Shapes and Structures 1 - Infrared Spectroscopy

Question 1

What information do IR spectra give?

Answer 1

Types of bonds present

Question 2

What is frequency plotted as?

Answer 2

Wavenumbers (cm^-1)

Question 3

Peaks over 1500 cm^-1 correspond to bonds doing what?

Answer 3

Stretching

Question 4

For a diatomic molecule, what can vibrations be modelled as?

Answer 4

A harmonic oscillator

Question 5

What is frequency proportional to for vibrations?

Answer 5

v ∝ sqrt(Kf/m)
v = frequency
Kf = force constant
m = mass

Question 6

How is the frequency equation adapted for stretching in diatomic molecules?

Answer 6

wavenumber = (1/2πc) sqrt(K(f)/μ)
K(f) is bond strength in N m^-1
μ is reduced mass in kg/molecule
c in cm s^-1 (so 3.00 x 10^10)

Question 7

For a diatomic molecule, what is the equation for reduced mass, μ?

Answer 7

μ = (m1m2)/(m1 + m2)

Question 8

For a diatomic molecule, what is the equation for reduced mass when m1&raquo_space; m2?

Answer 8

μ ≈ m1m2/m1

so μ ≈ m2 (lighter mass)

Question 9

How does the inverse relationship of μ and frequency explain the X-H stretch region being 2500-4000 cm^-1?

Answer 9

Smaller μ means higher frequency

μ(X-H) ≈ H so very small μ

Question 10

Aside from the X-H region, what do the other regions in IR spectra correspond to?

Answer 10

Different bond strengths

triple > double > single

Question 11

The frequency range for X-H?

Answer 11

2500-4000 cm^-1

Question 12

The frequency range for triple bonds?

Answer 12

2000-2500 cm^-1

Question 13

The frequency range for double bonds?

Answer 13

1500-2000 cm^-1

Question 14

The frequency range for the fingerprint region?

Answer 14

500-1500 cm^-1

Question 15

What is contained in the fingerprint region of an IR spectrum?

Answer 15

X-Y single bond stretches

Other vibrational modes such as bending

Question 16

How big the peak in IR spectra depends on what?

Give examples

Answer 16

The dipole moment of the bond
C=O, strong dipole moment, strong absorption
Symmetric C=C, no dipole, no absorption
Asymmetric C=C, weak dipole, weak absorption

Question 17

How can symmetrical vibrations with no change in dipole moment be determined?

Answer 17

Raman spectroscopy

Question 18

How does IR spectroscopy differ from Raman spectroscopy?

Answer 18

IR looks at frequencies of light absorbed by a sample

Raman looks at frequencies of light scattered by a sample

Question 19

For homonuclear diatomics such as O2 and N2, when will they be active?

Answer 19

Raman

Question 20

For heteronuclear diatomics such as HCl and CO, when will they be active?

Answer 20

IR and Raman

Question 21

Where are C-H stretches usually?

How does it change for tetrahedral and trigonal carbons?

Answer 21

2900 - 3200 cm^-1
Just less than 3000 cm^-1 for tetrahedral
Just over 3000 cm^-1 for trigonal

Question 22

Most C-H stretches are not mentioned as they feature in all organic compounds, what is the exception?

Answer 22

C≡C-H

Strong, sharp absorption around 3300 cm^-1

Question 23

Where are N-H stretches usually?

Answer 23

Sharp absorption around 3300 cm^-1

Question 24

NH2 groups often show two absorptions where?

Answer 24

Symmetric stretch: around 3300 cm^-1

Asymmetric stretch: around 3400 cm^-1

Question 25

Where are O-H stretches usually?

Answer 25

Alcohol: 3200-3600 cm^-1 broad absorption

Carboxylic acid: 2500-3300 cm^-1 broad absorption

Question 26

Why do O-H stretches have a broad range of values?

Answer 26

Presence/lack thereof hydrogen bonding

Hydrogen bonds weaken O-H bond and decrease the frequency

Question 27

Where are -C≡N stretches usually?

Answer 27

Strong absorption around 2250 cm^-1

Question 28

Where are -C≡C- stretches usually?

Answer 28

Weak absorption around 2100-2250 cm^-1

Question 29

Where are C=C stretches usually?

Answer 29

Weak absorption 1635-1690 cm^-1

Question 30

Where are stretches in aromatic rings usually?

Answer 30

Weak/medium absorption, number of peaks 1450-1625 cm^-1

Question 31

Where are -NO2 group stretches usually?

Answer 31

Symmetric stretch around 1350 cm^-1

Asymmetric stretch around 1530 cm^-1

Question 32

Where is the ketone C=O stretch?

Answer 32

1715 cm^-1

Question 33

Where is the acid chloride C=O stretch? Why is it different from that of a ketone?

Answer 33

1750-1820 cm^-1
Cl withdraws e- from C
Strengthens C=O bond

Question 34

Where is the amide C=O stretch? Why is it different from that of a ketone?

Answer 34

1640-1690 cm^-1
N donates e- into C=O π system
Weakens C=O bond

Question 35

Where is the carboxylic acid C=O stretch?

Answer 35

1730 cm^-1

Question 36

Where is the ester C=O stretch?

Answer 36

1745 cm^-1

Question 37

Where is the aldehyde C=O stretch? Why does it differ from a ketone?

Answer 37

1730 cm^-1

H instead of weakly e- donating alkyl group

Question 38

Where are the acid anhydride C=O stretches?

Answer 38

Symmetric stretch around 1820 cm^-1

Asymmetric stretch around 1750 cm^-1

Question 39

When does conjugation of a carbonyl occur?

Answer 39

One single bond between C=O and C=C

Question 40

What is the effect on frequency for a C=O stretch of a carbonyl by conjugation?

Answer 40

Lowers by 30 cm^-1

Question 41

Which effect is there on C=O stretching frequency if it is part of a ring?

Answer 41

Size of the ring
6 member 1715 cm^-1
5 member 1745 cm^-1
4 member 1780 cm^-1
3 member 1815 cm^-1

Question 42

Why does ring size have an effect on C=O stretching frequency?

Answer 42

Small CCC angle requires considerate C-C compression as C=O vibrates, harder so more energy required so higher frequency
Larger CCC angle requires less C-C compression, easier