Chapter 3 - Molecular symmetry

Question 1

Symmetry operation

Answer 1

An operation performed no an object which leaves it in a configuration that is indistinguishable from, and superimposable on, the original configuration.

Question 2

Symmetry element

Answer 2

A symmetry operation carried out with respect to points, lines, or planes.

Question 3

Principal axis

Answer 3

If a molecule contains more than one type of n-axis, the axis of highest value is the principal axis.

Question 4

Centrosymmetric

Answer 4

A molecule or ion that contains a center of symmetry.

Question 5

Non-centrosymmetric

Answer 5

A molecule or ion that does not contain a center of symmetry.

Question 6

To find the total number of operations in a point group:

Answer 6

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)

Question 7

In character tables, A and B indicate

Answer 7

non-degenerate

Question 8

In character tables, E refers to

Answer 8

doubly degenerate

Question 9

In character tables, T refers to

Answer 9

triply degenerate

Question 10

IR and Raman spectroscopy are _________ techniques.

Answer 10

vibrational

Question 11

Vibrational spectroscopy is concerned with the observation of

Answer 11

the degrees of vibrational freedom.

Question 12

Number of degrees of vibrational freedom for a non-linear molecule

Answer 12

= 3n - 6

Question 13

Number of degrees of vibrational freedom for a linear molecule

Answer 13

= 3n - 5

Question 14

For a mode of vibration to be IR active, it must

Answer 14

give rise to a change in the molecular electric dipole moment.

Question 15

For a mode of vibration to be Raman active, it must

Answer 15

give rise to a change in the polarizability of the molecule.

Question 16

Polarizability

Answer 16

the ease with which the electron cloud associated with the molecule is distorted.

Question 17

Rule of mutual exclustion

Answer 17

For centrosymmetric molecules, vibrations that are IR active are Raman inactive, and vice versa.

Question 18

Fundamental transition

Answer 18

The transition from the vibrational ground state to the first excited state.

Question 19

IR active symmetry labels

Answer 19

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.

Question 20

Raman active symmetry labels

Answer 20

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.

Question 21

A molecule is chiral if

Answer 21

it is non-superposable on its mirror image.

Question 22

A chiral molecule lacks ______________ axis of symmetry

Answer 22

an improper (Sn)