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4.2 UV-Visible Spectrophotometer
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1. function of light source: provide energy to excite the molecules of the substance to be measured, and make them produce electronic spectral bands (provide broadband radiation). Continuous light source: widely used in absorption and fluorescence spectrum (gas discharge light source) deuterium lamp, hydrogen lamp ultraviolet visible argon lamp vacuum ultraviolet xenon lamp vacuum ultraviolet, ultraviolet, visible (thermal radiation light source) tungsten lamp and halogen tungsten lamp visible region.
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2. Monochrome function: Separate monochromatic light from the synthetic light radiated by the light source. 3. The function of the absorption pool: to contain the analysis sample (usually liquid).
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4. Detector function: a device for detecting light signals and measuring the intensity change of monochromatic light after passing through the solution. 5. Signal display system
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6. Ultraviolet-visible spectrophotometer type (1) Single-wavelength single-beam spectrophotometer Disadvantages: The measurement result is greatly affected by power supply fluctuation, and the error is large.
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(2) Advantages of single-wavelength double-beam spectrophotometer: In addition to automatically scanning absorption spectrum, it can also automatically eliminate the influence of power supply voltage fluctuation and reduce the drift of amplifier gain.
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(3) Advantages of dual-wavelength spectrophotometer: In the presence of background interference or absorption interference of components, a component can be quantitatively determined. Differential spectra can also be obtained and measured by coefficient multiplication.
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(4) Multichannel spectrophotometer has the characteristics of fast scanning.
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4.3 Generation of electronic spectra of compounds 4.3. 1 UV-visible absorption spectra of organic compounds (1) The transition types are in the UV-visible region, and the absorption bands of organic compounds are mainly generated by σ→σ *, n→σ*, π→π * and n→π* transitions, and the order of their relative energies is σ→σ* & gt；; n→σ* & gt； π→π* & gt； n→π* .
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σ? π? nσσσ？ σ π? π σ? n σ？ π π? n π？ Electronic transition energy in organic compound molecules
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Σ→Σ * transition: The bonded Σ electrons in the molecule jump to the Σ * antibonding orbit, which is a type of electronic transition that all saturated organic compounds may produce. N→σ* transition: Unbound N electrons in the molecule are excited to σ * orbit, which can occur in all saturated hydrocarbon derivatives containing heteroatoms. Compounds with σ→σ * and n→σ* transitions, such as saturated alkanes, haloalkanes, alcohols and ethers, are good solvents for UV-Vis absorption spectrometry.
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π→π * transition: It can occur in any organic compound molecule containing unsaturated bonds, and its maximum molar absorption coefficient εmax is very large. ? N→π* transition: occurs in unsaturated compounds containing heteroatoms, and its maximum molar absorption coefficient εmax is relatively small. ? Charge transfer transition: when some compounds are irradiated by external radiation, electrons are transferred from the part with donor characteristics to the part with electron acceptor characteristics in the system. Its spectral bandwidth and absorption intensity are large, ε max >; 104。
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NR 1R2NR 1R2+-hυ electron acceptor electron donor CC+hυ electron acceptor electron donor OROR-
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② Common terms 1. Chromophore: an atomic group in a molecule that can absorb photons and produce electronic transitions. (unsaturated group with π bond) 2. Chromophore: Some groups have no chromogenic effect by themselves, but they can enhance the chromogenic ability of chromophores, that is, when they are connected with chromophores, their absorption bands will shift red at the maximum absorption wavelength, and their intensity will increase. Usually groups with non-bonded electron pairs. 3. Red shift and purple shift: The maximum absorption wavelength of the absorption band shifts, which is called red shift in the long wave direction and purple shift in the short wave direction.
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(3) Ultraviolet and visible spectra of organic compounds 1. Saturated hydrocarbons and their substituted derivatives σ→σ *, n→σ*2. Unsaturated hydrocarbons and conjugated olefins σ→σ *, π→π*3. Carbonyl compounds n→σ*, π→π * and n→π*4. Benzene and its derivative E64.
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4.3.2 UV-Vis absorption spectra of inorganic compounds (I) Charge transfer transition Some molecules have both electron donor and electron acceptor characteristics. When an electron transitions from a donor to an acceptor, it will produce strong absorption, and the resulting spectrum is called charge transfer spectrum. For example, in metal complexes, ligands have the properties of electron donors, and metal ions are electron acceptors. When electrons jump from the ligand orbit to the outer orbit of the central atom, the charge transfer spectrum can be generated.
