water molecule. Molecular Structure:.

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Group Theory and Symmetry .

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  1. Group Theory and Symmetry. two-fold rotational axis water molecule

  2. Molecular Structure: The most powerful idea in chemistry is the idea of the three-dimensional structures of molecules. Two techniques have been invaluable in this regard. One is NMR (Nuclear Magnetic Resonance), and the other is X-ray crystallography. X-ray crystallography has been intensively developed as a technique, which involves the ideas of symmetry of molecules. Understanding NMR also involves an understanding of symmetry. Group theory is also vital in understanding and predicting infra-red and Uv-visible (electronic) spectra. On the next two slides are structures of complexes of metal ions determined by X-ray crystallography. These are shown simply to illustrate the power of X-ray crystallography in determining molecular structure. Determining such structures relies heavily on a knowledge of symmetry and group theory.

  3. Crystals of [Cd(DPP)2](ClO4)2(viewed through a microscope)

  4. X-ray diffractometer: Monochromatic X-ray source, e.g. Cu Kα X-rays

  5. X-ray diffraction pattern:

  6. The actual structure of the [Cd(DPP)2]2+ complex cation: DPP Cd(II) cation DPP ligand G. M. Cockrell, R. D. Hancock, D. G. VanDerveer, G. Zhang, R. P. Thummel, J. Am. Chem. Soc., 2008, 130, 1420.

  7. Importance of X-ray crystallography N-U-O angle = 63.8(2)o oxo (O2-) anion PDA ligand uranium atom U-O bond = 2.279(6) Å Structure of [UO2(PDA)] determined by X-ray crystallography Nolan E. Dean, R. D. Hancock, M Frisch, C. Cahill, Inorg. Chem., 2008 in the press.

  8. Unit cell of [UO2(PDA)]

  9. Structures of proteins.

  10. Facial symmetry

  11. Invariance to transformation as an indicator of facial symmetry: Mirror image

  12. a a

  13. Transformations of the benzene molecule: The presence of a symmetry element is identified by the fact that we can carry out a symmetry operation without the molecule appearing to have changed. Thus, for the benzene molecule, rotation by 60o about the six-fold rotation axis does not change its appearance: six-fold rotation axis a rotate by 60o a The rotation axis is a six-fold rotation axis because we can repeat the operation six times before we get back to the original orientation of the benzene molecule

  14. C3 C3 or three-fold rotational axis of the ammonia molecule. If we rotate the ammonia molecule by 360/3 or 120º about this axis, its appearance is unchanged.

  15. Rotational axes of BF3 principal axis (highest value of Cn) C3 C3 C2 C2 . three-fold axis three-fold axis two-fold axis two-fold axis viewed from viewed from viewed from viewed from above the side the side above Note: there are 3 C2 axes

  16. Mirror planes (σ) of BF3: Mirror planes can contain the principal axis (σv) or be at right angles to it (σh). BF3 has one σh and three σv planes: (v = vertical, h = horizontal) σv mirror plane σh mirror plane C3 principal axis C3 principal axis σv mirror plane contains the C3 axis σh mirror plane is at right angles to the C3 axis

  17. center of symmetry center of symmetry (Note: The center of symmetry is important in deciding whether orbitals are g or u (lecture 2.))

  18. rotate by 360o/4 The S4 improper rotation axis here is also a C2 axis

  19. Rotational axes and mirror planes of the water molecule: C2 principal axis σv mirror plane C2 σv mirror plane C2 The water molecule has only one rotational axis, its C2 axis, which is also its principal axis. It has two mirror planes that contain the principal axis, which are therefore σv planes. It has no σh mirror plane, and no center of symmetry.

  20. Rotational axes and mirror planes of benzene C6 principal axis C2 C2 C2 C2 C6 σh σv σv C6 principal axis C6 principal axis

  21. Rotational axes and mirror planes of boron trifluoride C2 C3 principal axis C2 C2 σh σv σv σh boron trifluoride has a C3 principal axis and three C2 axes, a σh mirror plane three σv mirror planes, but no center of inversion C3 principal axis

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