Infrared and Raman spectroscopy analysis

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Data sharing, joint learning* Infrared and Raman spectroscopy Abstract: Both infrared and Raman spectroscopy are molecular vibration spectroscopies. Information on the molecular structure can be obtained through spectrum analysis. Infrared spectroscopy can be applied to any gaseous, liquid, or solid sample. Raman spectroscopy provides rapid, simple, repeatable, and, more importantly, non-destructive qualitative and quantitative analysis. It requires no sample preparation; the sample can be measured directly through a fiber optic probe or via glass, quartz, and fibers. Infrared and Raman spectroscopy are both important methods for analyzing the structure of organic compounds. Keywords: infrared spectroscopy, Raman spectroscopy. Spectral analysis is a major component of modern instrumental analysis; it serves as an important method for determining the structure of organic compounds, and compared to conventional chemical analysis, it offers advantages such as low sample requirements, speed, and high accuracy. With the advancement of science and technology, spectral analysis has become an important tool and modern analytical technique that chemists must master. The nature of light is electromagnetic radiation, and its fundamental property is wave-particle duality. Matter possesses energy and is a dielectric. The interaction between matter and light can be regarded as the transfer of energy by photons, that is, h=E1-E0; this principle is widely applied in spectral analysis. The interaction between electromagnetic radiation and matter essentially involves transitions that occur when matter absorbs light energy. Transition refers to the change in an object’s energy after it absorbs light energy. Since this change is quantized, it is called a transition. Light of different wavelengths has different energies and different types of transitions, which is why there are various spectroscopic analysis methods. The main techniques applied to the structural determination of organic compounds are ultraviolet spectroscopy (UV), infrared spectroscopy (IR), and Raman spectroscopy. 1. Infrared Spectroscopy 1.1 Historical Development In 1800, the British scientist Herschel discovered infrared radiation; at the beginning of the 20th century, people gained a more systematic understanding of the fact that different functional groups possess distinct infrared absorption frequencies. Automatic recording infrared spectrophotometers appeared after 1950. With the advances in computer science, Fourier-transform infrared spectrometers appeared after 1970. Infrared measurement techniques such as total reflection infrared, micro-infrared, photoacoustic spectroscopy, and chromatography-infrared coupling have also been continuously developed and improved, enabling the widespread use of infrared spectroscopy. 1.2 Basic principle: Infrared light with an energy range of 4000 ~ 400 cm-1 is not sufficient to induce transitions in the molecular electronic energy levels of the sample; it only causes transitions between vibrational and rotational energy levels. Since each change in the vibrational energy level is accompanied by changes in many rotational energy levels, the infrared spectrum is also a band spectrum. Molecules are only infrared active during vibration and rotation if the bonds involved experience a net change in dipole moment. It can be seen that for an infrared absorption spectrum to be generated, the following conditions must be met: (1) the energy of the emitted photons must match the energy required for vibration transitions, and (2) there must be a dipole interaction between the radiation and the molecular structure, meaning that molecular vibrations must be accompanied by changes in the dipole moment. Vibration of diatomic molecules: Molecular vibration can be approximated as the atoms in the molecule performing periodic harmonic vibrations around an equilibrium point with very small amplitudes. (Force constant of chemical bonds ； σ wave number cm-1 ； c speed of light μ reduced mass of an atom ； (The equivalent relative atomic mass) The greater the force constant of the chemical bond, the smaller the equivalent relative atomic mass; as a result, the vibration frequency or wavenumber of the chemical bond increases, and the absorption peak appears in the higher wavenumber region. It should be noted, however, that since the force constant of chemical bonds changes as the distance between atoms varies during vibration, molecular vibration is not a strictly harmonic vibration; this deviation is referred to as the anharmonicity of molecular vibration. Therefore, the value calculated using the above formula is only approximately equal to the value measured in reality. Vibration of atoms and molecules: Diatomic molecules can only undergo stretching vibrations, while polyatomic molecules have various other types of vibrations such as deformation vibrations (angle changes or bending vibrations). The number of vibration modes is called the degree of vibration freedom. For a polyatomic molecule with n atoms, the number of vibrational degrees of freedom for a linear molecule is 3n-5, whereas it is 3n-6 for a non-linear molecule. Each vibrational degree of freedom corresponds to a fundamental absorption band in the infrared spectrum (an absorption band resulting from the transition from the ground state to the first vibrational excited state; if the transition occurs to the second or third excited state, it is referred to as a second harmonic peak and a third harmonic peak, respectively). The fundamental