🌈Spectroscopy Unit 5 – Infrared Spectroscopy – Theory and Practice

Fundamentals of IR Spectroscopy

• Infrared (IR) spectroscopy is a powerful analytical technique that probes the interaction between IR radiation and matter
• Relies on the absorption of IR radiation by molecules, causing them to undergo vibrational transitions
• Vibrational transitions occur when the frequency of the IR radiation matches the natural vibrational frequency of a molecule
• The amount of IR radiation absorbed by a sample is proportional to the concentration of the absorbing species, following the Beer-Lambert law
  • Beer-Lambert law: $A = \varepsilon bc$, where $A$ is absorbance, $\varepsilon$ is molar absorptivity, $b$ is path length, and $c$ is concentration
• IR spectroscopy provides valuable information about the structure and functional groups present in a molecule
• Complementary to other spectroscopic techniques (NMR, UV-Vis, Mass Spectrometry) in elucidating molecular structure
• Non-destructive technique, allowing for the recovery of the sample after analysis

Electromagnetic Spectrum and IR Radiation

• The electromagnetic spectrum encompasses a wide range of wavelengths and frequencies, including radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays
• IR radiation lies between the visible and microwave regions of the electromagnetic spectrum
• Divided into three sub-regions: near-IR (12500-4000 cm^(-1)), mid-IR (4000-400 cm^(-1)), and far-IR (400-10 cm^(-1))
  • Mid-IR is the most commonly used region for structural elucidation and chemical analysis
• IR radiation has lower energy compared to visible light, with wavelengths ranging from 0.78 to 1000 μm
• The energy of IR radiation is sufficient to cause vibrational transitions in molecules but not electronic transitions
• The wavenumber ($\tilde{\nu}$), measured in cm^(-1), is often used to describe the energy of IR radiation
  • Wavenumber is the reciprocal of the wavelength: $\tilde{\nu} = 1/\lambda$
• The relationship between energy ($E$), frequency ($\nu$), and wavenumber ($\tilde{\nu}$) is given by: $E = h\nu = hc\tilde{\nu}$, where $h$ is Planck's constant and $c$ is the speed of light

Molecular Vibrations and IR Absorption

• Molecules absorb IR radiation when the frequency of the radiation matches the natural vibrational frequency of the molecule
• Molecular vibrations can be classified as stretching (changes in bond length) or bending (changes in bond angle) modes
  • Stretching modes: symmetric and asymmetric stretching
  • Bending modes: scissoring, rocking, wagging, and twisting
• The number of vibrational modes for a non-linear molecule is 3N-6, where N is the number of atoms
  • Linear molecules have 3N-5 vibrational modes due to their reduced degrees of freedom
• For a vibrational mode to be IR-active, it must cause a change in the dipole moment of the molecule
• The intensity of an IR absorption band depends on the magnitude of the change in dipole moment associated with the vibrational mode
• Homonuclear diatomic molecules (N2, O2) are not IR-active because they have no dipole moment
• The frequency of a vibrational mode depends on the masses of the atoms involved and the strength of the bond
  • Heavier atoms and weaker bonds result in lower vibrational frequencies
• Anharmonicity in molecular vibrations leads to the appearance of overtone and combination bands in IR spectra

IR Instrumentation and Sample Preparation

• Modern IR spectrometers are based on the Fourier Transform (FT) technique, which offers high sensitivity, resolution, and speed
• Key components of an FT-IR spectrometer include the source, interferometer, detector, and computer
  • Source: typically a glowing black-body source (Globar) that emits a broad spectrum of IR radiation
  • Interferometer: usually a Michelson interferometer, which creates an interference pattern by splitting and recombining the IR beam
  • Detector: measures the intensity of the IR radiation as a function of the interferometer mirror position (mercury cadmium telluride or deuterated triglycine sulfate)
  • Computer: performs the Fourier Transform to convert the interferogram into a spectrum
• Samples can be prepared for IR analysis in various forms, depending on their physical state and the desired measurement mode
  • Transmission mode: solid samples can be pressed into KBr pellets or dispersed in Nujol mull; liquids and gases can be placed in sealed cells with IR-transparent windows (NaCl, KBr)
  • Reflection mode: attenuated total reflectance (ATR) is commonly used for solid and liquid samples, utilizing a crystal with a high refractive index (diamond, ZnSe, Ge)
• Sample preparation should aim to minimize interference from atmospheric moisture and carbon dioxide, which have strong IR absorption bands
• Background spectra are collected before sample measurements to account for atmospheric absorption and instrument response

