Infrared spectroscopy (IR, Fourier)
Infrared spectroscopy (IR, Fourier)
In organic molecules, the atoms that make up the chemical bonds or functional groups are constantly vibrating at frequencies comparable to those of infrared light. Therefore, when the organic molecules are irradiated with infrared light, the chemical bonds or functional groups in the molecules may absorb vibration. The absorption frequencies of different chemical bonds or functional groups are different and will be in different positions in the infrared spectrum to obtain the chemical bonds or functional groups contained in the molecule Information.
When a bundle of infrared light with continuous wavelengths passes through a substance, the vibration frequency or the rotational frequency of a group of substance molecules is the same as the frequency of infrared light, and the molecule absorbs energy from the kinetic energy level of the original ground state transition to energy High vibration (rotation) kinetic energy level, the molecular absorption of infrared radiation occurs after the vibration and rotational energy level transition, where the wavelength of light is absorbed by the material. So, infrared
Spectroscopy is essentially a method of determining the molecular structure of a substance and identifying a compound based on information such as relative vibrations and molecular rotation between atoms in the molecule. The molecular absorption of infrared light recorded with the instrument, you get infrared spectroscopy. Infrared spectroscopy usually wavelength (λ) or wavenumber (σ) as abscissa, indicating the absorption peak position, the transmittance (T%) or absorbance (A) as the ordinate, said absorption intensity.
When an external electromagnetic wave irradiates a molecule, if the energy of the irradiated electromagnetic wave is equal to the two-level difference of the molecule, the electromagnetic wave of the frequency is absorbed by the molecule, thereby causing the transition of corresponding energy level of the molecule. Electromagnetic wave energy and molecular energy difference between the two equal to produce infrared absorption spectroscopy must meet one of the conditions, which determines the location of the absorption peak.
The second condition of infrared absorption spectroscopy is the coupling between infrared light and molecules. In order to meet this condition, the molecular vibration must change its dipole moment. This actually ensures that the energy of the infrared light can be transmitted to the molecule, which is achieved through the change of the molecular vibration dipole moment. Not all vibration will produce infrared absorption, only the dipole moment changes in the vibration can cause the observed infrared absorption, this vibration is called the infrared vibration; dipole moment is equal to zero molecular vibration can not produce infrared absorption, known as infrared Active vibration.
Molecular vibration can be divided into two categories: stretching vibration and bending vibration. The former refers to the reciprocating motion of atoms along the key axis direction, the key length changes during vibration. The latter refers to the vibration of the atom perpendicular to the direction of the chemical bond. Different symbols are usually used to represent different forms of vibration. For example, the telescopic vibration can be divided into symmetrical telescopic vibration and anti-symmetrical telescopic vibration, which are denoted by Vs and Vas, respectively. Bending vibration can be divided into in-plane bending vibration (δ) and out-of-plane bending vibration (γ). In theory, every fundamental vibration can absorb and
infrared spectrometer
The same frequency of infrared light in the infrared spectrum corresponding to the location of an absorption peak. In fact, some of the vibration molecules have no change in dipole moment and are infrared inactive. In addition, some vibrations have the same frequency and degenerate, and some vibration frequencies exceed the range that the instrument can detect. All these make the actual infrared spectrum The number of absorption peaks is much lower than the theoretical value.
Various groups of molecules have their own specific infrared absorption peaks. In different compounds, the absorption vibration of the same functional group always appears in a narrow wave number range. However, it does not appear on a fixed wave number. The specific wave number appears in the group and the environment in which the group is located. The factors that cause the frequency shift of the group are many, in which external factors are mainly the physical state and chemical environment in which the molecule is located, such as temperature effect and solvent effect. For the internal factors leading to the shift of the frequency of the groups, there are hitherto known the electrical effects of substituents in the molecule: induction, conjugation, mediation, dipolar field effects, etc .; mechanical effects such as mass effect, tension induced The key angle effect, the coupling effect between vibration and so on. Although many studies have been reported on these issues and have been systematically discussed, it is often difficult to predict the direction and magnitude of group frequency shifts quantitatively according to the result of some kind of effect, because these effects Mostly not a single appearance. In this way, it is very difficult to compare different molecules.
In addition, the hydrogen bond effect and coordination effect can also lead to the shift of the group frequency, which is an external factor if occurring in the intermolecular, and belongs to the intramolecular factor if occurring in the molecule.