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(2) Coordinate field conversion 1. F-f transition The absorption of ultraviolet and visible light by lanthanide and copper ions is based on internal F-electron transition, and its absorption spectrum consists of some narrow characteristic absorption peaks, which is not easily affected by the coordination environment of metal ions. 2. In the D-D transition, the D orbit of transition metal ions splits under the action of ligand field. D electrons jump between D orbitals with different energy levels, absorbing ultraviolet light or visible light to produce absorption spectra. The absorption band of this spectrum is relatively wide, and the absorption peak is strongly influenced by the coordination environment.
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4.3.3 Factors affecting absorption band 1. Changes in molecular structure: (1) introducing chromophores and chromophores into saturated compounds; (2) Changes of ligand field: octahedron, tetrahedron, square plane, etc.
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(3)*** Yoke Effect and Super * * * Yoke Effect Because the movement range of electrons behind the * * * Yoke increases, the energy required for the transition decreases, so the absorption peak wavelength generated by the * * * Yoke Effect is larger and the absorption intensity is increased. * * * The more unsaturated bonds in the yoke, the more obvious the redshift phenomenon. λ max = 2 1 7 nm and ε max = 2 1 0,000 for 3- butadiene in hexane; For 1, 3,5-hexatriene in isooctane, λmax= 268nm, ε max = 43,000; For 1, 3,5,7,9-pentadecene in isooctane, λmax= 334nm, εmax = 12 1 000.
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(4) steric hindrance ch 3c H3 H3 c(a)(b)λmax(b)>λmax(a) Generally speaking, the steric hindrance of trans isomer is smaller than that of cis isomer, and the * * * planarity of its yoke system is better than that of cis isomer, with lower transition energy and larger λ max.
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2. Solvent effect When the polarity of the solvent changes from nonpolar to polar, the fine structure disappears and the absorption band changes smoothly.
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Changing the polarity of solvent will also change the maximum absorption wavelength: when the polarity of solvent increases, the absorption band produced by n→π* transition will shift purple, while the absorption band produced by π→π * transition will shift red.
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3. With the increase of temperature, the frequency of molecular collision increases, the band widens and the fine structure of the band disappears (thermochromic effect).
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4.4 Analytical method and its application 4.4. 1 Qualitative analysis The absorption spectrum is relatively simple, and the characteristics are not strong, and most simple functional groups have only weak absorption or no absorption in the near ultraviolet region, so the application has certain limitations. However, it can be used to identify the chromophore of * * * and infer the structural skeleton of unknown objects, which is a useful auxiliary method when cooperating with other structural analysis methods.
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(1) qualitative method 1. Compare the absorption spectrum curves: shape, peak number, λmax position and corresponding εmax. 2. Calculate λmax with empirical rules, and then compare it with the measured value.
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(2) Woodward's rule of thumb: calculate the maximum wavelength of π→π * transition of conjugated diene, polyene and conjugated ketene.
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Oαβδδ+ 1γδ+2 parent cyclohexanone 2 15nm homocyclic diene 39nm added two * * * yoke double bonds 60nm, a β alkyl group 12nm, an exocyclic double bond 5nm and three γ+alkyl groups 54 nm * * 385nm.
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AcOABCR parent homocyclic diene added two * * * yoke double bonds at 253nm, three extra-ring double bonds at 60nm and five alkyl groups substituted at 15nm at 25 nm.
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4.4.2 Quantitative analysis (I) Single component quantitative method 1. Calibration curve method 2. Standard comparison method as = kcxbax = kcxbcx = csax/as.
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(2) Multi-component quantitative method A1A+B = ε1ABC1A+ε1bc2b2a+B = ε 2abc1A+ε 2bc2b.
