What are the characteristics of the information analyzed by ultraviolet absorption spectroscopy?

The ultraviolet band is mainly due to the absorption of organic molecules (conjugation) and n* transitions, which mainly correspond to the absorption of unsaturated bonds or conjugated bonds, and the study of structures with such types of information is studied. The main four types of absorption bands produced are:

1.r absorption band: groups containing double bonded heteroatoms such as o, s, n, etc.

n Absorption bands generated.

Features: The maximum absorption wavelength is larger, smaller (1-10)2kAbsorption band: The absorption band produced by the open-chain conjugated double bond.

Features: The maximum absorption wavelength is more than 200nm, 10003b Absorption band: The absorption band produced by the closure of the annular conjugated double bond.

Characteristics: The maximum absorption wavelength is larger, smaller (but stronger than the R-band, 10-100)4e absorption band: the absorption band produced by the three alkene double bonds in the benzene ring, and it can be.

It is divided into e1 and e2(k).

Features: E1: The maximum absorption wavelength is less than 200nm (184nm, =680) E2 (K): the maximum absorption wavelength is slightly greater than 200nm, 203nm, =880) are about 100---1000

Fast, convenient and easy to operate.

1. Ultraviolet absorption spectroscopy.

and visible absorption spectra both belong to the molecular spectrum, and they are both due to valence electrons.

of the jump. The ultraviolet-visible spectrum is produced by the absorption of ultraviolet and visible light by molecules or ions of a substance.

and the degree of absorption can be analyzed, measured and inferred about the composition, content and structure of the substance.

2. In the molecules of organic compounds, there are orange pep old electrons that form single bonds, electrons that form double bonds, and lone pair n electrons that are not bonded and rounded. When a molecule absorbs a certain amount of radiant energy, these electrons will jump to a higher energy level, and the orbital occupied by the electrons is called the antibond orbital, and this electronic transition is closely related to the internal structure.

Characteristics of UV-Vis absorption spectroscopy

1.In instrumental analysis, UV-Vis spectrophotometry is one of the oldest and most widely used optical analysis methods. It is the use of molecules or ions of substances to absorb a certain wavelength range of light, qualitative analysis, quantitative analysis and structural analysis of substances, based on the spectrum is the absorption spectrum produced by molecules or ions absorbing a specific wavelength of light in the incident light.

2.Compared with other spectral analysis methods, the equipment and operation of the Tomb are relatively simple, the cost is less, and the analysis speed is faster.

Applications of UV-Vis absorption spectroscopy

1.Qualitative analysis.

Determine comodal yoke relationships and certain functional groups. If there is no absorption peak between (200 400) nm, it means that the unknown substance has no conjugation relationship, and it will not be aldehydes or ketones, and it is likely to be a saturated compound.

2.Quantitative analysis.

It is used to determine the concentration or content of a substance, and the determination steps are the same as those of a visible spectrophotometer.

3.Determination of isomers.

For example, ethyl acetoacetate has a keto-enol tautomer. The ketone formula has no conjugated double bond and has weak absorption at 204 nm; The enol formula has a conjugated double bond and has strong absorption at 245 nm. Therefore, their presence or absence can be judged according to their ultraviolet absorption spectrum.

Selective absorption of light by a substance.

Ultraviolet-visible absorption spectroscopy of molecules is a commonly used spectroscopy method for analysis based on the absorption spectra generated by electron transitions within molecules. When a substance is irradiated with light, the molecules of the substance collide with the light, and as a result, the energy of the photons is transferred to the molecules. In this way, the molecule in the ground state in a stable state transitions to an unstable high-energy state, i.e., the excited state

m (ground state) + hv --- m * (excited state).

This is the absorption of light.

Since the energy of matter is discontinuous, i.e., the energy is quantized. Absorption can occur only when the energy (HV) of the incident light is equal to the energy difference between the excited state and the ground state of the molecule of the substance.

e=e2-e1=

hv=hc/λ

Different material molecules have different quantization energy levels due to their different structures, that is, e is different, so the absorption of light is also different.

Noun: Absorption Spectrum Curve (Light Absorption Curve) PPP7: It reflects the absorption of light of different wavelengths by a substance. Figure 2-1 of PPP7 shows the absorption spectra of different concentrations of potassium permanganate solution.