Interpreting IR Spectra

• IR spectra plot absorbance or transmittance as a function of wavenumber (cm^(-1))
• Absorbance (A) and transmittance (T) are related by: $A = -\log_{10}(T)$
• The x-axis of an IR spectrum represents the wavenumber, with high wavenumbers (shorter wavelengths) on the left and low wavenumbers (longer wavelengths) on the right
• The y-axis represents the amount of IR radiation absorbed or transmitted by the sample
• IR absorption bands are characterized by their position (wavenumber), intensity, and shape
  • Position: determined by the type of vibration and the atoms involved
  • Intensity: depends on the change in dipole moment and the concentration of the absorbing species
  • Shape: influenced by the chemical environment and intermolecular interactions
• Functional groups have characteristic IR absorption bands that can be used for their identification
• The fingerprint region (1500-400 cm^(-1)) contains a complex pattern of absorption bands that is unique to each molecule
• Hydrogen bonding can cause broadening and shifting of IR absorption bands, particularly for O-H and N-H stretching vibrations
• Overtone and combination bands appear at higher wavenumbers than the fundamental vibrations and are usually weaker in intensity

Common Functional Groups and Their IR Signatures

• Alkanes: C-H stretching (2950-2850 cm^(-1)), C-H bending (1450-1375 cm^(-1))
• Alkenes: =C-H stretching (3100-3000 cm^(-1)), C=C stretching (1680-1620 cm^(-1)), =C-H bending (1000-650 cm^(-1))
• Alkynes: ≡C-H stretching (3300 cm^(-1)), C≡C stretching (2260-2100 cm^(-1))
• Aromatics: C-H stretching (3100-3000 cm^(-1)), C=C stretching (1600-1450 cm^(-1)), C-H bending (900-650 cm^(-1))
• Alcohols and phenols: O-H stretching (3600-3200 cm^(-1)), C-O stretching (1300-1000 cm^(-1))
  • Hydrogen bonding can cause significant broadening of the O-H stretching band
• Amines: N-H stretching (3500-3300 cm^(-1)), C-N stretching (1350-1000 cm^(-1)), N-H bending (1650-1550 cm^(-1))
• Carbonyls (aldehydes, ketones, carboxylic acids, esters): C=O stretching (1850-1650 cm^(-1))
  • The exact position of the C=O stretching band depends on the type of carbonyl compound and its chemical environment
• Nitriles: C≡N stretching (2260-2200 cm^(-1))
• Nitro compounds: asymmetric and symmetric NO2 stretching (1550-1350 cm^(-1))

Applications in Chemical Analysis

• IR spectroscopy is widely used for the identification of organic compounds and functional groups
• Valuable tool for the characterization of polymers, including composition, structure, and degree of polymerization
• Used in the pharmaceutical industry for drug identification, purity assessment, and polymorph screening
• Quality control and process monitoring in various industries (food, cosmetics, petrochemicals)
  • Rapid and non-destructive analysis of raw materials, intermediates, and final products
• Environmental monitoring and analysis of pollutants, such as atmospheric gases and water contaminants
• Forensic applications, including the identification of drugs, fibers, and paint chips
• Coupled with microscopy (IR microspectroscopy) for the analysis of small samples and spatial distribution of components
• Quantitative analysis using the Beer-Lambert law, with appropriate calibration standards
• Complementary to other analytical techniques (NMR, MS, UV-Vis) for comprehensive structural elucidation

Advanced Techniques and Recent Developments

• Two-dimensional IR (2D-IR) spectroscopy: provides information about the coupling and dynamics of vibrational modes
• Time-resolved IR spectroscopy: allows the study of fast chemical processes and reaction intermediates
• IR imaging and mapping: enables the spatial distribution analysis of components in heterogeneous samples
• Nanoscale IR spectroscopy (AFM-IR): combines atomic force microscopy with IR spectroscopy for nanometer-scale chemical analysis
• Quantum cascade laser (QCL) based IR spectroscopy: offers high power, tunable, and narrow-bandwidth IR sources for improved sensitivity and selectivity
• Cavity ring-down spectroscopy (CRDS): a highly sensitive technique for trace gas analysis and weak absorptions
• Surface-enhanced IR absorption spectroscopy (SEIRAS): utilizes metal nanostructures to enhance the IR absorption of adsorbed molecules
• Combining IR spectroscopy with computational methods (DFT) for the prediction and interpretation of IR spectra
• Developing advanced data analysis techniques (chemometrics, machine learning) for the interpretation of complex IR datasets
• Expanding the application of IR spectroscopy to new fields, such as biomedical diagnostics, cultural heritage, and space exploration