Raman spectroscopy is a method of molecular process spectroscopy based on the interaction of light with matter. It allows getting data about the material structure or its characteristics, and in this regard, it is similar to the method of FTIR spectroscopy. Raman process spectroscopy is based on the study of scattered light, while IR spectroscopy is based on the absorption of the light. Raman spectroscopy provides information about intramolecular and intermolecular vibrations and helps to get a more complete data of the reaction.
Both Raman and FTIR spectroscopy gives a spectral characteristic of molecular vibrations (the “molecular imprint”) and are used to identify substances. Herewith, Raman spectroscopy can provide additional information on low-frequency modes and vibrations, which indicate the features of the crystal lattice and molecular structure.
Raman spectroscopy is used to monitor crystallization processes, mechanisms and reaction kinetics. In combination with analytical tools, this data allows better understanding and optimizing the response.
The principle of Raman process spectroscopy is based on the interaction of light with molecules in a gas, liquid or solid, while the vast majority of photons are scattered, having the same energy as the incident photons. The Raman effect is widely applied in various fields, from medical diagnostics to materials science and reaction analysis. The Raman effect allows studying the vibration characteristics of the molecule, giving information about how it is arranged and how it interacts with other molecules.
In contrast to Fourier-transform infrared spectroscopy, Raman process spectroscopy demonstrates changes in the polarizability of molecular bonds. The interaction of light with a molecule can cause deformation of its electronic cloud. This deformation is called a change in polarizability. Under certain energy transitions, accompanied by changes in the polarizability of molecular bonds, active Raman modes arise.
It should be noted that since the Raman effect is weak, the optical components of the Raman spectrometer should be specially optimized and well-adjusted. In addition, since organic molecules may cause fluorescence under the influence of short-wave radiation, monochromatic sources with a long wavelength are commonly used, such as solid-state diode lasers that emit light at a wavelength of 785 nm.
Raman spectroscopy is used in industry for solving various problems, including:
- Study of crystallization processes;
- Identification of polymorphic forms;
- Study of polymerization reactions;
- Study of the reactions of hydrogenation;
- Chemical synthesis;
- Biocatalysis and enzymatic catalysis;
- Chemical processes in the flow;
- Monitoring of biological processes;
- Study of synthesis reactions.
Although the methods of FTIR and Raman process spectroscopy are interchangeable in many cases and complement each other well, there are differences that should be considered when choosing one method or another in practice. Most molecules with symmetry can be identified both in the infrared and Raman spectra. A special case is represented by molecules with the center of inversion.
If the molecule has an inversion center, then the Raman scattering bands and the IR bands will be mutually exclusive, that is, the link will be active either in the Raman or in the IR spectrum. There is a general rule: functional groups with strong changes in the dipole moment are clearly visible in the IR spectrum, whereas functional groups with weak changes or with a high degree of symmetry are better seen in the Raman spectra.
Raman spectroscopy is recommended in the following cases:
- if it is required to examine carbon bonds in aliphatic and aromatic rings;
- if it is necessary to identify bonds that are difficult to see in the IR spectra (for example, O–O, S–H, C=S, N=N, C=C, etc.);
- if the study of particles in solution is carried out, for example in the study of polymorphism;
- if low-frequency modes are studied (e.g. in inorganic oxides);
- to study reactions in the water environment;
- if it is easier and safer to observe the reaction through a viewing window (for example, catalytic reactions under high pressure, polymerization);
- to study the low-frequency vibrations of the crystal lattice;
- to determine the beginning and end of the reaction, to study the stability of the product in two-phase and colloidal reactions.
FTIR spectroscopy is recommended in the following cases:
- for the study of liquid-phase reactions;
- if the reactants, reagents, solvents and other components, involved in the reaction, fluoresce;
- if connections with strong dipole moment change are important (for example, C=O, O–H, N=O);
- if the reagents and the reactants have a low concentration;
- if the solvent bands appear strongly in the Raman spectrum and can suppress the signal of the main components;
- if the intermediate reaction products are active in the IR spectrum.
Raman spectroscopy has many advantages. Since visible-light lasers are used in Raman spectrometers, flexible fiber optic cables made from quartz glass fibers can be used to excite a sample and collect scattered radiation. If necessary, these fiber cables can be quite long.
Since visible light is used, samples can be placed in glass or quartz containers. Therefore, a Raman spectroscopy probe can be put into the reaction medium or Raman spectra can be recorded through a window, for example, in an external sampling loop or in a flow cell during studying chemical reactions. The latter method eliminates the possibility of sample contamination.
Since quartz or high-quality sapphire can be used as a window material, Raman spectra of catalytic reactions can be observed in high-pressure cells. During the study of catalysts, the operative process spectroscopy using the Raman effect is useful for studying in situ reactions on catalytic surfaces in real-time.
Another advantage of the Raman process spectroscopy is that hydroxyl bonds are not very active in the Raman spectrum, and therefore, this sensing technique is suitable for aqueous media. Raman spectroscopy is considered to be non-destructive, although laser radiation may affect some samples. It is necessary to consider how specific a sample may tend to fluorescence when choosing this method. Raman spectroscopy scattering is a weak effect, and fluorescence can suppress the signal, making it difficult to obtain high-quality data. This problem can be easily solved using an excitation source with a longer wavelength.
As for the analysis of reactions, Raman process spectroscopy is sensitive to many functional groups but it is particularly effective in obtaining information about the molecular structure. The Raman spectrum uniquely defines molecules. Since Raman spectroscopy is based on the polarizability of bonds and is capable to measure low frequencies, the process spectroscopy is sensitive to lattice vibrations, which provide information about polymorphs. FTIR process spectroscopy is less informative there. This makes it possible to use Raman spectroscopy with great efficiency in the study of crystallization and other complex processes.
A modern compact Raman spectrometer consists of several main components, including a laser, which serves as a source of molecule excitation for inducing Raman scattering. Usually, modern Raman spectrometers use solid-state laser systems with wavelengths of 532, 785, 830 and 1064 nm. Lasers with shorter wavelengths have a larger scattering area, so the signal is ultimately more powerful, but fluorescence occurs more often at such lengths.
Fiber optic cables are used to transmit laser energy. Band-pass or edge filters are used to eliminate Rayleigh and anti-Stokes scattering, and the remaining light that has undergone Stokes scattering is transmitted to the dispersion element — usually a holographic grating.
The following types of Raman spectroscopy techniques are identified:
- Resonance Raman scattering spectroscopy, where the frequency of the laser radiation is selected in accordance with the electronic transitions in the molecule or crystal, which correspond to the excited electronic states. This approach allows for obtaining high scattering intensity in the absence of unwanted fluorescent interference, the frequency of which is lower than the frequency of exciting radiation.
- Coherent anti-Stokes Raman spectroscopy. This method requires the use of two lasers, one of which has a fixed and the other a variable generation frequency. A spectrum of resonant Raman scattering is achieved by varying the frequency of the tunable laser.
- Surface-enhanced Raman scattering spectroscopy, in particular, for the study of biomolecules imparted to nanoparticles of noble metals.
- Tip-enhanced Raman scattering spectroscopy is a special type of surface-enhanced Raman spectroscopy, in which the SPM probe is applied to amplify the signal.
- Optical Tweezers Raman Spectroscopy is used to study individual particles, as well as biochemical processes in cells captured by optical tweezers – a device that allows for manipulating microscopic objects using laser light.
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