1、Hello everyone, welcome to the organic spectroscopic analysis course. In last lecture, we have learned how to produce an IR spectrum. 大家好，欢迎学习今天的课程 有机波谱分析。 在上一节课中，我们学习了如何产生红外光谱。 In IR spectroscopy, an organic molecule is exposed to infrared radiation. When the radiant energy matches the energy of a
2、specific molecular vibration, absorption occurs. 在 IR光谱学中，有机分子暴露于红外辐射。当辐射能 量 与特定分子振动的能量匹配时，就会发生 红外 吸收。 P3: A typical IR spectrum is shown below. The wavenumber, plotted on the X-axis, is proportional to energy; therefore, the highest energy vibrations are on the left. 典型的红外光谱如下图所示：在 X轴上绘制的波数与能量成正比。因
3、此，最大的能量振动在左侧。 The percent transmittance (%T) is plotted on the Y-axis. An absorption of radiant energy is represented by a “through” in the curve: zero transmittance corresponds to 100% absorption of light at that wavelength. 透光率百分比（ T）绘制在 Y轴上。 曲线表示辐射能 “通过”样品池 的吸收：零透 光 率对应于该波长下100 吸光度 。 P4: How to c
4、alculated the number of vibration modes? A molecule consisting of n atoms has a total of 3n degrees of freedom, corresponding to the Cartesian coordinates of each atom in the molecule. 如何计算振动模式的数量？ 由 n个原子组成的分子总共具有 3n个自由度，对应于分子中每个原子的笛卡尔坐标。 P5: In a nonlinear molecule, 3 of these degrees are rotationa
5、l and 3 are translational and the remaining correspond to fundamental vibrations; 在非线性分子中，这些度数中的 3个是旋转的，而 3个是平移的，其余对应于基本振动； in a linear molecule, 2 degrees are rotational and 3 are translational. The net number of fundamental vibrations for nonlinear and linear molecules is (3n-6), and (3n-5) respec
6、tively. 在线性分子中， 2个 旋转 自由度 ， 3个 平 动自由度 。 非线性和线性分子的基本振动 自由度 分别为（ 3n-6）和（ 3n-5）。 P6: Lets have a practice here: 1) What are the Vibration Degrees of Freedom of H2O? 2) What are the Vibration Degrees of Freedom of CO2? 让我们 来 练习一下： 1） H2O的振动自由度是多少？ 2）二氧化碳的振动自由度是多少？ P7: The four Vibration Degrees of Freed
7、om of CO2 correspond to two stretching vibrations, symmetric and asymmetric stretches, and two bending vibrations. CO2的四个振动自由度分别对应于两个 伸缩 振动，对称和非对称 伸缩以及两个弯曲振动。 P8: Among these vibration degrees of freedom, some vibrations are infrared active modes, and some are inactive modes. 在这些振动自由度中，有些振动是红外活性模式，有
8、些是非活性模式。 For a vibration mode to be infrared active, it must be accompanied by a change in the molecular electric dipole moment. 为了使振动模式具有红外 活性，必须伴随分子电偶极矩的变化。 For example, as for linear CO2, symmetric stretch is IR inactive, because of no change in dipole moment. Asymmetric stretch is IR active beca
9、use of change in dipole moment. 例如，对于线性 CO2，由于偶极矩没有变化，所以对称 伸缩 IR 非活性 。 由于偶极矩 发生 变化，不对称 伸缩 具有 IR活性。 P9: A polar bond is usually IR-active. A nonpolar bond in a symmetrical molecule such as homonuclear species of O2, N2, or Cl2, will absorb weakly or not at all. This is a change in dipole of a vibrati
10、on to be IR active. 极性化学键通常是红外活性的。 对称分子（例如 O2， N2或 Cl2的 双原子同核分子 ）中的非极性键 弱 吸收或根本不吸收 IR。 振动的偶极 距 发生变化，以激活 IR。 P10: IR Selection Rule (IR active and IR inactive)： For infrared the selection rule states that in order for the photon to be absorbed, the molecule must undergo a change in dipole moment. 红外选
11、律（红外激活和红外不激活）： 对于红外，选律指出，为了吸收光子，分子必须有偶极矩的变化。 Vibration Spectroscopy Selection Rule is follows: V 1, 2, 3。 At room temperature, the vast majority of molecules are in the vibrational ground state of V = 0, so the main observed band is the transition from V = 0 to the fundamental frequency absorption b
12、and of V = l. 振动光谱选 率 如下 ： V 1， 2， 3。 在室温下，绝大多数分子处于 V = 0的振动基态，因此观察到的主要谱带是从 V = 0到 V = 1的基频吸收带的跃迁。 P12: Four factors tend to produce fewer experimental peaks than would be expected. 1. No change in dipole of a particular vibration; 2. The energies of two or more vibrations are identical or nearly id
13、entical; 3. The absorption intensity is too low; 4. The vibrational energy is in a wavelength beyond the range of the instrument. 四个因素 使实验 会产生比预期更少的峰 数 。 1.特定振动的偶极 距 没有变化； 2.两个或多个振动的能量相同或几乎相同； 3.吸收强度太低； 4.振动能量的波长超出了仪器的范围。 P13: Occasionally more peaks are found than are expected. For example, Overton
14、e and Combination Bands. 有时发现峰比预期的多。 例如，倍频和合频 P14: Over tone V0-V2 vibration absorption band appears at about twice the frequency of the strong base band (actually lower than twice), generally weak absorption band. V0 V2的倍频带振动吸收带，出现在强的基频带频率的大约两倍处(实际上比两倍要低 )，一般都是弱吸收带。 Combination band is also a weak
15、absorption band, which appears near the sum or difference of two or more fundamental frequencies. 合 频带也是弱吸收带，出现在两个或多个基频频率之和或频率之差附近。 The transition probability of over tone or combination band is small and the intensity is weak, and usually cannot be detected. 倍频带与合频带统称为泛频带，其跃迁几率小，强度弱，通常不能检出。 So in this lecture, we have learned about the selection rule of IR spectroscopy. Thank you for watching. See you next time. 在本 节课中 ，我们 学习 了红外光谱的 选律 。谢谢 ，再见！
