第3单元 uv final.pptVIP

第3章 紫外-可见光谱(Ultraviolet/visible spectroscopy) 3.1 Background Basics of UV Light Absorption Ultraviolet/visible spectroscopy involves the absorption of ultraviolet/visible light by a molecule causing the promotion of an electron from a ground electronic state to an excited electronic state. Ultraviolet/Visible light: wavelengths (l) between 190 and 800 nm. Peak Broadening UV absorptions are generally broad because vibrational and rotational levels are superimposed on top of the electronic levels. Peak broadening 电子跃迁规则 自旋多重性 Pauli原理,同一轨道上2个自旋电子方向相反,自旋量子数 s=±1/2, 自旋多重性S = 2s+1 = 1,称为单充态, 用S表示 (singlet). 当电子由基态激发到另一个能量较高的激发态时,自旋方向可以改变,也可以不变. 自旋方向保持, s=0, S=1,仍是单重态; 自旋方向反转, 此时,量子数之和的绝对值为1, 自旋多重性为3, 为3重态, 用T表示. 电子跃迁选择规则 电子自旋容许跃迁 容许的跃迁要求电子自旋方向不变, s=0, 即跃迁只能在自旋多重性相同的能级之间发生, 如 S0 与 S1, T1与T2之间跃迁为容许跃迁, 跃迁几率大,紫外吸收强. 对称性容许跃迁 对称性容许跃迁,跃迁几率达, 吸收强度大. 如p to p* B. Terminology The following definitions are useful in a discussion of UV/Vis spectroscopy. chromophore： Any group of atoms that absorbs light whether or not a color is thereby produced. auxochrome ： A group which extends the conjugation of a chromophore by sharing of nonbonding electrons. Bathochromic shift： The shift of absorption to a longer wavelength. Hypsochromic shift ： The shift of absorption to a shorter wavelength. Hyperchromic effect ： An increase in absorption intensity. Hypochromic effect ： A decrease in absorption intensity. C. Laws of Light Absorption Beer-Lambert Law The intensity of the absorption band is measured by the percent of the incident light that passes through the sample: Transmittance % = (I / I0) × 100% Because light absorption is a function of the concentration of the absorbing molecules, a more precise way of reporting intensity of absorption is by use of the Beer-Lambert Law: 3.2 Types of Transitions s to s* (alkanes) s to p* (carbonyl compounds) p to p* (alkenes, carbonyl compounds, alkynes, azo compounds) h to s* (oxygen, nitrogen,