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Determination of manganese and chromium in alloy steel by spectrophotometry. Weigh 1.000g steel sample, dissolve it and dilute it to 50.00mL. Cr is oxidized to Cr2O72- and Mn is oxidized to MnO4-. Then the absorbance values measured by 1.0cm absorption cell at 440nm and 545nm are 0.204 and 0.860 respectively. It is known that ε 440mn = 95.0 L mol-1cm-1,ε 440cr = 369.0 L mol-1,ε 545mn = 2.35×103 L mol-. Find the mass fraction of manganese and chromium in this alloy steel. (MMn=54.94 g mol-1MCr=52.00 g mol-1)
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Solution According to the additivity of absorbance, the simultaneous equations are listed as follows: A440 = A440mn+A440cr = ε 440mnbcmn+ε 440rbccra 545 = A545mn+A545cr = ε 545mnbcmn+ε 545crbccr, that is, 0.204 = 95.0×/kloc-0. The solution of 38+0× CCR 0.860 = 2.35×103×/kloc-0 /× CMN+11.0×1× CCR gives CMN = 3.64×1. l . 10% 100% 1.00054.94 1050 103.64(Mn)-3-4 =×××××=？ 0.24% 100% 1.000252.00 1050 104.59(Cr)-3-4 =××××××=？
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(3) Dual wavelength method 1. Equal absorption wavelength method a1= a1a+a1a2 = a2a+a2b δ a = a2-a1= (a2a-a1a)+(a2b-a655438)
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2. Coefficient ratio method This formula shows that the signal S is only related to the absorbance value of the measured component. aabbbbbabaakakakakakakaakkakakakakakaks 12 12 122 12 12 122 1 1 1 122 120λλλλλλλλλ？ ==? ==+=? The differential amplifier = λ λ can obtain the differential signal S with mixed samples at wavelengths λ2 and λ 1.
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3. Determination of turbid samples In the dual-wavelength method, if λ2 is set at the absorption peak of the sample and λ 1 is set at the wavelength where the sample has no characteristic absorption, the background absorption of λ 1 and λ2 should be equal. Measuring the difference of absorbance between two places by dual-wavelength method can eliminate background absorption and make quantitative analysis.
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(4) Derivative spectrophotometry takes the first or higher derivative of the absorption spectrum curve to obtain various derivative spectrum curves. Advantages: 1 It can distinguish two or more absorption peaks that completely overlap or have a small difference in overlapping wavelengths. 2. It can distinguish the weak absorption peak when the absorbance rises sharply with the wavelength. 3. It can determine the maximum absorption wavelength of broad absorption band. The resolution increases with the increase of derivative order, and the signal-to-noise ratio decreases with the increase of derivative order.
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Quantitative analysis: Aλ= ελbc, wavelength λ is differentiated by n times. Only Aλ and ε λ are functions of wavelength λ, so it can be seen that the absorbance value is still proportional to the absorbent concentration after n times of differentiation. bcdddadnnnnnλλλλε=
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4.4.3 Apply UV-Vis absorption spectrum 1. Determination of relative molecular weight M = εmb/A m: The sample weight in UV-Vis absorption spectrum, λmax and εmax of absorption peaks are almost the same as long as the compounds have the same color skeleton. Therefore, as long as the ε value of the known compound with the same chromogenic skeleton as the test object is found, the relative molecular weight of the test compound can be obtained.
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2. Examples of determination of equilibrium constant: The equilibrium constant λmax of ZnL22- in the following reactions was determined by spectrophotometry to be 480nm, in which ε max = 3.00×103 L. mol-1.cm-1,and Zn2 ++ and L2- had no absorption at 480nm. The absorbance of the solution containing 2.30×10-4 mol l-1Zn 2+and 5.00×10-4 mol l-1l 2-is 0.540, measured with an absorption cell of1.00 cm. +? +22222ZnLLZn
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Solution [] [] [] [] [] [] [] 8245422222214442221544222143221084. ×=+=? ×=×? ×=? =? × =×× = ε =++lznnllmolznlcllznlcznlmolznllzn = then the equilibrium constant is
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3. Structural analysis H3C CH2CC H3OOHH OH H3CCC H3OOH For example, there are two isomers of acetylacetone, ketone isomer and enol isomer, which are mainly ketone isomers in polar solvent water, forming intermolecular hydrogen bonds with λmax of 277nm; ; In the nonpolar solvent hexane, enol isomers predominate, forming intramolecular hydrogen bonds with λmax of 269nm.
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4. Determination of hydrogen bond strength [Example] The absorption band of acetone n→π* is 265.4 nm, that is, 452.96 kJ mol-1hexane (nonpolar) is 279.0 nm, that is, the hydrogen bond strength of 429.40 kJ mol-is 452.96-429.40 = 23.56 kJ.
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Thinking 1. The generation of ultraviolet-visible absorption spectrum is briefly described. 2. Lambert-Beer Law and its mathematical expression. 3. What is the main function of light source? 4. Characteristics of various UV-Vis spectrophotometers. 5. What kind of transition mainly produces the absorption band of organic compounds? 6. What are the factors that affect the absorption band? 7.Woodward rule is used to calculate the maximum wavelength of π→π * transition of * * conjugated dienes, polyenes and * * conjugated ketenes. 8. Multi-component quantitative method and characteristics of dual-wavelength method and derivative method 9. Application of ultraviolet-visible absorption spectrometry