1、 I 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 as the Larmor
2、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 energy spin s
3、tate(+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(emitting a very prec
4、ise 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 should the pro
5、ton 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 applied fiel
6、d.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 effective field at
7、 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)of the proton
8、s 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 exam
9、ple 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 sh
10、ift 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)/
11、0 106(ppm)0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 compounds.Tetramethylsilane,(CH3)4Si,usually referred to as TMS,meets（附和）all these characteristics（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)
