有机波谱分析双语课程 (19).pdf

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1、核磁共振氢谱 III 有机波谱分析 Organic Spectroscopic Analysis 1H NMR Spectroscopy III 2 Number of Signals Equivalent hydrogens have the same chemical shift.Chemical Shift&SpinChemical Shift&Spin-spin Splittingspin Splitting 3 Spin-Spin Coupling 1,2-dichloroethane 1,1-dichloroethane 4 What Causes This Signal Spli

2、tting?Nuclei which are close to one another exert an influence on each others effective magnetic field.5 What Causes This Signal Splitting?Nuclei experiencing the same chemical environment or chemical shift are called equivalent.They do not exhibit splitting.Those nuclei experiencing different envir

3、onment or having different chemical shifts are nonequivalent.If the distance between non-equivalent nuclei is less than or equal to three bond lengths,they influence each other and exhibit spin-spin coupling.6 Spin-spin Coupling the N+1 Rule Shielding Deshielding Chemical shift of A when B is not pr

4、esent 7 Spin-Spin Coupling 1,2-dichloroethane 1,1-dichloroethane Hydrogens in 1,1-dichloroethane are equivalent and do not exhibit splitting.The two signals from the different hydrogens in 1,1-dichloroethane are split into close groupings of two resonances.8 The N+1 Rule If a signal is split by N eq

5、uivalent protons,it is split into N+1 peaks.9 The N+1 Rule 10 The N+1 Rule 11 Spin-Spin Coupling 12 Spin-Spin Coupling 13 The(n+1)(n+1)Rule If a given nucleus is spin-coupled to two or more sets of nonequivalent neighboring nuclei.1 n+1(n+1)(n+1)=4(n+1)(n+1)(n+1)=8 14 Complex Coupling Patterns In re

6、ality,coupling patterns are often more complex than the simple N+1 rule since the neighbouring protons are often not equivalent to each other(i.e.there are different types of neighbours).15 Complex Coupling Patterns 16 Complex Coupling Patterns However,in many cases the lines overlap with each other

7、 and the result is further distortion from the ideal pattern.17 The distance between two split absorption lines is called the J coupling constant or the spin-spin splitting constant and is a measure of the magnetic interaction between two nuclei.Coupling Constant 18 The coupling constant originates

8、from the interaction between the spin nuclei,so its value has nothing to do with the strength of the applied magnetic field,and is only determined by the molecular structure.Coupling Constant 19 Coupling Constant Take HF for example 20 偶合常数偶合常数J J的含义的含义 In the figure,the solid arrow()represents the

9、energy level transition of the hydrogen nucleus when the external magnetic field is not disturbed,and the dashed arrow represents the hydrogen level energy transition when the fluorine nucleus spins the interference.Characters of Coupling constant J Coupling constant J is a very important parameter

10、in nuclear magnetic resonance spectroscopy.1.The size of the J value has nothing to do with the magnetic field strength B0.2.The size of the J value mainly depends on the magnetic and molecular structure and conformation of the atomic nucleus.3.It is an attribute of the molecular structure of a comp

11、ound.22 Relationship between coupling constant and molecular structure Therefore,the molecular structure can be inferred according to the size of the coupling constant and its changing law.23 Values for Coupling Constants 24 偶合常数（偶合常数（J J）与化学位移（）与化学位移（）Conversion between coupling constant J(Hz)and c

12、hemical shift (ppm):J=Oscillator frequency（e.g.400M、800M）/106 Hz ppm 25 Intensity of Signals The area under each peak is proportional to the number of protons.(The Integral Trace)=26 Summary:Parameters in NMR 1.Chemical shift(d)ppm Structural information 2.Coupling constant(J)Hz Structural and conformational information 3.Intensity of Signals(Area of peaks)Relative proportion of nuclei