1.2 Infrared (IR) spectroscopy

Infrared spectroscopy is a powerful tool for identifying organic compounds by analyzing their molecular vibrations. It provides crucial information about functional groups and molecular structure, making it an essential technique in organic chemistry.

IR spectroscopy works by measuring how molecules absorb infrared radiation, causing them to vibrate in specific ways. By interpreting the resulting spectrum, chemists can determine the presence of various functional groups and gain insights into a compound's overall structure.

Principles of IR spectroscopy

• Infrared spectroscopy exploits molecular vibrations to identify and analyze organic compounds
• IR spectroscopy provides valuable information about functional groups and molecular structure in organic chemistry

Electromagnetic spectrum

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• IR radiation occupies the region between visible light and microwaves (wavelength range 0.78 to 1000 μm)
• IR spectrum divided into near-IR (0.78-2.5 μm), mid-IR (2.5-50 μm), and far-IR (50-1000 μm)
• Mid-IR region most commonly used for organic compound analysis
• Energy of IR radiation corresponds to vibrational transitions in molecules

Molecular vibrations

• IR spectroscopy detects molecular vibrations caused by absorption of IR radiation
• Two main types of molecular vibrations stretching (change in bond length) and bending (change in bond angle)
• Stretching vibrations classified as symmetric or asymmetric
• Bending vibrations include scissoring, rocking, wagging, and twisting modes
• Number of vibrational modes for a molecule with N atoms 3N-6 for non-linear molecules, 3N-5 for linear molecules

Selection rules

• IR-active vibrations must cause a change in the molecular dipole moment
• Symmetric vibrations of non-polar molecules are IR-inactive
• Rule of mutual exclusion states vibrations cannot be both IR and Raman active in centrosymmetric molecules
• Intensity of IR absorption proportional to the magnitude of dipole moment change

IR instrumentation

• IR spectroscopy instruments have evolved from dispersive to Fourier transform (FT-IR) spectrometers
• Modern FT-IR spectrometers offer improved sensitivity, speed, and data processing capabilities

IR spectrometer components

• Light source typically a heated ceramic source (Globar) emitting continuous IR radiation
• Interferometer (in FT-IR) consists of a beam splitter, fixed mirror, and moving mirror
• Sample compartment holds the sample and allows IR beam to pass through
• Detector converts IR radiation into an electrical signal (common types thermal and photonic)
• Computer processes the detector signal and generates the IR spectrum

Sample preparation techniques

• Solid samples prepared as KBr pellets, Nujol mulls, or thin films
• Liquid samples analyzed as thin films between NaCl or KBr plates
• Gas samples measured in specialized gas cells with long path lengths
• Attenuated Total Reflectance (ATR) technique allows direct analysis of solids and liquids without extensive sample preparation

IR spectrum interpretation

• IR spectra plot transmittance or absorbance versus wavenumber (cm⁻¹)
• Interpretation involves identifying characteristic absorption bands and their intensities

Absorption bands

• IR absorption bands result from specific molecular vibrations
• Band position (wavenumber) indicates the energy of the vibration
• Band intensity relates to the change in dipole moment and concentration
• Band shape affected by factors like hydrogen bonding and molecular environment
• Overtone bands appear at approximately twice the frequency of fundamental vibrations

Functional group regions

• 4000-1400 cm⁻¹ region contains stretching vibrations of functional groups
• O-H stretch (3200-3600 cm⁻¹), N-H stretch (3300-3500 cm⁻¹), C-H stretch (2850-3300 cm⁻¹)
• C=O stretch (1650-1800 cm⁻¹), C=C stretch (1620-1680 cm⁻¹), C=N stretch (1610-1680 cm⁻¹)
• NO₂ asymmetric stretch (1530-1560 cm⁻¹) and symmetric stretch (1310-1360 cm⁻¹)

Fingerprint region

• 1400-400 cm⁻¹ region contains complex absorption patterns unique to each molecule
• Includes bending vibrations and skeletal vibrations of the molecular framework
• Used for compound identification by comparison with reference spectra
• C-O stretching vibrations of alcohols and ethers appear in this region (1000-1300 cm⁻¹)
• Aromatic ring vibrations produce characteristic bands around 1600, 1500, and 1450 cm⁻¹

Common functional groups

• IR spectroscopy excels at identifying functional groups in organic molecules
• Each functional group has characteristic absorption bands in specific regions

Alkyl groups

• C-H stretching vibrations appear in the 2850-3000 cm⁻¹ region
• Methyl (CH₃) groups show asymmetric stretch (~2960 cm⁻¹) and symmetric stretch (~2870 cm⁻¹)
• Methylene (CH₂) groups exhibit asymmetric stretch (~2925 cm⁻¹) and symmetric stretch (~2850 cm⁻¹)
• C-H bending vibrations appear in the fingerprint region (1350-1480 cm⁻¹)

Carbonyl compounds

• C=O stretching vibration produces a strong band in the 1650-1800 cm⁻¹ region
• Exact position depends on the type of carbonyl compound (aldehyde, ketone, carboxylic acid, ester)
• Aldehydes show additional C-H stretching band around 2720-2820 cm⁻¹ (aldehyde C-H)
• α,β-unsaturated carbonyls exhibit a lower C=O stretching frequency due to conjugation

Alcohols and phenols

• O-H stretching vibration appears as a broad band in the 3200-3600 cm⁻¹ region
• Hydrogen bonding affects the O-H band shape and position
• C-O stretching vibration occurs in the 1000-1300 cm⁻¹ region
• Phenols show a characteristic O-H bending vibration around 1360-1380 cm⁻¹

