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Chemistry > Infrared Spectroscopy > Flashcards

Infrared Spectroscopy Flashcards

1
Q

What sort of energy transitions are used in infrared (IR) spectroscopy?

A

Vibrational energy.

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2
Q

In transmission spectra, do the peaks point up or hang down?

A

Hang down.

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3
Q

In absorption spectra, do the peaks point up or hang down?

A

Point up.

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4
Q

What are the units in IR spectra?

A

Wavenumber (cm^-1)

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5
Q

What is the equation for wavenumber?

A

1/λ

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6
Q

What is the equation for frequency that relates the speed of light to wavenumber?

A

frequency (ν) = the speed of light (c) x wavenumber

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7
Q

Describe the symbol for wavenumber.

A

Greek letter nu with a line above it.

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8
Q

What happens when a bond is stretched or compressed?

A

The energy rises and there is a restoring force that attempts to return the bond to its equilibrium length.

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9
Q

What is the bond length when the bond is at its lowest energy?

A

The equilibrium bond length (r_e)

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10
Q

What is a useful model for describing extending and compressing bonds?

A

The harmonic oscillator.

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11
Q

What is the frequency of oscillation dependent on?

A

The mass of the weight on the ends of the bond, and how hard it is to stretch the bond.

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12
Q

What is the equation, based on Hooke’s Law, for the frequency of vibration (wavenumber) of a diatomic?

A

(1/2πc) x sqrt(k_f/μ)

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13
Q

Give the equation for the reduced mass in kg.

A

μ= (m1 x m2)/(m1 + m2)

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14
Q

What are the units for the force constant?

A

N m^-1

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15
Q

What are the units for the speed of light?

A

cm s^-1

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16
Q

What can the vibrations of a molecule be broken down into?

A

Normal modes

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17
Q

Why does the size of the peak depend on the dipole moment of the bond?

A

Light is an oscillating magnetic field, so can interact with a vibrating dipole.

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18
Q

What happens to the size of the peak the larger the dipole moment is?

A

The peak increases in size.

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19
Q

If there is no dipole moment, even though the bond is still vibrating, does the bond show any absorption?

A

No.

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20
Q

What technique can be used to determine the frequencies of symmetrical vibrations?

A

Raman Spectroscopy.

21
Q

Are homonuclear diatomics Raman active or IR active?

A

Raman active

22
Q

Are heteronuclear diatomics Raman active or IR active?

A

Both.

23
Q

In what range of frequencies do C-H stretches usually fall?

A

2900-3200 cm^-1.

24
Q

In what part of the C-H stretch range do bonds on tetrahedral carbons fall?

A

Just under 3000cm^-1

25
Q

In what part of the C-H stretch range do bonds on trigonal carbons fall?

A

Just over 3000cm^-1

26
Q

What is the peak like for C-H bond attached to an alkyne group?

A

Unusually strong, sharp peak at about 3300cm^-1.

27
Q

Why are there two peaks for amine groups?

A

There are symmetric and anti-symmetric stretches.

28
Q

Where are the two peaks for amine groups?

A

Symmetric stretch has a peak at about 3300cm^-1 and the anti-symmetric stretch gives a peak at 3400cm^-1.

29
Q

Why are there broad peaks for O-H absorptions when hydrogen bonding takes place?

A

When there is hydrogen bonding, there are lots of different bond strengths in the sample due to different degrees of hydrogen bonding.

30
Q

What is the shape of the peak for an O-H group in a carboxylic acid?

A

V-shaped.

31
Q

Why might hydrogen bonding not be possible even when there is an O-H bond present? What effect does this have on the peak?

A

Steric hindrance.

Makes the peak sharper.

32
Q

What is the absorption for a carbon-carbon triple bond?

A

Weak absorption at about 2100 - 2250cm^-1.

33
Q

What is the absorption for the C-N bond in a nitrile?

A

Strong absorption at about 2250cm^-1.

34
Q

What are the absorptions for carbon-carbon double bonds?

A

Weak absorption at 1635-1690cm^-1.

35
Q

What are the absorptions for double bonds in benzene rings?

A

Weak to medium intensity absorptions at 1450-1625cm^-1.

36
Q

What are the two absorptions for the NO2 group?

A

Symmetric stretch is at 1350cm^-1, and the anti-symmetric stretch is at 1530cm^-1.

37
Q

Which groups strengthen carbonyl groups?

A

Electron-withdrawing groups.

38
Q

Which groups weaken carbonyl groups?

A

Electron-donating groups.

39
Q

What is the absorption for a ketone?

A

1715cm^-1.

40
Q

What is the absorption for the carbonyl group in an acid chloride?

A

1750-1820cm^-1.

41
Q

What is the absorption for the carbonyl group in an amide?

A

1640-1690cm^-1.

42
Q

What is the absorption for the carbonyl group in a carboxylic acid?

A

1730cm^-1.

43
Q

What is the absorption for the carbonyl group in an ester?

A

1745cm^-1.

44
Q

What is the absorption for the carbonyl group in an aldehyde?

A

1730cm^-1.

45
Q

Why does an aldehyde have a higher absorption than a ketone?

A

The alkyl group in ketones is weakly electron-donating.

46
Q

What are the absorptions for an acid anhydride?

A

Symmetric stretch is 1820cm^-1, and anti-symmetric stretch is 1750cm^-1.

47
Q

What is the effect of conjugation on the carbonyl bond and its absorption?

A

It weakens it by 20-30cm^-1.

48
Q

If the carbonyl group forms part of a ring, what is the effect of decreasing the ring size by one carbon on the stretching frequency?

A

It increases the stretching frequency by about 30cm^-1.

49
Q

Why does decreasing ring size increase frequency?

A

The smaller the angle in the ring, the more the carbon-carbon bonds need to be compressed in a vibration, which requires more energy, increasing the frequency.

Chemistry (9 decks)

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