IR Spectroscopy: Understanding Infrared Spectra

Infrared (IR) Spectroscopy

• Infrared (IR) spectroscopy deals with the infrared region (710 nm - 1 mm) of the electromagnetic spectrum.
• Infrared light has a longer wavelength and lower frequency than visible light.
• IR spectroscopy studies how molecules interact with infrared light.
• IR spectroscopy is an analytical technique that relies on the vibrational transitions of a molecule when it interacts with infrared light.
• Each molecule produces a characteristic IR spectrum when exposed to infrared light.
• The IR spectrum is a fundamental measurement obtained in IR spectroscopy.

IR Spectrum

• An IR spectrum is a graph that plots the infrared light absorbed on the Y-axis against frequency or wavelength on the X-axis.
• Molecules can be identified by comparing their absorption peak with spectral libraries.
• IR spectroscopy is mainly used in inorganic and organic chemistry to determine the functional groups in molecules.
• Different functional groups absorb different frequencies of IR radiation.
• Chemical compounds have different chemical properties due to the presence of different functional groups.
• IR spectroscopy is useful for identifying and analyzing the structure of various compounds and complex mixtures of similar compounds.

Principle of IR Spectroscopy

• Covalent bonds in organic molecules are like stiff springs that can be bent and stretched (vibrations).
• Molecular vibration energy is quantized, meaning a molecule can only stretch and bend at certain "allowed" frequencies.
• Molecules absorb specific light frequencies that correspond to the frequency of their bond vibrations.
• These frequencies lie in the infrared region of the electromagnetic spectrum.
• Transitions occur from a ground vibrational state to an excited vibrational state when molecules absorb IR radiation.
• The absorption of these frequencies is measured using the IR spectroscopic technique.
• Polychromatic light (light having different frequencies) is passed through a sample.
• The intensity of the transmitted light is measured at each frequency in IR spectroscopy.
• For a molecule to be IR active, there must be a change in dipole moment when IR radiation is absorbed.
• The dipole moment changes as the bond expands and contracts.

IR Regions

• The IR region is subdivided into three regions: near IR, mid IR, and far IR.
• The mid IR region is most commonly used.
• Near infrared region: 0.75-2.5 μm wavelength, 14000-4000 cm⁻¹ wavenumber.
• Mid infrared region: 2.5-50 μm wavelength, 4000-400 cm⁻¹ wavenumber.
• Far infrared region: 50-300 μm wavelength, 400-40 cm⁻¹ wavenumber.
• In IR spectroscopy, the MIR region is mostly utilized because absorption bands corresponding to different individual components of a mixture are usually isolated and quantification is easy.
• In the NIR region, the absorption bands corresponding to the different components are weak and tend to be superimposed, making component quantification with NIR is more complex.
• The FIR region is useful for inorganic studies due to stretching and bending vibrations of bonds between the metal atoms and ligands.
• Frequencies at which these vibrations are observed are usually lower than 650 cm-1.

IR Instrumentation

• Infrared spectrometers or spectrophotometers are used to produce infrared spectra.
• There have been three generations of IR spectrometers or spectrophotometers.
• First generation IR spectrometers were invented in the late 1950s and use a prism optical splitting system with NaCl prisms.
• The requirement of the sample's water content and particle size is extremely strict for first generation spectrometers, and they are no longer in use due to narrow scan range and poor repeatability.
• The second generation IR spectrometer was introduced in 1960s and utilizes gratings as the monochromator.
• Second generation IR spectrometers have better performance compared with IR spectrometers with prism monochromator, but still has low sensitivity, low scan speed and poor wavelength accuracy.
• The third generation IR spectrometer Fourier transform infrared (FTIR) spectrometer, marked the abdication of monochromator and the prosperity of interferometer.
• IR spectrometers became exceptionally powerful with the replacement of the monochromator, leading to various applications.
• An FTIR spectrophotometer consists of a radiation source, interferometer, sample chamber/compartment, detector, and electronic display/computer.