The basis for the qualitative analysis of ultraviolet-visible absorption spectrum: the wavelength at the maximum degree of light absorption is called the maximum absorption wavelength, which is represented by max, when the concentration of the same absorbent substance is different, the shape of the absorption curve is different, and the max is unchanged, but the corresponding absorbance is different, which is the basis for qualitative analysis.

Basis for quantitative analysis of UV-Vis absorption spectroscopy: Lambert-Beale law.

Lambert-Beale Law.

The quantitative analysis of UV-Vis spectrophotometers is based on the Lambert-Beale law.

When the monochromatic light passes through the solution containing the absorbent substance with a certain thickness of the liquid layer, the absorbance of the solution A is proportional to the concentration c of the solution, and the physical meaning of this formula is that when a beam of parallel monochromatic light passes through the solution containing the absorbent substance uniformly, the absorbance of the solution is proportional to the concentration of the absorbent substance and the thickness of the absorbing layer.

Reasons for deviating from Beer's Law.

The main reason: the current instrument does not provide true monochromatic light and changes in the properties of light-absorbing substances.

1) Deviations caused by non-monochromatic light.

2) Deviations due to chemical and physical factors of the solution itself.

UV-Vis absorption spectroscopy: The spectrum of a molecule due to the transition of valence electrons.

The factors influencing UV-Vis absorption spectra mainly include the following aspects:

Molecular structure: The electronic structure and type of chemical bonds of a molecule have a great influence on the absorption of the UV-Vis spectrum. Compounds with conjugated structures typically have strong absorption capacity in the UV-Vis spectral range.

Molecular. Solvent effect: The solvent has an effect on the molecular structure and electronic state distribution of the compound in solution, thus affecting its UV-Vis absorption spectrum. Factors such as the polarity of different solvents, the dielectric constant, and the structure of solvent molecules affect the absorption behavior of compounds.

pH: The pH of a solution has an effect on the absorption spectrum of some compounds. Specific implicit acid-base conditions can cause changes in the charge state of a molecule, which can affect its absorption properties.

pH comparison table.

Temperature: Temperature has an effect on the molecular vibration and rotation rate of compounds in solution, which in turn affects the shape and intensity of the absorption spectrum.

Concentration: The concentration of a compound has an effect on the absorption spectrum. Typically, solutions with higher concentrations show stronger absorption peaks.

Ultraviolet. It should be noted that different compounds exhibit different absorption characteristics in the UV-Vis spectrum, so the influencing factors of the split hall will vary from compound to compound. For specific compounds and experimental conditions, specific experiments and studies are required to determine the specific effects of the influencing factors.

Both infrared absorption spectroscopy and ultraviolet-visible absorption spectroscopy can be used for the qualitative and quantitative determination of substances. It's just that the spectrum required is different.

Ultraviolet: 180 380, visible 380 750, infrared, 750 2000 nm, in different bands.

Infrared absorption spectroscopy is referred to as infrared spectroscopy. In general, the wavelength position of the infrared absorption band and the intensity of the absorption number collection band reflect the characteristics of the molecular structure, which can be used to identify the structural composition of the unknown substance or determine its chemical group. The absorption intensity of the absorption band, on the other hand, is related to the molecular composition or the content of chemical groups, and can be used for quantitative analysis and purity identification. It is often used for structural analysis of chemical components of traditional Chinese medicine.

Infrared spectroscopy analysis has strong characteristics, gas, liquid, and solid samples can be measured, and has the characteristics of low dosage, fast analysis speed, and no destruction of samples. As a result, infrared spectroscopy is not only qualitative and quantitative, like many other analytical methods, but it is also one of the most useful methods for identifying compounds and determining molecular structure.

Ultraviolet-visible absorption spectroscopy is a method to study the composition and structure of substances based on the absorption of radiant energy by the molecules in solution in the ultraviolet and visible spectral regions. Also known as UV and Vis absorbance photometry, it includes colorimetric analysis and UV-Vis spectrophotometry. It is widely used for the qualitative and quantitative determination of organic and inorganic substances.

Ultraviolet-visible absorption spectrometry is a method with high sensitivity, good accuracy, excellent selectivity, easy operation and good analysis speed.