frequency peak is the strongest, the second harmonic peak is next strongest, and the third harmonic peak is weak. If a molecule absorbs an infrared photon, thereby stimulating two transitions with fundamental frequencies of v1 and v2 respectively, the resulting absorption frequency should equal the sum of the absorption frequencies of these two transitions; this is why it is called a combination frequency. ). Group frequency and fingerprint region: The vibration frequencies of the same type of groups in different molecules are very similar; absorption bands appear within a certain frequency range, and the frequency of these absorption bands is referred to as the group frequency of the corresponding functional group. It serves as a basis for identifying functional groups. The region where differences in absorption bands occur due to variations in molecular structure is called the fingerprint region. 1.2.4 Vibration coupling: Vibration coupling occurs when two groups are adjacent and their fundamental vibration frequencies differ little; the absorption frequency resulting from vibration coupling is called the coupling frequency. The coupling frequencies deviate from the fundamental frequency, one moving towards higher frequencies and the other towards lower frequencies. When a harmonic or combined frequency is close to a certain fundamental frequency, the absorption bands that arise as a result of their interaction, or the splitting of peaks that occurs, is known as Fermi resonance. Fermi resonance is a universal phenomenon; it exists not only in infrared spectroscopy but also in Raman spectroscopy. Features: Strong specificity, rapid measurement, no damage to the sample, low amount of sample required, simple operation, capable of analyzing samples in various states, low analytical sensitivity, and relatively large errors in quantitative analysis. Infrared spectrometer: An infrared spectrometer has a basic structure similar to that of an ultraviolet-visible spectrophotometer, consisting of a light source, a sample chamber, a monochromator, and a detector. The two instruments differ significantly in the specific materials of their various components. The following is a brief introduction to the light source and detector: 1.4.1 Light Source A light source with low energy is required for measuring infrared absorption spectra. Blackbody radiation is the continuous radiation that is closest to an ideal light source. Infrared light sources that meet this requirement are the radiation produced by stable solids when heated; common examples include Nernst lamps, silicon carbide rods, and incandescent coils. 1.4.2 Detectors There are three types of infrared detectors: thermal detectors, thermoelectric detectors, and photoconductive detectors. The first two are used in dispersive instruments, while the latter two are commonly found in Fourier-transform infrared spectrometers. Infrared spectrometers include dispersive infrared spectrometers and interference spectroscopic infrared spectrometers (Fourier transform infrared spectrometers); the following mainly focuses on Fourier transform infrared spectrometers. It features the advantages of no need for spectroscopy, fast scanning speed, and high performance. Interferometer, sample chamber, detector, computer, light source, interference pattern, spectrum, monitor, plotter. The diagram above shows its working principle; the core component is the Michelson interferometer. The infrared light emitted from the light source is divided into two beams by a beam splitter; after being reflected by the fixed mirror and the moving mirror, these beams reach the detector where interference occurs. When the optical path difference between the moving mirror and the fixed mirror reaches an even multiple of l/2, the coherent lights overlap with each other, resulting in a maximum intensity; when the optical path difference is an odd multiple of l/2, the coherent lights cancel each other out, leading to a minimum intensity. By continuously changing the position of the moving mirror, it is possible to obtain on the detector a graph showing the interference intensity as a function of the optical path difference and the infrared light frequency. 1.5 Applications 1.5.1 Qualitative analysis Infrared spectroscopy is primarily used for the structural identification of organic compounds; by analyzing the spectrum of the sample and comparing it with standard spectra, it is possible to determine the structure of the compound with accuracy. The horizontal axis of an infrared spectrum represents the wavenumber (cm-1, the most common) or wavelength (mm), while the vertical axis represents the transmittance or absorbance. When analyzing infrared spectra, attention should be paid to both the position, intensity, and shape of the absorption peaks. Several vibration-related peaks of the same group should be present simultaneously. The sequence for spectrum analysis is: (1) Check whether the spectrum meets the requirements ； (2) Understand the source of the sample, its physical and chemical properties, other analysis data, the recrystallization solvent used for the sample, and its purity ； (3) Exclude possible “false bands”” ； (4) The molecular formula is determined based on the mass spectrometry and elemental analysis results. Calculate the degree of unsaturation U from the molecular formula ； (5) Determine the types of groups and chemical bonds present in the molecule (functional group region 4000–1330 cm-1 and fingerprint region 1330–650 cm-1) ； (6) By combining other analytical data, determine the structural units of the compound and derive possible structural formulas ； (7) Verification of the molecular structure of the known compound ； (8) Standard spectrum comparison ； (9) Computerized spectral library search. The main standard spectral databases include: the Sadtler standard infrared spectrum database, the Aldrich infrared spectrum library, and the Sigma Fourier infrared spectrum library. 