IR Spectroscopy Notes

General IR Rules

• Polar Bonds and Intensity
  • More polar bonds result in more intense absorption bands.
• Bond Strength and Frequency
  • Stronger bonds lead to higher frequency vibrations.
• Atomic Mass and Wavenumbers
  • Lighter atoms correlate with higher wavenumbers; stronger, shorter bonds absorb at higher wavenumbers.

Specific Functional Groups and Absorption Characteristics

Alkynes

• Terminal Alkyne (R–C≡C–H)
  • sp C–H stretch: ~3300 cm⁻¹
  • C≡C stretch: <2200 cm⁻¹

Alkenes

• C=C stretch: Weak to moderate at >1600 cm⁻¹
• sp² C–H stretch: >3000 cm⁻¹

Aromatics

• C=C stretches:
  • 1600–1585 cm⁻¹
  • 1500–1400 cm⁻¹
• sp² C–H stretch: >3000 cm⁻¹

Alkanes

• sp³ C–H stretch: <3000 cm⁻¹

Alcohols

• O–H stretch:
  • Large, rounded, broad peak
  • Range: 3500–3200 cm⁻¹

Carboxylic Acids

• O–H stretch:
  • Very broad, range: 3300–2500 cm⁻¹
• C=O stretch:
  • Approximate value: ~1710 cm⁻¹

Amines (Primary & Secondary)

• N–H stretch:
  • Not super broad
  • Strong, “pointy” peaks
  • Range: <3500 cm⁻¹

Aldehydes

• C–H stretches:
  • Range: ~2700–2800 cm⁻¹
• C=O stretch:
  • Strong, sharp peak
  • Range: 1820–1630 cm⁻¹

Ketones

• C=O stretch:
  • Strong, sharp peak
  • Ranges: 1820–1630 cm⁻¹ and ~1710 cm⁻¹

Amides

• C=O stretch
• N–H stretch

Esters

• C=O stretch
• C–O stretches:
  • Range: 1000–1200 cm⁻¹

Ethers

• C–O stretches:
  • Located in ester-like region

Nitriles and Imines

• Nitriles (R–C≡N) and Imines (R₂C=NR′)
  • Similar to C≡C: ~2200 cm⁻¹
  • C=C (imines): ~1600 cm⁻¹
  • Presence of N results in more intense absorption and higher wavenumbers.