IR Absorption Spectroscopy

Infrared Spectroscopy Overview

• Infrared spectroscopy measures the interaction of infrared radiation with matter via absorption, emission, or reflection for identifying chemical substances.
• The technique yields an IR absorption spectrum, which serves as a unique fingerprint for chemical identification through characteristic functional groups and bonds.

Infrared Spectrum Characteristics

• Simple absorption of IR radiation causes molecular vibrations, resulting in various close-packed absorption bands.
• The frequencies (ν) are expressed in Hertz (Hz), with 1 Hz being one cycle per second.
• Wavelength (λ) is measured in centimeters, showing an inverse relationship with frequency: ν = c/λ.

Energy Relationships

• Energy (E) correlates with frequency and wavelength using the equations:
  • E = hν
  • E = hc/λ
• Planck’s constant (h) is 6.6 x 10^-34 joules-sec.
• Energy is directly proportional to frequency and inversely proportional to wavelength.

Infrared Regions

• The IR spectrum is divided into three main sections: near-IR, mid-IR, and far-IR.
• The mid-IR region (3 x 10^-4 to 3 x 10^-3 cm) is most valuable for organic chemistry.
• Wavenumbers, defined as the inverse of wavelength, are preferred for reporting spectra, particularly in the mid-IR range (4000–400 cm^-1).

Instrumentation

• An infrared spectrometer (or spectrophotometer) produces the infrared spectrum.
• Common units in IR spectra include reciprocal centimeters (cm^-1) for frequency and micrometers (μm) for wavelength.

Functional Group Analysis

• Certain atom groups generate absorption bands at consistent frequencies, regardless of other molecular structure aspects.
• Near-infrared (NIR) reflects deeper into samples due to low absorptivity, enhancing analysis accuracy.

Applications in Analyzing Amines

• Primary aromatic amines show two key absorption bands near 1.97 μm (N-H bending/stretching) and 1.49 μm (symmetric N-H stretching).
• Secondary amines display overtone bands but minimal absorption in combination regions.
• Tertiary amines have negligible absorption at these wavelengths, with aliphatic amines shifted to ~1.525 μm and 2.000 μm.

Other Applications

• NIR reflectance spectra are widely used in food and grain industries to analyze protein, fat, moisture, sugars, and oils.
• The technique also applies to wood substance determination, polymer composition, and geological exploration via aircraft.

Mid-Infrared Region Breakdown

• Divided into two categories:
  • Group frequency region: 4000-1300 cm^-1 (2.50-7.69 μm).
  • Fingerprint region: 1300-650 cm^-1 (7.69-15.38 μm).

This quiz covers the principles of infrared absorption spectroscopy, including selective absorption, molecular vibrations, and IR absorption spectra. Test your knowledge of this analytical technique!