1.5.2 Quantitative analysis: Quantitative analysis by infrared spectroscopy involves determining the composition content by measuring the intensity of characteristic absorption bands. Its theoretical basis is the Lambert-Beer law. Infrared spectra are complex, with many overlapping peaks, making it difficult to identify suitable detection peaks. The infrared spectrum features narrow peak shapes, low light source intensity, and low detector sensitivity; as a result, wider slits must be used, and these factors lead to deviations from Beer’s law. During infrared measurement, it is difficult to determine the thickness of the absorption cell, and it is hard to eliminate the effects of the absorption cell and solvent using a reference cell. All of the above factors mean that infrared spectroscopy is far less suitable for quantitative analysis than ultraviolet-visible spectroscopy. 1. Raman spectroscopy 2.1 Historical development: In 1928, Indian physicist C. V. Raman discovered that when light passes through a transparent solution, a portion of the light is scattered, with a frequency different from that of the incident light; this frequency shift is related to the structure of the molecules that cause the scattering. This scattering is called Raman scattering, and the frequency shift is called the Raman shift. In recent years, thanks to the high sensitivity of CCD detection systems in the near-infrared region, the development of diode lasers that are small in size but high in power, as well as fiber probes that integrate excitation lasers and signal filtering, Raman spectroscopy has seen wider application. 2.2 Basic principle When a gas, liquid, or transparent sample is illuminated with monochromatic light whose wavelength is much smaller than the particle size of the sample, most of the light passes through in its original direction, while a small portion is scattered at different angles, resulting in scattered light. When viewed in the vertical direction, in addition to Rayleigh scattering with the same frequency as the original incident light, there is also a series of weak Raman spectral lines symmetrically distributed at frequencies shifted from that of the incident light; this phenomenon is known as the Raman effect. The number of Raman lines, the magnitude of their shifts, and the length of the lines are directly related to the vibrational or rotational energy levels of the molecules in the sample. Therefore, similar to infrared absorption spectroscopy, the study of Raman spectroscopy can also provide information on molecular vibrations or rotations. When measuring Raman scattering spectra, it is generally chosen that the energy of the excitation light is greater than the energy of the vibrational levels but lower than the energy difference between electronic levels, and it should be far away from the UV-visible absorption peaks of the analyte. When the excitation light interacts with the sample molecules, these molecules are excited to higher-energy virtual states (represented by dashed lines in the diagram). The set of lines on the left represents the energy changes of molecules after interacting with light; thick lines indicate a higher probability of occurrence, while thin lines indicate a lower probability, as most molecules at room temperature are in the lowest vibrational energy level of their ground state. The middle set of lines represents Rayleigh scattering, in which photons undergo elastic collisions with molecules; during these collisions, only the direction of the photons changes, with no exchange of energy. The set of lines on the right represents Raman scattering: when photons collide with molecules, energy is exchanged, with the photons transferring part of their energy to the sample molecules or acquiring some energy from them, thereby changing the frequency of the light. The change in the frequency of the scattered light caused by a change in energy is called the Raman shift. Since at room temperature the greatest number of molecules are in the lowest vibrational level of the ground state, and the greatest number of molecules return to the same vibrational level after interacting with photons, the order of probability for the occurrence of the aforementioned scattering is: Rayleigh scattering > Stokes line > anti-Stokes line. As the temperature increases, the intensity of the anti-Stokes line increases. Its schematic diagram is as follows: ground state, virtual state, Rayleigh scattering, Raman scattering. E=hv, E=hv±ΔE. file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtmlclip1/01/clip_image019.gif file:///C:/DOCUME~1/ADMINI~1/LOCALS~1/Temp/msohtmlclip1/01/clip_image020.gif 2.3 Characteristics: (1) The wavelength shift occurs in the mid-infrared region. Molecules with infrared and Raman activity have infrared spectra and Raman spectra that are similar. (2) Various solvents can be used, especially those suitable for measuring aqueous solutions, and sample preparation is simple. (3) Measurement in the low wave number range is easy (such as the vibrations of bonds between metals and oxygen or nitrogen, e.g.,M-O, M-N). The far-infrared region of the infrared spectrum is not suitable for aqueous solutions, making it difficult to select window materials and detectors. (4) The temperature of the sample system