Infrared Spectroscopy Flashcards by Ellie Kennedy

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

Vibrational energy.

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

Hang down.

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

Point up.

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What are the units in IR spectra?

Wavenumber (cm^-1)

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What is the equation for wavenumber?

1/λ

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

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

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Describe the symbol for wavenumber.

Greek letter nu with a line above it.

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What happens when a bond is stretched or compressed?

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

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

The equilibrium bond length (r_e)

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

The harmonic oscillator.

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What is the frequency of oscillation dependent on?

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

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

(1/2πc) x sqrt(k_f/μ)

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Give the equation for the reduced mass in kg.

μ= (m1 x m2)/(m1 + m2)

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What are the units for the force constant?

N m^-1

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What are the units for the speed of light?

cm s^-1

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

Normal modes

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

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

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

The peak increases in size.

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

No.

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

Raman Spectroscopy.

Are homonuclear diatomics Raman active or IR active?

Raman active

Are heteronuclear diatomics Raman active or IR active?

Both.

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

2900-3200 cm^-1.

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

Just under 3000cm^-1

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

Just over 3000cm^-1

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

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

Why are there two peaks for amine groups?

There are symmetric and anti-symmetric stretches.

Where are the two peaks for amine groups?

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

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

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

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

V-shaped.

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

Steric hindrance. | Makes the peak sharper.

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

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

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

Strong absorption at about 2250cm^-1.

What are the absorptions for carbon-carbon double bonds?

Weak absorption at 1635-1690cm^-1.

What are the absorptions for double bonds in benzene rings?

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

What are the two absorptions for the NO2 group?

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

Which groups strengthen carbonyl groups?

Electron-withdrawing groups.

Which groups weaken carbonyl groups?

Electron-donating groups.

What is the absorption for a ketone?

1715cm^-1.

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

1750-1820cm^-1.

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

1640-1690cm^-1.

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

1730cm^-1.

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

1745cm^-1.

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

1730cm^-1.

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

The alkyl group in ketones is weakly electron-donating.

What are the absorptions for an acid anhydride?

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

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

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

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?

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

Why does decreasing ring size increase frequency?

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.