Amines and amides

• Primary amines show two N-H stretching bands (3300-3500 cm⁻¹)
• Secondary amines exhibit a single N-H stretching band
• Amides display a strong C=O stretching band (Amide I, 1630-1690 cm⁻¹)
• N-H bending vibration (Amide II) appears around 1550 cm⁻¹ for primary and secondary amides

Factors affecting IR spectra

• Various molecular and environmental factors can influence the appearance of IR spectra
• Understanding these effects crucial for accurate spectrum interpretation

Hydrogen bonding

• Hydrogen bonding causes broadening and shifting of O-H and N-H stretching bands
• Lowers the stretching frequency and increases the absorption intensity
• Affects alcohols, phenols, carboxylic acids, amines, and amides
• Concentration and solvent effects can influence the extent of hydrogen bonding

Conjugation effects

• Conjugation of C=C or C=O bonds with other π systems lowers the stretching frequency
• α,β-unsaturated carbonyls show C=O stretch at lower wavenumbers compared to saturated analogs
• Aromatic compounds exhibit characteristic ring stretching vibrations (~1600, 1500, 1450 cm⁻¹)
• Conjugated dienes show C=C stretching at lower frequencies than isolated double bonds

Steric hindrance

• Steric effects can influence the frequency and intensity of certain IR bands
• Bulky substituents near a carbonyl group can increase the C=O stretching frequency
• Steric hindrance can affect hydrogen bonding, leading to changes in O-H and N-H band shapes
• Ring strain in cyclic compounds can alter characteristic absorption frequencies

Quantitative IR analysis

• IR spectroscopy can be used for quantitative analysis of organic compounds
• Requires careful sample preparation and calibration

Beer-Lambert law

• Relates absorbance (A) to concentration (c) and path length (l) $A = εcl$
• ε molar absorption coefficient, specific to each compound and wavelength
• Linear relationship between absorbance and concentration allows quantitative analysis
• Deviations from linearity can occur at high concentrations or due to instrumental limitations

Calibration methods

• External standard method uses a series of known concentration standards
• Internal standard method adds a fixed amount of reference compound to samples and standards
• Standard addition method involves adding known amounts of analyte to the sample
• Multivariate calibration techniques (PLS, PCR) used for complex mixtures or overlapping bands

Advanced IR techniques

• Modern IR spectroscopy employs various advanced techniques to enhance sensitivity and applicability

Attenuated total reflectance

• ATR-IR allows direct analysis of solids and liquids with minimal sample preparation
• Sample placed in contact with an internal reflection element (IRE) (ZnSe, Ge, diamond)
• IR beam undergoes total internal reflection, creating an evanescent wave that interacts with the sample
• Particularly useful for analyzing polymers, coatings, and aqueous solutions

Fourier transform IR

• FT-IR spectrometers use an interferometer instead of a monochromator
• Collects all wavelengths simultaneously, improving signal-to-noise ratio (Fellgett advantage)
• Higher throughput of radiation compared to dispersive instruments (Jacquinot advantage)
• Enables rapid data collection, signal averaging, and advanced data processing techniques

IR spectroscopy vs other methods

• IR spectroscopy complements other analytical techniques in organic chemistry
• Each method provides unique information about molecular structure and composition

IR vs NMR spectroscopy

• IR focuses on functional groups and molecular vibrations
• NMR provides detailed information about molecular connectivity and structure
• IR requires smaller sample sizes and shorter analysis times compared to NMR
• NMR offers better structural elucidation capabilities for complex molecules
• Combination of IR and NMR data provides comprehensive structural information

IR vs mass spectrometry

• IR identifies functional groups and molecular vibrations
• Mass spectrometry determines molecular mass and fragmentation patterns
• IR non-destructive technique, while MS typically involves sample ionization and fragmentation
• MS provides information about molecular formula and structural fragments
• IR and MS often used together for complete characterization of organic compounds

Applications in organic chemistry

• IR spectroscopy finds widespread use in various aspects of organic chemistry research and industry

Structure determination

• Rapid identification of functional groups in unknown compounds
• Confirmation of product formation in organic synthesis reactions
• Distinguishing between structural isomers based on characteristic absorption patterns
• Complementary technique to NMR and MS for complete structure elucidation

Reaction monitoring

• In-situ monitoring of reaction progress by tracking changes in functional group absorptions
• Identification of reaction intermediates and side products
• Kinetic studies of organic reactions by following time-dependent spectral changes
• Quality control in industrial processes to ensure consistent product composition

Quality control

• Verification of raw material purity in pharmaceutical and chemical industries
• Identification of contaminants or adulterants in organic products
• Monitoring product stability and degradation during storage
• Ensuring batch-to-batch consistency in manufacturing processes

Limitations and troubleshooting

• Understanding limitations and common issues in IR spectroscopy crucial for accurate data interpretation

Spectral interferences

• Atmospheric CO₂ and H₂O can produce interfering absorption bands
• Solved by purging the instrument with dry nitrogen or using a background subtraction
• Solvent absorption bands may overlap with sample peaks
• Choose appropriate solvents or use techniques like ATR to minimize solvent interference
• Impurities in the sample can lead to unexpected absorption bands

Sample preparation issues

• Inconsistent sample thickness in transmission measurements affects band intensities
• Ensure uniform sample thickness and use internal standards for quantitative analysis
• Poor contact between sample and ATR crystal results in weak or distorted spectra
• Apply consistent pressure and ensure full coverage of the ATR crystal
• Decomposition of heat-sensitive samples during pellet preparation
• Use alternative techniques like ATR or prepare samples at lower temperatures