Ultraviolet-visible absorption spectrum is produced by valence electron transition after molecules (or ions) absorb ultraviolet or visible light (usually 200-800 nm). Because the energy level transition between electrons is always accompanied by the transition of vibration energy level and rotation energy level, the ultraviolet-visible spectrum presents a wide band.
The abscissa of UV-Vis absorption spectrum is wavelength (nm) and the ordinate is absorbance. Ultraviolet-visible absorption spectrum has two important characteristics: the position of the maximum absorption peak (λmax) and the molar absorption coefficient of the maximum absorption peak (κmax). The wavelength corresponding to the maximum absorption peak represents the characteristic absorption of the compound in the ultraviolet-visible spectrum. The corresponding molar absorption coefficient is the basis of quantitative analysis. An important concept in UV-Vis absorption spectrum: chromophore: an unsaturated group that produces UV or Vis absorption, usually a group with n electrons and π electrons, such as C=O, C=N, etc. When several chromophore yokes appear, the absorption bands produced by several chromophores will disappear and be replaced by a new * * * yoke absorption band, whose wavelength is longer than that of a single chromophore and its intensity is also enhanced. Chromophore: a group that has no ultraviolet absorption, but can strengthen or (and) redshift the absorption peak of chromophore, such as OH, Cl, etc. ; The maximum absorption peak shifts to the long wavelength direction. Blue shift: the maximum absorption peak moves to the short wave direction. Color increasing (decreasing) effect: the effect of increasing (decreasing) absorption intensity. 2. Valence electron transition and types of absorption bands
σ→σ * transition: It has high absorption energy and generally occurs in the vacuum ultraviolet region. C-C in saturated hydrocarbon belongs to this transitional type. For example, the transition of ethane C-C bond σ→σ *, λmax is 135nm. (Note: Because the general ultraviolet-visible spectrophotometer can only provide monochromatic light in the range of 190 ~ 850 nm, it can't detect σ→σ * transition) n→σ* transition: groups containing heteroatoms such as O, N and S, such as -NH2, -OH- and -SH, may produce n→σ* transition. π→π * transition: π→π * transition will occur in groups with π electrons, such as C=C, C≡C, C=O, etc. , generally located in the near ultraviolet region, about 200 nm, ε max ≥104 l mol-1cm-65438. K-band: π→π * transition of * * yoke system is also called K-band, which is related to the number, position and substituent type of * * * yoke system. B-band: The fine structure absorption band produced by π→π * transition of aromatic compounds is called B-band.