Shapes and Structures 1 - Infrared Spectroscopy Flashcards

(42 cards)

1
Q

What information do IR spectra give?

A

Types of bonds present

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2
Q

What is frequency plotted as?

A

Wavenumbers (cm^-1)

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3
Q

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

A

Stretching

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4
Q

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

A

A harmonic oscillator

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5
Q

What is frequency proportional to for vibrations?

A

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

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6
Q

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

A

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)

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7
Q

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

A

μ = (m1m2)/(m1 + m2)

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8
Q

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

A

μ ≈ m1m2/m1

so μ ≈ m2 (lighter mass)

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9
Q

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

A

Smaller μ means higher frequency

μ(X-H) ≈ H so very small μ

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10
Q

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

A

Different bond strengths

triple > double > single

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11
Q

The frequency range for X-H?

A

2500-4000 cm^-1

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12
Q

The frequency range for triple bonds?

A

2000-2500 cm^-1

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13
Q

The frequency range for double bonds?

A

1500-2000 cm^-1

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14
Q

The frequency range for the fingerprint region?

A

500-1500 cm^-1

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15
Q

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

A

X-Y single bond stretches

Other vibrational modes such as bending

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16
Q

How big the peak in IR spectra depends on what?

Give examples

A

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

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17
Q

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

A

Raman spectroscopy

18
Q

How does IR spectroscopy differ from Raman spectroscopy?

A

IR looks at frequencies of light absorbed by a sample

Raman looks at frequencies of light scattered by a sample

19
Q

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

20
Q

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

21
Q

Where are C-H stretches usually?

How does it change for tetrahedral and trigonal carbons?

A

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

22
Q

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

A

C≡C-H

Strong, sharp absorption around 3300 cm^-1

23
Q

Where are N-H stretches usually?

A

Sharp absorption around 3300 cm^-1

24
Q

NH2 groups often show two absorptions where?

A

Symmetric stretch: around 3300 cm^-1

Asymmetric stretch: around 3400 cm^-1

25
Where are O-H stretches usually?
Alcohol: 3200-3600 cm^-1 broad absorption | Carboxylic acid: 2500-3300 cm^-1 broad absorption
26
Why do O-H stretches have a broad range of values?
Presence/lack thereof hydrogen bonding | Hydrogen bonds weaken O-H bond and decrease the frequency
27
Where are -C≡N stretches usually?
Strong absorption around 2250 cm^-1
28
Where are -C≡C- stretches usually?
Weak absorption around 2100-2250 cm^-1
29
Where are C=C stretches usually?
Weak absorption 1635-1690 cm^-1
30
Where are stretches in aromatic rings usually?
Weak/medium absorption, number of peaks 1450-1625 cm^-1
31
Where are -NO2 group stretches usually?
Symmetric stretch around 1350 cm^-1 | Asymmetric stretch around 1530 cm^-1
32
Where is the ketone C=O stretch?
1715 cm^-1
33
Where is the acid chloride C=O stretch? Why is it different from that of a ketone?
1750-1820 cm^-1 Cl withdraws e- from C Strengthens C=O bond
34
Where is the amide C=O stretch? Why is it different from that of a ketone?
1640-1690 cm^-1 N donates e- into C=O π system Weakens C=O bond
35
Where is the carboxylic acid C=O stretch?
1730 cm^-1
36
Where is the ester C=O stretch?
1745 cm^-1
37
Where is the aldehyde C=O stretch? Why does it differ from a ketone?
1730 cm^-1 | H instead of weakly e- donating alkyl group
38
Where are the acid anhydride C=O stretches?
Symmetric stretch around 1820 cm^-1 | Asymmetric stretch around 1750 cm^-1
39
When does conjugation of a carbonyl occur?
One single bond between C=O and C=C
40
What is the effect on frequency for a C=O stretch of a carbonyl by conjugation?
Lowers by 30 cm^-1
41
Which effect is there on C=O stretching frequency if it is part of a ring?
``` Size of the ring 6 member 1715 cm^-1 5 member 1745 cm^-1 4 member 1780 cm^-1 3 member 1815 cm^-1 ```
42
Why does ring size have an effect on C=O stretching frequency?
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