Radiation Source

• IR instruments require a source of radiant energy that emits IR radiation.
• The radiation source must be steady and intense enough for detection and should extend over the desired wavelength.
• Radiation sources in IR spectroscopy vary according to the IR region to be measured (NIR, MIR, or FIR).
• Various sources of IR radiations include Nernst glower (rare-earth oxides), Globar (silicon carbides), Laser, incandescent lamp, Mercury arc, Tungsten lamp, and Nichrome wire.

Interferometer

• FTIR utilizes the Michelson Interferometer to modulate the incoming optical radiation.
• The interferometer consists of a beam splitter placed in the path of the incoming radiation.
• The interferometer consists of two mirrors that a perpendicular to each other (one fixed or stationary and one movable).
• The beam splitter transmits half of the light and reflects the other half.
• The transmitted light strikes the stationary mirror while the reflected light strikes the movable mirror.
• The two beams of light recombine at the beam splitter when reflected back by the mirrors.

Sample Chamber

• Sample containers and handling can present a challenge in the infrared region.
• Most materials used to produce cuvettes are not IR-transparent and cannot be used.
• Cells made from alkali halides such as sodium chloride are commonly used due to their transparency in the infrared region.
• A problem with sodium chloride cells is that they absorb moisture and can become fogged; NaCl discs also absorb below 625 cm⁻¹.
• KBr discs are mostly suitable but expensive.
• IR spectroscopy can be used to characterize solid, liquid, or gas samples.
• Liquids may be analyzed in their neat form by placing a small amount of sample on a sodium chloride plate and then placing a second plate on top to form a.
• Place the plates in an appropriate holder in the sample compartment of the instrument.
• This technique provides adequate spectra for qualitative use.
• The use of silver chloride or silver bromide “disposable” windows has gained widespread acceptance for use with liquids
• Sampling of gas is similar to that of liquids.
• Various techniques are used for preparing solid samples including pressed pellet, solid run in solution, and mull techniques.
• Aqueous solvents cannot be used as they dissolve alkali halides; only organic solvents like chloroform can be used.
• Modern FTIR Spec uses ATR accessory to make sampling easier.

ATR-FTIR Spectroscopy

• In ATR-FTIR spectroscopy the sample, evanescent wave and ATR crystal are used with a Puck.
• An IR Beam is also used with reference to the ATR Crystal.

Detectors

• Detectors are used to measure the intensity of unabsorbed infrared radiation.
• Detectors are temperature-sensitive devices that undergo large changes in properties (electrical resistance, temperature-dependent potential) upon small temperature variations.
• Detectors can be classified into thermal detectors, pyroelectric detectors, and photo conducting detectors.
• Thermal detectors can be used over a wide range of wavelengths and they operate at room temperature.
• Pyroelectric detectors consist of a pyroelectric material that is an insulator with special thermal and electric properties.
• Photo conducting detectors are the most sensitive detectors and rely on interactions between photons and a semiconductor material
• The detector consists of a thin film of a semiconductor material such as lead sulphide, mercury cadmium telluride (MCT) or indium antimonide.

Electronic Display

• Recorders are used to record the IR spectrum as a plot of frequency of absorbed radiation and intensity of absorption in terms of transmittance.
• The radiant energy received by the detector is converted into a measurable electrical signal and is amplified by an amplifier.
• The amplified signals are recorded and plotted.
• The recorder will record transmittance of a sample as well as function of wave number.

Advantages of IR Spectrophotometer

• Advantages include high scan speed, high resolution, high sensitivity, wide range of application, large amount of information, and being non-destructive.

Disadvantages of IR Spectrophotometer

• Disadvantages include sample constraint, spectrum complication, and quantification.

Applications of IR Spectroscopy

• Applications include determination of functional groups, constitution and interaction of molecules, identification of chemical species and compounds, determination of purity of compounds, and studying the development and progress of chemical reactions.
• The development and progress of chemical reactions can be determined by studying the IR spectra of reactant and product molecules.