Vibrations of Polyatomic Molecules (4)

Question 1

How many fundamental molecular vibrations are possible for a nonlinear polyatomic molecule?

Answer 1

3n-6

Question 2

How many fundamental molecular vibrations are possible for a linear polyatomic molecule?

Answer 2

3n-5

Question 3

how many of these vibrations will be caused by bond stretching

Answer 3

n-1

Question 4

How many vibrations are caused by bond bending motions for nonlinear molecules

Answer 4

2n-5

Question 5

How many vibrations are caused by bond bending motions for linear molecules

Answer 5

2n-4

Question 6

What is a normal mode of vibration

Answer 6

fundamental molecular vibrations; molecular motion where all atoms move in phase and at the same frequency

Question 7

What is a polar bond?

Answer 7

unequal distribution of electrons (ex: c-cl)

Question 8

what is a non polar bond?

Answer 8

even distribution of electrons (ex: c-c)

Question 9

What is the dipole moment of a bond?

Answer 9

strength of polarity

Question 10

how can chemical bonds and their vibrations be described in terms of molecular symmetry?

Answer 10

Stretching

Question 11

Why are dipole moments important in IR spectroscopy?

Answer 11

Charge separation with magnitude and direction changes symmetry, therefore identify bond stretching

Question 12

In order to absorb IR radiation, what must happen to the dipole moment during the molecular vibration?

Answer 12

vibration must cause permanent change in dipole moment required for IR absorption

Question 13

What are degenerate vibrations

Answer 13

modes with equivalent energy

Question 14

how is IR activity related to the permanent dipole moment of the chemical bond

Answer 14

activity requires permanent dipole moment

Question 15

What is the IR activity of polar bonds with large values of µ?

Answer 15

very strong

Question 16

What is the IR activity of nonpolar bonds with small values of µ?

Answer 16

weak or absent

Question 17

What are group frequencies used for in IR

Answer 17

makes IR very sensitive probe of structure

Question 18

What are group frequencies

Answer 18

certain sub molecular groups of atoms consistently produce vibrational bands in characteristic frequency regions of the vibrational spectrum

Question 19

how do group frequencies originate

Answer 19

due to coupling oscillating dipole moments

Question 20

group frequencies can be very sensitive to

Answer 20

configuration and conformation of local molecular groupings

Question 21

Is symmetric stretching IR active

Answer 21

No

Question 22

Sulfur dioxide is a nonlinear molecule. How many vibrational modes will the compound have? How many IR absorption bands?

Answer 22

3N-6 or (3x3) -6 = 3 vibrational modes. Includes symmetric and anti symmetric stretching and a bending/scissoring

Question 23

Of these vibrations how many are bending motions

Answer 23

Linear 2N - 4, nonlinear 2N-5