Chapter 2b IR Spectroscopy

2.1 Introduction to Infrared Absorption Spectroscopy

• Infrared absorption spectroscopy involves the absorption of radiation in the infrared range.

• It is a widely used method to investigate the structures of organic substances.

• The instrument used is called an infrared spectrophotometer.

  • Typical range: 2.5 µm (4000 cm^-1) to 17.2 µm (580 cm^-1).

  • Near infrared: Below 2.5 µm.

  • Far infrared: Above 17.2 µm.

• Organic molecules typically absorb in the mid-infrared region.

• Absorption bands can be referred to by wavelength (µm) or wave number (cm^-1).

  • Band intensity is expressed in absorbance (A) or transmittance (T); related as A = log(1/T).

2.2 Basic Theory of IR Spectroscopy

2.2.1 Molecular Vibrations

• Molecules undergo continuous rotation and vibrations.

• Comparisons can be made with a weightless spring connecting two balls (atoms).

• Two main types of molecular vibrations:

  1. Stretching Vibrations

     • Diatomic Molecules: Periodic compression and extension along the bond.

     • Triatomic or Higher Molecules:

       • Symmetric Stretching: Atoms move together in the same direction around a central atom.

       • Asymmetric Stretching: One atom approaches the center while another moves away.

  2. Bending Vibrations

     • Changes in bond angles between non-linked atoms.

     • Types include:

       • Scissoring: In-plane bending.

       • Rocking: In-plane bending in the same direction.

       • Wagging: Out-of-plane bending.

       • Twisting: One atom moves above while the other moves below.

• Bending vibrations require less energy than stretching vibrations.

2.2.3 Origin of IR Modes

• Infrared spectra arise from the quantization of vibrational energy levels.

• A molecule absorbs infrared radiation that matches the energy required for specific vibrational transitions.

• The absorption wavelengths depend on the vibrational energy levels.

2.3 Calculation of Vibrational Frequency: Hooke's Law

• The force constant (k) relates to bond strength; higher k = stronger bonds.

• Formula: v = (1/2π)√(k/μ)

  • μ = reduced mass of the diatomic system equation: μ = (m1*m2)/(m1+m2)

• Using Hooke's law, vibrational frequency can be approximated for single bonds.

Factors Affecting Vibrational Frequency

1. Atom Mass: Lower mass = higher frequency.

2. Force Constant (k): Higher k (stronger bond) = higher frequency.

3. Dipole Moment: Intensity of absorption is influenced by the bond's polarity.

2.4 Number of Fundamental Vibrations

• Fundamental vibrational modes depend on molecule geometry:

  • Non-linear polyatomic: 3n - 6 modes.

  • Linear polyatomic: 3n - 5 modes.

• Presence of weak vibrations and overlaps may lead to fewer observable bands.

2.5 Selection Rules

• For infrared activity, vibrations must result in a change in dipole moment.

• Symmetric stretches of centro-symmetric molecules do not exhibit IR activity.

2.6 Position and Intensity of Bands

2.6.1 Position of Absorption Bands

• The position reflects bond character; distinct bands correspond to specific functional groups.

• Divided into functional group region (2.5 to 8 µm) and fingerprint region (below 8 µm).

2.6.2 Intensity of Absorption Bands

• Depends on the character of bonds and change in dipole moments.

• Increased band intensity is often a function of higher dipole moments in stronger polar bonds.

2.7 Factors Influencing Vibrational Frequency

1. Electronic Effect

• Inductive effects and resonance can modify bond strength.

• Higher inductive effects usually lower the force constant, affecting frequency.

• Conjugation can lead to decreased frequency shifts due to increased bond length.

2. Hydrogen Bonding

• Strength of hydrogen bonds alters the absorption frequencies:

  • Strong hydrogen bonding causes shifts to lower wave number.

  • Broadening observed in intermolecular bonding; sharper bands in intramolecular.

2.8 Instrumentation: Mechanics of Recording IR Spectra

• Parts of an infrared spectrophotometer:

  • Source, sample containers, monochromator, detector, recorder.

• Common infrared sources: Nernst glower, silicon carbide (Globar).

• FT-IR spectrometer provides better sensitivity and faster recording times.

2.9 Characteristic Absorption Regions of Various Bonds

• Correlation tables show expected frequencies and intensities for various bonds and functional groups.

2.10 Spectral Features of Some Classes of Compounds

2.10.1 Hydrocarbons

• Alkanes: Peaks at 2960-2850 cm^-1 (C-H stretching).

• Alkenes: Vinylic C-H at 3000 cm^-1.

• Alkynes: Strong C-H stretching near 3300 cm^-1; weak C≡C at 2100-2260 cm^-1.

2.10.2 Functional Group Identification

• Alcohols: O-H stretching at 3640-3610 cm^-1 (free), 3400-3200 cm^-1 (hydrogen bonded).

• Aldehydes: C=O stretching at 1740-1720 cm^-1.

• Ketones: Lower frequency for C=O than aldehydes.

• Carboxylic Acids: Broad O-H stretching at 3000-2500 cm^-1.

2.11 Interpretation of IR Spectra

2.11.1 Specific Absorption Regions

• Absorption bands categorized for quick identification based on frequency.

2.11.2 Practical Tips

• Begin with high-frequency bands and use absence of bands as a tool for identification.

2.12 Applications of Infrared Spectroscopy

• Identification of organic compounds, functional groups, reaction progress, purity assessment.

Example Interpretations

Example 2.2: Expected Absorption for Compounds

• Isopropyl alcohol: O-H stretching (3200-3400 cm^-1), C-H stretching (2850-2950 cm^-1).

• Dimethyl ether: C-O stretching (1060-1150 cm^-1).

• Toluene: C-H stretching (3000-3100 cm^-1).

Example 2.3: Absorption Bands for Carbonyl Compounds

• Approximate positions highlighted for carbonyl stretching in aldehydes and ketones.