can be determined from the intensity ratio of the Stokes and anti-Stokes lines. (5) The spatial resolution of micro-Raman is very high, at 1 mm. (6) Time-resolved measurements can track dynamic reaction processes on the order of 10–12 s. (7) Resonance Raman and surface-enhanced Raman can improve the detection sensitivity. The resonant Raman effect can be used to selectively enhance the vibration of specific chromophores in large biomolecules, allowing the Raman intensity of these chromophores to be increased selectively by a factor of 1000 to 10,000. (8) The diameter of the laser beam at its focal point is usually only 0.2–2 millimeters, so conventional Raman spectroscopy requires only a small amount of sample. This is a major advantage of Raman spectroscopy over conventional infrared spectroscopy. Furthermore, the objective lens of a Raman microscope can further focus the laser beam to 20 microns or even smaller, allowing the analysis of samples with even smaller areas. However, laser light sources may damage the sample; fluorescence-based measurements are generally not applicable, and near-infrared laser excitation must be used instead. 2.4 Raman Spectrometer A Raman spectrometer consists of a laser source, a collection system, a spectroscopy system, and a detection system. The laser source typically uses a laser with high energy concentration and high power density. The collection system is made up of lens arrays, while the spectroscopy system employs gratings or notch filters in combination with gratings to eliminate Rayleigh scattering and stray light. The detection system uses photomultiplier tube detectors, semiconductor array detectors, or multi-channel charge-coupled devices. Since Raman scattering is very weak, a high intensity light source is required, and laser light sources are generally used. There are visible and infrared laser light sources, etc. Such as ultraviolet lasers with 308nm and 351nm emission lines ； Ar+ lasers generally emit light in the visible range at wavelengths such as 488.0 nm and 514.5 nm ； The Nd:YAG laser, on the other hand, is used in the near-infrared region at 1064 nm. 2.5 Applications of Raman spectroscopy 2.5.1 Qualitative analysis Different substances have distinct characteristic spectra, so qualitative analysis can be carried out using these spectra. 2.5.1.1 Raman spectrogram: The abscissa represents the Raman shift, expressed in wavenumbers, where and are the Stokes shift and the wavenumber of the incident light, respectively. The vertical axis represents the Raman intensity. Since the Raman shift is independent of the excitation light, only the Stokes shift component is generally used. For fluorescent molecules, the anti-Stokes shift is sometimes used. 2.5.1.2 Raman Spectroscopy Databases The existing Raman spectroscopy databases include: GRAMS (Infrared/Raman Database for Pharmaceutical Excipients), GRAMS (Raman Polymer Database), GRAMS (Raman Forensic Analysis Database), GRAMS (Raman Organic Chemistry Database), and GRAMS (Nicolet Collection of Raman Spectroscopy). 2.5.2 Quantitative analysis: The quantitative analysis of Raman spectroscopy is based on the following parameters: I, which represents the intensity of the Raman signal from the sample surface collected by the optical system; K, the Raman scattering cross-section of the molecules; Φ, the laser power incident on the sample surface; k and k’, the absorption coefficients of the incident light and the scattered light respectively; Z, the distance traveled by the incident light and the scattered light; h(z), the transfer function of the optical system; and b, the thickness of the sample cell. ) Under certain conditions, the intensity of the Raman signal is proportional to the concentration of the sample that causes the Raman scattering. 2.5.3 Application Technologies Conventional Raman spectroscopy can be used to analyze inorganic materials such as semiconductors and ceramics. Such as residual stress analysis, crystal structure determination, etc. Raman spectroscopy is also an important tool for the analysis of synthetic polymers and biological macromolecules. Such as the analysis of molecular orientation, protein thiol groups, porphyrin rings, etc. Furthermore, Raman spectroscopy has important applications in the analysis of combustion products and atmospheric pollutants. 2.5.3.1 Resonant Raman RRS uses the region adjacent to the UV-vis absorption peak of the analyte as the excitation wavelength. After absorbing light, the sample molecules transition to a higher electronic energy level and immediately return to a certain vibrational energy level of the ground state, resulting in resonant Raman scattering. This process is very short, about 10-14 seconds. Fluorescent emission occurs when, after molecular absorption, vibration relaxation takes place, returning to the first vibrational level of the first electron excited state, and light is emitted as it returns to the ground state. The fluorescence lifetime is generally 10-6 to 10-8 seconds. The resonance Raman intensity can be 102–106 times higher than that of conventional Raman spectroscopy, with a detection limit of 10–8 moles per liter; in contrast, conventional Raman spectroscopy can only be used to measure samples with concentrations of 0.1 moles per liter or higher. Therefore, the RRS method is used for high-sensitivity determination and state analysis, such as the determination of aqueous solutions of low-concentration biomacromolecules. The main drawback of resonant Raman is fluorescence interference. 