Ir Spectroscopy

Introduction

IR spectroscopy: analytical method for determining molecular structure via absorption of infrared radiation. Used extensively in organic chemistry to identify functional groups and study molecular vibrations. Provides qualitative and quantitative data by measuring frequency-specific absorption. Non-destructive, rapid, and sensitive technique.

"Infrared spectroscopy remains one of the most powerful tools for elucidating organic molecular structure." -- Pavia, Lampman, Kriz, Vyvyan

Principles of IR Spectroscopy

Electromagnetic Spectrum and IR Region

IR region: 4000–400 cm⁻¹ (wavenumber). Subdivided into near-IR, mid-IR, and far-IR. Mid-IR most useful for organic compounds. IR radiation causes vibrational excitation in molecules with dipole moment changes.

Molecular Vibrations

Two main types: stretching (bond length changes) and bending (bond angle changes). Vibrations quantized; energy absorbed matches vibrational transition energy. Only vibrations altering dipole moment absorb IR radiation.

Selection Rules

IR active vibrations: require change in dipole moment. Homonuclear diatomic molecules IR inactive. Complex molecules exhibit multiple vibrational modes depending on atomic count (3N-6 or 3N-5).

Instrumentation and Techniques

Dispersive IR Spectrometers

Use prism or grating to disperse IR radiation. Monochromatic light scanned over range. Slower, less sensitive than FTIR.

Fourier Transform IR (FTIR) Spectrometers

Interferometer generates interferogram; Fourier transform converts to spectrum. Advantages: faster, higher resolution, better signal-to-noise ratio.

Detectors and Sources

Common sources: Globar (silicon carbide), Nernst glower. Detectors: thermocouples, mercury cadmium telluride (MCT) for mid-IR, deuterated triglycine sulfate (DTGS).

Sample Preparation Methods

Liquid Samples

Thin films between NaCl or KBr plates for transmission. Solvents: non-IR absorbing like CCl₄, CS₂.

Solid Samples

KBr pellet method: sample ground with KBr powder, pressed into transparent pellet. Nujol mull: sample suspended in mineral oil.

Gas Samples

Gas cells with long path lengths used; IR absorption measured in gaseous state for volatile organics.

IR Spectral Regions

Functional Group Region (4000–1500 cm⁻¹)

Contains characteristic absorption bands of functional groups. High diagnostic value for organic analysis.

Fingerprint Region (1500–400 cm⁻¹)

Complex absorption pattern unique to each molecule. Useful for identification by comparison with reference spectra.

Far-IR Region (< 400 cm⁻¹)

Used for metal-ligand vibrations and inorganic complexes, less common in organic chemistry.

Molecular Vibrations and Modes

Stretching Vibrations

Symmetric: bonds stretch in phase. Asymmetric: bonds stretch out of phase. Frequencies depend on bond strength and atomic masses.

Bending Vibrations

Includes scissoring, rocking, wagging, and twisting. Lower energy than stretching, appear at lower wavenumbers.

Normal Modes and Degrees of Freedom

Molecules with N atoms have 3N total degrees of freedom: 3 translational, 3 rotational (linear molecules 2 rotational), remainder vibrational modes.

Functional Group Identification

O-H and N-H Groups

O-H stretch: broad band 3200-3600 cm⁻¹. N-H stretch: sharper bands ~3300-3500 cm⁻¹. Hydrogen bonding affects shape.

C-H Stretching

Alkanes: 2850-2960 cm⁻¹. Alkenes: 3020-3100 cm⁻¹. Aromatics: multiple C-H stretches around 3000 cm⁻¹.

Carbonyl Groups (C=O)

Strong, sharp absorption 1650-1750 cm⁻¹. Position varies with conjugation, ring strain, and substituents.

Interpretation of IR Spectra

Peak Positions

Reported in wavenumbers (cm⁻¹). Correlate with specific bond vibrations. Shifts indicate electronic environment changes.

Peak Intensities and Shapes

Intensity depends on dipole moment change magnitude. Broad peaks indicate hydrogen bonding or complex interactions.

Comparison with Reference Spectra

Fingerprint region used to confirm molecular identity. Databases and spectral libraries aid interpretation.

Quantitative Analysis

Beer-Lambert Law Application

Absorbance proportional to concentration and path length. Used for determining concentration of IR active species.

Calibration Curves

Constructed from standards with known concentrations. Linear range depends on sample and instrument.

Limitations in Quantification

Overlapping bands, scattering, and baseline drift can interfere. Requires careful sample prep and data correction.

Applications in Organic Chemistry

Functional Group Identification

Rapid screening of organic compounds. Verification of synthetic intermediates and products.

Reaction Monitoring

Real-time observation of bond formation/breaking. Used in polymerization and organic synthesis kinetics.

Structural Elucidation

Complements NMR and MS data. Confirms presence/absence of key functional groups.

Limitations and Challenges

Complex Spectra in Large Molecules

Overlapping bands hinder analysis. Requires complementary techniques.

IR Inactivity of Symmetrical Molecules

Vibrations lacking dipole moment change not detected. Limits applicability.

Sample Preparation Constraints

Moisture sensitivity, sample homogeneity, and thickness affect spectral quality.

Recent Advances and Hybrid Techniques

Attenuated Total Reflectance (ATR)

Minimal sample prep. Measures surface layers. Widely adopted in organic analysis.

Two-Dimensional IR Spectroscopy

Provides information on vibrational coupling and molecular dynamics.

Hyphenated Techniques

IR combined with GC (GC-IR) and microscopy (IR microscopy) for complex mixture analysis and spatial resolution.

References

• Pavia, Lampman, Kriz, Vyvyan, Introduction to Spectroscopy, 5th ed., Cengage Learning, 2015, pp. 1-400.
• Silverstein, Webster, Kiemle, Spectrometric Identification of Organic Compounds, 7th ed., Wiley, 2005, pp. 50-300.
• Smith, B.C., Infrared Spectral Interpretation: A Systematic Approach, CRC Press, 1999, pp. 10-220.
• Stuart, B.H., Infrared Spectroscopy: Fundamentals and Applications, Wiley, 2004, pp. 33-180.
• Harrick, N.J., Internal Reflection Spectroscopy, Wiley-Interscience, 1967, pp. 75-150.