紫外可见光谱法？ 一、紫外可见吸收光谱与分子结构的关系（UV-Vis absorbed spectrum –molecular structure relation） 1. UV-Vis吸收的化学由来（Chemical origins of UV-Vis absorption）分子外层电子的分子轨道可以分为五种,即σ成键与σ* 反键轨道,π成键与π*反键轨道,ｎ非键轨道。 A. σ分子轨道(σ(bonding) molecular orbital), 如: │ │ -C--C- │ ︱ B.π键轨道(π(bonding) molecular orbital), 如: C＝C C＝O -N＝N- -C≡C- C.ｎ键轨道((non-bonding) molecular orbital), 如： -C--Br: -C—O:H -C--N:H In addition, two typed of antibonding orbital may be involved in the transitions: D. σ*反键轨道(sigma star)orbital E. π*反键轨道(pi star)orbital σ，π，n 键轨道为基态轨道（ground state orbital） ，σ*，π*为激发态轨道(excited state orbital). 2、分子电子能级和跃迁（Molecular-electronic orbital and transition） The following electronic transition can therefore occur by the absorption of ultraviolet and visible light : σ→σ*，ｎ→σ*，π→σ*,ｎ→π* , π→π*, σ→π* . A. σ→σ* 跃迁（ σ→σ* transition） △E 较大,跃迁发生在远紫外区,波长范围低于 200nm。如甲烷(125nm),乙烷 (135 nm)。 B.ｎ→σ* 跃迁(ｎ→σ* transition) △E 较σ→σ* 跃迁要小,跃迁发生在150--250nm波长范围内。如含有杂原子饱和烃衍生物。摩尔吸收系数一般在100-300范围内。 由ｎ→σ* 跃迁而产生吸收的一些例子化合物 最大波长 摩尔吸收系数 化合物 最大波长 摩尔吸收系数 (nm) (nm) H2O 167 1480 (CH3)2S 229 140 CH3OH 184 150 (CH3)2O 184 2520 CH3Cl 173 200 CH3NH2 215 600 CH3Br 204 200 (CH3)2NH 220 100 CH3I 258 365 (CH3)3N 227 900 C. ｎ→π* 和 π→π* 跃迁 这两类跃迁是最有用的。△E 比较少,最大吸收波长均大于200 nm 。这两类跃迁的差别在于吸收峰的强度不同。ｎ→π* 跃迁摩尔吸收系数很少,仅在10-100范围内。而π→π*跃迁摩尔吸收系数很大,比ｎ→π* 跃迁大100-1000倍,达到1000-100000。 ｎ→π* 和π→π* 跃迁的吸收特征 生色团 例 子 溶 剂 λmax(nm) 摩尔吸收系数 跃迁类型烯烃 C6H13CH＝CH2 正庚烷 177 13000 π→π* 178 10000 炔 C5H11≡CCH3 正庚烷 196 2000 π→π* 225 165 酮 (CH3)2C=O 280 16 n→π* 醛 CH3CH=O 293 12 n→π*其它 CH3C(NH2)=O 214 60 n→π* CH3NO2 280 22 n→π* CH3N=NCH3 339 5 n→π* 3.分子结构和光谱的相互关系（Correlation of molecular structure and spectrum） A.共轭效应（Conjugation effect ） 当分子含有多个π键,并且被单键隔开时,共轭效应增加,π→π* 跃迁能量更低,吸收光谱最大吸收峰向长波方向移动,摩尔吸收系数增大。称红移效应(red shift effect)。 π→π*transition, when occurring in isolated groups in a molecule, give to absorptions of fairly low intensity. However, conjugation of unsaturated groups in a molecule produces a remarkable effect upon the absorption spectrum. The wavelength of maximum absorption moves to a longer wavelength and the absorption intensity often increases greatly. 如: λmax(nm) εmax C＝C 180--200 10000 C＝C－C＝C 217 21000 C＝C－C＝C－C＝C 258 35000 The same effect occurs when groups containing n electrons are conjugated with a πelectron group,e.g. Acetone O O ‖ ‖ CH3-C-CH3 CH3-C-CH＝CH2 λmax(nm) 290 λmax(nm) 325 含有π电子芳香体系,最大吸收向紫外方向移动。称蓝移效应(blue shift effect)。 Aromatic systems, which contain πelectrons, absorb strongly in the ultraviolet:C. 助色团: 一些原子和原子团不吸收200-800nm范围内的光,但与生色团结合后,具有能使生色团的吸收峰向长波或短波方向移动的作用,这样的原子或原子团称为助色团。 Auxochromes: An auxochrome is a group which does not absorb significantly in the region 200-800nm, but which affect the spectrum of the chromophore to which it is attached. Examples of auxochromes are : -CH3 -OH -NH2 -NO2 Auxochromes can have the following effects: λmax shifted to longer wavelength, “red shift”-Bathochromic effect. λmax shifted to shorter wavelength, “blue shift”-Hypsochromic effect. εmax increased, (increased intensity)- Bathochromic effect. εmax decreased, (decreased intensity)-Hypsochromic effect.查看更多
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