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2.2: Point Groups

  • Page ID
    360810
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    Introduction

    A Point Group describes all the symmetry operations that can be performed on a molecule that result in a conformation indistinguishable from the original. Point groups are used in Group Theory, the mathematical analysis of groups, to determine properties such as a molecule's molecular orbitals.

    Assigning Point Groups

    While a point group contains all of the symmetry operations that can be performed on a given molecule, it is not necessary to identify all of these operations to determine the molecule's overall point group. Instead, a molecule's point group can be determined by following a set of steps which analyze the presence (or absence) of particular symmetry elements.

    Steps for assigning a molecule's point group:

    1. Determine if the molecule has an infinite rotation axis.
    2. If yes, determine if there is an inversion center. If no, find the highest order rotation axis, Cn, and continue.
    3. If the molecule does not contain a rotation axis, look for either a mirror plane or an inversion center. If you find at least one rotation axis, look for multiple rotation axes where the n value is greater than 2.
    4. Determine if the molecule has any C2 axes perpendicular to the principal Cn axis. If so, then there are n such C2 axes, and the molecule is in the D set of point groups. If not, it is in either the C or S set of point groups.
    5. Determine if the molecule has a horizontal mirror plane (σh) perpendicular to the principal Cn axis. If so, the molecule is either in the Cnh or Dnh set of point groups.
    6. Determine if the molecule has a vertical mirror plane (σv) containing the principal Cn axis. If so, the molecule is either in the Cnv or Dnd set of point groups. If not, and if the molecule has n perpendicular C2 axes, then it is part of the Dn set of point groups.
    7. Determine if there is an improper rotation axis, S2n, collinear with the principal Cn axis. If so, the molecule is in the S2n point group. The S2n point group is extremely unlikely to appear in this course. If not, the molecule is in the Cn point group.
    Flow Chart.pngFigure \(\PageIndex{1}\): Decision tree for determining a molecule's point group (CC-BY-NC-SA; Chip Nataro)

     

    Example \(\PageIndex{1}\)

    Find the point group of borane (BH3).

    Answer

    Solution

    1. Borane does not have an infinite rotation axis.
    2. Highest order rotation axis: C3
    3. There are not multiple axes with n greater than 2.
    4. There are 3 C2 axes perpendicular to the principal axis
    5. There is a horizontal mirror plane (σh)

    Borane is in the D3h point group.

    See Also

    Symmetry Challenge


    2.2: Point Groups is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.