Chapter 3 - Molecular symmetry Flashcards
Symmetry operation
An operation performed no an object which leaves it in a configuration that is indistinguishable from, and superimposable on, the original configuration.
Symmetry element
A symmetry operation carried out with respect to points, lines, or planes.
Principal axis
If a molecule contains more than one type of n-axis, the axis of highest value is the principal axis.
Centrosymmetric
A molecule or ion that contains a center of symmetry.
Non-centrosymmetric
A molecule or ion that does not contain a center of symmetry.
To find the total number of operations in a point group:
assign 1 for C or S, 2 for D, 12 for T, 24 for O, and 60 for I
multiply by n for a numerical subscript
multiply by 2 for a letter subscript (s, v, d, h, i)
(For example, C3v is 1 x 3 x 2 = 6)
In character tables, A and B indicate
non-degenerate
In character tables, E refers to
doubly degenerate
In character tables, T refers to
triply degenerate
IR and Raman spectroscopy are _________ techniques.
vibrational
Vibrational spectroscopy is concerned with the observation of
the degrees of vibrational freedom.
Number of degrees of vibrational freedom for a non-linear molecule
= 3n - 6
Number of degrees of vibrational freedom for a linear molecule
= 3n - 5
For a mode of vibration to be IR active, it must
give rise to a change in the molecular electric dipole moment.
For a mode of vibration to be Raman active, it must
give rise to a change in the polarizability of the molecule.
Polarizability
the ease with which the electron cloud associated with the molecule is distorted.
Rule of mutual exclustion
For centrosymmetric molecules, vibrations that are IR active are Raman inactive, and vice versa.
Fundamental transition
The transition from the vibrational ground state to the first excited state.
IR active symmetry labels
If the symmetry label (A1, B1, E) of a normal mod of vibration is associated with x, y, or z in the character table, then the mode is IR active.
Raman active symmetry labels
If the symmetry label (A1, B1, E) of a normal mod of vibration is associated with a product term (x^2, xy, etc.) in the character table, then the mode is Raman active.
A molecule is chiral if
it is non-superposable on its mirror image.
A chiral molecule lacks ______________ axis of symmetry
an improper (Sn)
