IR Spectroscopy Concepts

Infrared Spectroscopy and Mass Spectrometry

• Infrared (IR) spectroscopy is a technique used to determine the structure of a compound by identifying functional groups.
• In IR spectroscopy, bonds in molecules absorb specific frequencies of infrared radiation.
• Each type of bond absorbs a unique characteristic frequency.
• This allows for the determination of the types of bonds present in a molecule.
• IR spectroscopy uses wavenumbers (cm⁻¹), which are proportional to frequency and energy.
• The signals in an IR spectrum point downwards.
• No two molecules (except enantiomers) have the same IR spectrum.

Regions of the Spectrum

• The wavenumber range of an IR spectrum is from 400 to 4000 cm⁻¹.
• The region between 400 and 1500 cm⁻¹ is called the fingerprint region.
• This region has the most complex vibrations.
• The region between 1500 and 4000 cm⁻¹ is called the diagnostic region.
• This region contains useful information for identifying functional groups in a molecule.

Characteristics of IR Spectrum

• Each signal (peak, band) in an IR spectrum has three characteristics:
  • Intensity
  • Shape
  • Wavenumber

IR Spectrum: Signal Intensity

• Some signals are stronger than others.
• The intensity of a signal is related to the strength of bond dipole moment; stronger bonds produce stronger signals.
• Bonds with larger dipole moments create stronger signals, conversely, bonds with smaller dipole moments produce weaker signals.

IR Spectrum: Signal Shape

• Signals can be broad or narrow.
• Compounds with hydrogen bonding usually produce broad signals.

IR Spectrum: Wavenumber

• The frequency (wavenumber) of a stretching vibration depends on the bond strength and the masses of the atoms involved.
  • Stronger bonds produce higher frequencies (higher wavenumbers)
  • Weaker bonds produce lower frequencies (lower wavenumbers)
  • Lighter atoms produce higher frequencies (higher wavenumbers)
  • Heavier atoms produce lower frequencies (lower wavenumbers)
    • For example, C-H is around 3000 cm⁻¹

Carbon-Carbon Double Bond Stretching

• Conjugation lowers the frequency of a C=C bond.
  • Isolated C=C: 1640–1700 cm⁻¹
  • Conjugated C=C: 1600–1640 cm⁻¹
  • Aromatic C=C: ~1600 cm⁻¹

Carbon-Carbon Triple Bond Stretching

• Terminal alkynes: signal around 2200 cm⁻¹.
• Internal alkynes: No signal around 2200 cm⁻¹. or a very weak signal

Carbon-Hydrogen Bond Stretching

• Bond strength and s character affect the frequencies of the C-H stretches:
  • sp³ C-H: 2850-3000 cm⁻¹ (Alkanes)
  • sp² C-H: 3000-3100 cm⁻¹ (Alkenes)
  • sp C-H: ~3300 cm⁻¹ (Alkynes)

IR Spectrum of Alkanes

• Alkanes show frequencies for C–H and C–C bonds.
• C–H stretching is a broad band between 2850 and 3000 cm⁻¹.

IR Spectrum of Alkenes

• Alkenes show frequencies for C=C, and C–H bonds.
• C=C signal at 1600–1700 cm⁻¹.
• C–H signal at 3000–3100 cm⁻¹.

IR Spectrum of Alkynes

• Alkynes show frequencies for C≡C, and C–H bonds.
• C≡C signal at 2100-2200 cm⁻¹.
• C–H signal at 3300 cm⁻¹ (terminal alkynes)

O–H and N–H Bond Stretching

• O–H bonds produce broad signals around 3300 cm⁻¹, with a rounded tip.
• N–H bonds produce broad signals with sharp spikes around 3300 cm⁻¹, and the number of spikes depends on the type of amine (primary, secondary, or tertiary).

IR Spectrum of Alcohols

• Alcohols show a broad, rounded O–H signal between 3200 and 3600 cm⁻¹.

IR Spectrum of Amines

• Amines show broad N–H signals between 3350 and 3500 cm⁻¹ with sharp spikes.
  • primary amines produce two sharp spikes
  • secondary amines produce one sharp spike
  • tertiary amines have no N-H signal

Carbonyl Compounds

• Carbonyl groups (C=O) show a characteristic peak around 1700 cm⁻¹.
• The carbonyl group is usually the strongest IR signal.
• The exact frequency of the C=O bond depends on the functional group.

Conjugated Carbonyl Compounds

• Conjugation lowers the carbonyl frequency.

IR Spectrum of Amides

• Amides show a strong C=O signal around 1640–1680 cm⁻¹.
• Amides show a broad N–H stretching signal around 3300 cm⁻¹, with sharp spikes.

IR Spectrum of Nitriles

• Nitriles show an intense, sharp C≡N signal around 2200-2300 cm⁻¹.

When Problem Solving

• Not all signals need to be assigned.
• Some signals may be ambiguous.
• Bond stretching or bending must involve a change in dipole moment for a signal to be observed in an IR spectrum.
• Bonds with no dipole moment don't show a signal.

Class Activities

• Several class activities are listed in the text. These generally involve correctly identifying a compound or specific bond type.
• The user should focus on applying the concepts to solve the problems.