《有机波谱分析双语课程 (15).pdf》由会员分享，可在线阅读，更多相关《有机波谱分析双语课程 (15).pdf（13页珍藏版）》请在taowenge.com淘文阁网|工程机械CAD图纸|机械工程制图|CAD装配图下载|SolidWorks_CaTia_CAD_UG_PROE_设计图分享下载上搜索。
1、核磁共振氢谱 I 有机波谱分析 Organic Spectroscopic Analysis 1H NMR Spectroscopy I 2 How to produce NMR phenomena?1.Nuclei I0 2.There is a external magnetic field B0.Radio frequency electromagnetic waves in the vertical direction of B0.3.When the frequency of the radio frequency electromagnetic wave is the same a
2、s the Larmor frequency of the spin nucleus,the nucleus absorbs the energy of the radio frequency wave and transitions from the low energy state to the high energy state,that is,nuclear magnetic resonance occurs.Resonance Frequency:=Bo/2 Nuclear magnetic dipole Oscillator coil Precessional orbit Low
3、energy spin state(+1/2)Precessional orbit High energy spin state(-1/2)Oscillator coil transmits radiofrequency 3 A Simple NMR Machine An NMR machine consists of:(1)A powerful,supercooled magnet(stable,with sensitive control,producing a precise magnetic field).(2)A radio-frequency transmitter(emittin
4、g a very precise frequency).(3)A detector to measure the emission of radiofrequency by the sample.(4)A recorder(to plot the output).4 NMR System 5 NMR Machine 6 The NMR magnet is of superconducting(超导的)type and has to be immersed in liquid helium.It is the most expensive component.NMR System 7 Why s
5、hould the proton nuclei behave differently in the NMR experiment?The answer to this question lies with the electron(s)surrounding the proton：Since electrons are charged particles,they move in response to the external magnetic field(0)so as to generate a secondary field that opposes the much stronger
6、 applied field.This secondary field shields the nucleus from the applied field Chemical Shift Phenomenon 8 Chemical Shift Phenomenon The electron density around each nucleus in a molecule varies according to the types of nuclei and the bonds in the molecule.The opposing field and therefore the effec
7、tive field at each nucleus will vary.This is called the chemical shift phenomenon.9 The external field(B0)must be increased to compensate for the induced shielding field.As a consequence,the NMR equation need be modified to this format:0=B0(1-)/2,where is the shielding tensor.The 0(Larmor frequency)
8、of the protons in the electron rich chemical environments（氢核外电子云密度大）,has bigger B0,，,共振吸收峰出现在高场，则反之。0 不变，改变B0（称为扫场），分子中不同环境的原子核会因不同而出现不同共振吸收峰。Chemical Shift Phenomenon 10 Field Dependence and Chemical Shift Consider the methanol molecule.The resonance frequency of two types of nuclei in this example
9、 differ.This difference will depend on the strength of the applied magnetic field,B0.The greater the value of B0,the greater the frequency difference.This relationship could make it difficult to compare NMR spectra taken on spectrometers operating at different field strengths.The term chemical shift
10、 was developed to avoid this problem.0=B0(1-)/2 11 Field Dependence of a Spectrum 12 Reference Standard The location of an NMR signal in a spectrum is to be reported relative to a reference signal from a standard compound added to the sample.Chemical Shift:=(sample-STD)/STD 106 (ppm)=(sample-STD)/0
11、106 (ppm)0为仪器的射频频率。为仪器的射频频率。为无因次数值与该处的化学环境有关故称化学位移值。13 Such a reference standard should be chemically unreactive（化化学惰性学惰性）,and easily removed from the sample after the measurement.Also,it should give a single sharp NMR signal that does not interfere with the resonances normally observed for organic
12、compounds.Tetramethylsilane,(CH3)4Si,usually referred to as TMS,meets（附和附和）all these characteristics（特征特征）,12H and 4C suffer the same chemical enviroment and has become the reference compound of choice for 1H and 13C NMR.Reference Standard S i C C C C H H H H H H H H H H H H Chemical Shift:=（样-TMS)/TMS 106 (ppm)