The intensity of the infrared band is a measure of the probability of vibration transition, and the probability of transition is related to the change of dipole moment when the molecule vibrates. The greater the change of dipole moment, the larger the intensity of the band. The change of dipole moment is related to the intrinsic dipole moment of the group itself, so the stronger the group polarity is, the larger the change of dipole moment during vibration is, and the stronger the absorption band is. The higher the symmetry of the molecule is, The smaller the moment change, the weaker the absorption band.
1. Infrared spectral partition
The infrared spectrum is usually divided into three regions: the near-infrared region (0.75-2.5μm), the mid-infrared region (2.5-25μm) and the far-infrared region (25-300μm). In general, near-infrared spectroscopy is produced by multiplying and combining frequency of molecules. The mid-infrared spectrum belongs to the fundamental frequency vibrational spectrum of the molecule. The far-infrared spectrum belongs to the rotational spectrum of the molecule and the vibrational spectra of certain groups.
Since most of the fundamental absorption bands of organic and inorganic species are present in the mid-infrared region,
Near infrared spectrometer
Infrared is the most studied and applied areas, the accumulation of the most information, the most sophisticated instrument technology. The so-called infrared spectrum refers to the mid-infrared spectrum.
2 infrared spectrum partition
According to the source of the absorption peak, the infrared spectrum of 2.5 ~ 25μm can be roughly divided into two regions, the characteristic frequency region (2.5 ~ 7.7μm) and the fingerprint region (7.7 ~ 16.7μm).
Among them, the absorption peak in the characteristic frequency region is basically generated by the stretching vibration of the group, but the number is not very large, but it is very characteristic. Therefore, it is of great value in the group identification and is mainly used for the identification of functional groups. Such as carbonyl, whether it is in ketones, acids, esters or amides and other compounds, stretching vibration always appears in a strong absorption peak around 5.9μm, 5.9μm or so there is a strong absorption peak in the spectrum, you can roughly determine There are carbonyl molecules.
The fingerprint area is different, the area is more complex and has no strong characteristic. The area is mainly composed of the stretching vibration of some single bonds CO, CN and CX (halogen atoms) and the bending vibration of hydrogen-containing groups such as CH, OH And CC skeleton vibration. When the molecular structure is slightly different, the absorption of the area there are subtle differences. This situation is like everyone has a different fingerprint, which is called the fingerprint area. The fingerprint zone is useful for distinguishing similar compounds.
Spectral classification
Infrared spectrum can be divided into two types of emission spectra and absorption spectra.
The infrared emission spectrum of an object mainly depends on the temperature and chemical composition of the object. Due to the difficulty of testing, the infrared emission spectrum is only a new experimental technique under development, such as laser-induced fluorescence. Will be a bunch of different wavelengths of infrared radiation to the material molecules, some of the specific wavelengths of infrared radiation is absorbed, the formation of the molecular infrared absorption spectrum. Each molecule has a unique infrared absorption spectrum determined by its composition and structure, which is a molecular spectrum.
For example, water molecules have a wide absorption peak, so the molecular infrared absorption spectrum belongs to the banded spectrum. Atoms also have infrared emission and absorption spectra, but they are all linear spectra.
Infrared absorption spectra are generated by the molecules constantly vibrating and rotating. Molecular vibration refers to the relative movement of each atom in the molecule near the equilibrium position. Polyatomic molecules can be composed of a variety of vibration patterns. When the atoms in the molecule to the same frequency, the same phase near the equilibrium position for simple harmonic vibration, this vibration mode called Jane vibration.
The molecule containing n atoms should have 3n-6 normal vibrational modes; for linear molecules, there are only 3n-5 normal vibrational modes. Taking non-linear triatomic molecules as an example, there are only three simple vibration modes. In v1 and v3 vibration, only the chemical bond elongation and shortening, known as the stretching vibration, and v2 vibration mode changes the chemical bond between molecules called angular vibration, they are the main way of molecular vibration. The molecular vibrational energy corresponds exactly to the photon energy of the infrared ray. Therefore, when the vibrational state of the molecule changes, the infrared spectrum can be emitted, and the infrared absorption spectrum can also be generated due to the vibration of the molecule excited by the infrared radiation.
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