Kimia Tanah

The Basis of Infrared Spectroscopy

Infrared (IR) spectroscopy is a powerful technique for identifying organic compounds by detecting the vibrations of chemical bonds. When infrared radiation (wavelength 2.5–25 μm; wavenumber 4000–400 cm⁻¹) is shone on a molecule, bonds absorb radiation at frequencies that match their natural vibrational frequencies. The result is a spectrum — a plot of transmittance (or absorbance) vs. wavenumber — that acts as a molecular fingerprint.

Every bond type has a characteristic vibration frequency determined by the masses of the atoms and the bond strength (analogous to a spring: stiffer springs and lighter masses vibrate faster). The two main types of molecular vibrations are:

• Stretching: the bond length changes (symmetric and asymmetric)
• Bending: the bond angle changes (in-plane and out-of-plane)

The IR Spectrum: Key Regions

The IR spectrum is conventionally divided into functional regions:

Functional Group Region (4000–1500 cm⁻¹)

This region contains absorptions characteristic of specific functional groups and is used for compound identification:

The carbonyl stretch at ~1700–1750 cm⁻¹ is one of the most useful IR signals. Its exact position reveals the carbonyl type: - Ketone: ~1715 cm⁻¹ - Aldehyde: ~1725 cm⁻¹ (plus C–H stretch near 2720, 2820 cm⁻¹) - Carboxylic acid: ~1710 cm⁻¹ (with broad O–H) - Ester: ~1735 cm⁻¹ - Amide: ~1650 cm⁻¹ (lowered by resonance) - Acid chloride: ~1800 cm⁻¹

Fingerprint Region (1500–400 cm⁻¹)

This region contains complex overlapping bands from C–C, C–N, C–O single bond stretches and bending vibrations. While individual peaks are difficult to assign, the overall pattern is unique to each molecule — hence "fingerprint." Comparing the fingerprint region to a reference spectrum is the most reliable way to confirm compound identity.

Instrumentation: FTIR

Modern IR spectroscopy uses Fourier Transform Infrared (FTIR) spectrometers. Unlike older dispersive instruments that scanned wavelength-by-wavelength, FTIR uses an interferometer (Michelson design) to collect all wavelengths simultaneously, then applies a mathematical Fourier transform to convert the raw interferogram into a spectrum.

Advantages of FTIR: - Speed: A full spectrum is acquired in seconds - Sensitivity: Signal-to-noise improves with multiple scans (co-adding) - Wavenumber accuracy: Typically ±0.01 cm⁻¹ (internal calibration with HeNe laser) - Versatility: Compatible with numerous sampling accessories

ATR (Attenuated Total Reflectance)

The most common modern sampling technique is ATR. The sample (liquid or solid) is pressed against a crystal (typically diamond, ZnSe, or germanium). The IR beam undergoes total internal reflection inside the crystal, and an evanescent wave penetrates a few micrometers into the sample, causing absorption. ATR requires minimal sample preparation — solids and liquids can be measured directly without dissolving or pressing into pellets.

Traditional sample preparation methods include: - KBr pellet: grind solid with dry KBr and press into a transparent disk - Nujol mull: grind solid with mineral oil (Nujol) - Thin film: cast liquid or dissolved sample onto NaCl plates and evaporate solvent

Interpreting an IR Spectrum: A Systematic Approach

When analyzing an unknown IR spectrum:

1. Check for broad O–H around 3300 cm⁻¹: if present, suspect alcohol or carboxylic acid. Carboxylic acids extend the broad absorption down to ~2500 cm⁻¹.
2. Check for C=O at 1700–1750 cm⁻¹: if present, identify the exact position and look for supporting features (aldehyde C–H, O–H for acid, N–H for amide).
3. Check N–H at 3300–3500 cm⁻¹: primary amines show two N–H bands; secondary amines show one; amides show N–H and carbonyl.
4. Note C–H stretch region: sp³ C–H just below 3000 cm⁻¹; sp² and sp C–H at 3000–3300 cm⁻¹.
5. Look at fingerprint region: match pattern to a reference database (SDBS, NIST).

Quantitative IR Spectroscopy

IR can be used quantitatively via Beer's Law (A = ε × c × l), though UV-Vis is more commonly used for concentration measurements. IR quantitation is valuable when: - Compounds lack UV-Vis chromophores - Distinguishing components in mixtures (e.g., gasoline blending, monitoring polymer curing) - Process analytical technology (PAT): in-line FTIR monitors reactions in real time

Applications

Polymer characterization: FTIR distinguishes polymer types (PE vs. PP vs. PVC), detects degradation (carbonyl formation), and quantifies blend composition. Recycling facilities use FTIR to sort plastic waste.

Pharmaceutical analysis: Verifying raw material identity ("right first time" in GMP manufacturing), detecting polymorphic forms of drugs (different crystal structures give different IR spectra), and monitoring manufacturing processes.

Environmental monitoring: Portable FTIR instruments measure combustion gases, atmospheric CO₂ and methane, and contamination in soil extracts.

Food authentication: Detecting adulteration in olive oil, honey, and spices. The complex fingerprint region reveals subtle compositional differences invisible to simpler tests.

Art conservation: FTIR identifies pigments, binders, and varnishes in historical paintings without damaging the artwork, guiding restoration decisions.

Forensic chemistry: Identifying controlled substances, accelerants in arson debris, and fibers from crime scenes.

Summary

IR spectroscopy identifies functional groups through characteristic absorption frequencies and confirms compound identity through fingerprint matching. The shift from dispersive to FTIR instruments, combined with ATR sampling, has made IR one of the most accessible and rapid routine analytical tools available — capable of delivering structural information in seconds from a milligram of sample.