Deck 3: Molecular Symmetry

Question 1

Select the correct statement:

A) Symmetry operations are actions that leave the molecule apparently unchanged.
B) Reflection operation has inversion centre as its symmetry element.
C) Identity operation involves rotating an object for 360 around a vertical axis.

Answer 1

A

Question 2

Select the correct statement about symmetry elements:

A) Centre of inversion is a point that is found in between the object and its inverse image; its symbol is C_i.
B) Rotation about a two-fold symmetry axis by 180 leaves the molecule unchanged. The symbol for this axis is C₂.
C) One mirror plane, , cuts the molecule in two or more identical parts. Only planar molecules have mirror planes.

Answer 2

B

Question 3

What is the point group of a AB₂ molecule?

A) If the molecule is linear, the point group is D_∞h, but if it is bent then the point group is C_2v.
B) If the molecule is linear, the point group is C_∞h, but if it is bent then the point group is D_2h.
C) If the molecule is linear, the point group is C_∞h, but if it is bent then the point group is D_∞h.

Answer 3

A

Question 4

To which point group does the tetrahedral SiF₂Cl₂ molecule belong?

A) D_4h
B) C_2v
C) T_d

Question 5

Looking at the character table for C_3v point group, we see that z² quadratic function has A₁ symmetry. What does this tell us?

A) The z-axis has the highest group order.
B) The rotation around z axis is a symmetrical operation.
C) The d_z2 atomic orbital remains unchanged under all symmetry operations of this group.

Question 6

Is tetrahedral SiF₂Cl₂ molecule polar and why?

A) Yes, because it has a centre of inversion.
B) No, because it does not have a centre of inversion.
C) Yes, because it does not have a centre of inversion.

Question 7

The molecule SiFClBrI is chiral because:

A) it is has both tetrahedral geometry and tetrahedral symmetry but four different substituents on Si.
B) it has tetrahedral geometry, but not tetrahedral symmetry and hence has no S_n axis.
C) It has tetrahedral symmetry but not tetrahedral geometry and hence lacks the centre of inversion.

Question 8

A molecule with general formula AB₂ has:

A) 3N-5 vibrational modes if it is linear and 3N-6 vibrational modes if it is non-linear.
B) 5N-3 vibrational modes if it is linear and 6N-3 vibrational modes if it is non-linear
C) 3N-3 vibrational modes if it is linear and 3N-5 vibrational modes if it is non-linear

Question 9

How can we differentiate bent and linear structures for AB₂ molecules using IR spectroscopy?

A) Bent structure will show both symmetric and antisymmetric vibrations, linear only antisymmetric.
B) Linear structure will show both symmetric and antisymmetric vibrations, bent only symmetric.
C) We cannot do conclusive differentiation without Raman spectroscopy.

Question 10

When is a vibrational mode of a molecule Raman active?

A) When the vibrational mode has the same symmetry as a component of the molecule's electric dipole moment vector.
B) When the vibrational mode has the same symmetry as a component of the molecule's polarizability.
C) Vibrational modes of molecules are Raman active only; they cannot be IR active at the same time