2.5.3.2 Surface-Enhanced Raman SERS Surface-enhanced Raman is a technique that uses conventional Raman spectroscopy to analyze samples adsorbed on the surfaces of metallic particles such as silver, gold, or copper, or on the rough surfaces of these metal sheets. Although the reason remains unclear, it has been found that the intensity of the Raman spectrum of the adsorbed samples can increase by a factor of 103–106. If surface-enhanced Raman spectroscopy is combined with resonant Raman spectroscopy, the net increase in spectral intensity is almost the sum of the enhancements from both methods. The detection limit can be as low as 10-9 to 10-12 moles/liter. Surface-enhanced Raman is mainly used for analyzing the state of adsorbed species, etc. 2.5.3.3 Electrochemical In-situ Raman Spectroscopy Electrochemical in-situ Raman spectroscopy makes use of the scattering phenomenon in which the frequency of incident light undergoes significant changes due to interaction with molecular structures. Monochromatic incident light (including circularly polarized and linearly polarized light) is used to excite the electrode surface, whose potential is modulated. By measuring the changes in the Raman spectroscopic signals that are scattered back (such as frequency, intensity, and polarization properties), it is possible to determine the relationship between these changes and parameters such as electrode potential or current intensity. The Raman spectra of ordinary molecular substances are very weak; to obtain an enhanced signal, the surface of the electrode can be roughened, which yields surface-enhanced Raman scattering spectra with intensities 104–107 times higher. When molecules with a resonance Raman effect adsorb onto the roughened electrode surface, surface-enhanced resonance Raman scattering spectra are produced, whose intensities can be increased by another factor of 102–103. The current research advancements in the use of electrochemical in-situ Raman spectroscopy include: expanding the applicable systems to transition metal and semiconductor electrodes through surface enhancement treatments; providing a molecular-level description of electrochemical adsorption by analyzing the structure and orientation of species adsorbed on the electrode surfaces as well as the relationship between their SERS spectra and electrochemical parameters; and obtaining \"time-resolved spectra\" that vary with two different potentials by changing the frequency of the modulation potential, thereby enabling the analysis of the relationship between the SERS peaks of the system and the potential, and addressing the issues arising from changes in the SERS-active sites on the electrode surface due to variations in potential. 2. Comparison between infrared spectroscopy and Raman spectroscopy 3.1 Similarities For a given chemical bond, its infrared absorption frequency is equal to the Raman shift; both represent the energy of the first vibrational level. Therefore, for a given compound, the infrared absorption wavenumbers of certain peaks are exactly the same as the Raman shifts; both the infrared absorption wavenumbers and the Raman shifts lie in the infrared region, and both provide information on the molecular structure. 3.2 Differences: (1) In infrared spectroscopy, both the incident light and the detected light are infrared light, whereas in Raman spectroscopy, the incident light is mostly visible light, and the scattered light is also visible light ； (2) Infrared spectroscopy measures the absorption of light, with the abscissa expressed in wavenumber or wavelength, whereas Raman spectroscopy measures the scattering of light, with the abscissa being the Raman shift ； (3) Their formation mechanisms are different. Infrared absorption is caused by changes in the molecular dipole moment or charge distribution resulting from vibrations. Raman scattering is caused by a temporary polarization resulting from the instantaneous deformation of the electron cloud distribution around bonds; it represents a change in polarizability that generates an induced dipole, and the scattering occurs as the system returns to its ground state. While scattering occurs, the electron cloud returns to its original state ； (4) Infrared spectroscopy uses a Nernst lamp, silicon carbide rod, or incandescent coil as the light source, while Raman spectrometers use a laser as the light source ； (5) No pretreatment is required for the sample when using Raman spectroscopy. When analyzing samples using infrared spectroscopy, the samples need to undergo pretreatment; for liquid samples, the liquid film method is commonly used, while for solid samples the paste preparation method is applicable. For polymer compounds, the film method is typically used. The measurement of bulk samples can be carried out using large-volume gas cells with window gaps of 2.5–10 cm ； (6) Infrared spectroscopy primarily reflects the functional groups of molecules, whereas Raman spectroscopy mainly reflects the molecular skeleton and is used primarily for analyzing biological macromolecules ； (7) Raman spectroscopy and infrared spectroscopy can complement each other; for molecules with a symmetry center, there is a rule of exclusion: vibrations that are symmetric with respect to the symmetry center are invisible in infrared spectroscopy but visible in Raman spectroscopy ； Vibrations that have no symmetry relationship with the center of symmetry are visible in infrared but not in Raman spectroscopy. 4. Summary Infrared and Raman spectroscopy are important analytical methods; they share common features as well as differences, and they can complement each other. Both are widely used for determining the structure of compounds as well as for qualitative analysis.