Chapter11-NMR.ppt

单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,Chapter 11 Nuclear Magnetic Resonance Spectroscopy,Many atomic nuclei have the property of nuclear spin.When placed between the poles of a magnet,the axis of rotation of the spinning nuclei,precess,around the axis of the field.This precession can be detected by irradiation with energy in the radiofrequency region of the spectrum.When the precession and irradiation frequencies are equal the system is said to be in,resonance,.,Nuclear Magnetic Resonance(NMR)-Absorption of,radiowave,EM energy,by nuclei,in a magnetic field.,Background and Theory,Electron spin observed-1922,By 1926,became apparent that nuclear spin also existed.,1939,Rabi observes absorption of radio frequency(RF)energy by nuclei of H,2,gas;gets Nobel prize in physics in 1944.,第一节 核磁共振基本原理,principle of nuclear magnetic resonance,第二节 核磁共振与化学位移,nuclear magnetic resonance and chemical shift,第三节 自旋偶合与自旋裂分,spin coupling and spin splitting,第四节 谱图解析与结构确定,analysis of spectrograph and,structure determination,第五节,13,C,核磁共振波谱,13,C,nuclear magnetic resonance,Nuclei are charged and if they have spin,they are magnetic,No Field,Applied Magnetic Field=B,Energy of transition=energy of,radiowaves,Higher energy state:magnetic field opposes applied field,Lower energy state:magnetic field aligned with applied field,1,Principle of nuclear magnetic resonance,1.1 Nuclear Spin,Do all nuclei have spin movement?,Nuclear spin depends on its,spin angular momentum number,I,Protons,Neutrons,I,Examples,Even,Even,0,12,C,16,O,32,S,no magnetic moment,Odd,Even,1/2,3/2,1,H,19,F,1,P,11,B,79,Br,With,magnetic moment,Even,Odd,1/2,3/2,13,C,127,I,With,magnetic moment,Odd,Odd,1,2,H,14,N,With,magnetic moment,All nuclear spin states are degenerate in magnetic field,A spinning charge generates a magnetic dipole,m,The dipole can take certain allowed orientations in a magnetic field,where they split into,2I+1,states(limited to 2I+1 discrete values):,m=I,I-1,I-2.-I,For,I=0:,m=0(only 1 state),NMR inactive,For,I=:,m=(2 states),NMR Active,1.2,自旋核在外加磁场中的取向数和能级,图,10.3,能级裂分与外加磁场强度的关系,两种取向间的能级差，可用,E,来表示：,E,=,E,2,E,1,=+,B,0,(-,B,0,)=2,B,0,(10.3),氢核由低能级,E,1,向高能级,E,2,跃迁时需要的能量,E,与外加磁场强度,B,0,及氢核磁矩,成正比,1.3,核的回旋,Larmor,方程,=2v,=,B,0,v=B,0,/2,-,角速度；,v,-,进动频率（回旋频率）；,-,旋磁比,当电磁波的频率,射,与该核的回旋频率,回,相等时，电磁波的能量就会被吸收，核的自旋取向就会由低能态跃迁到高能态，即发生核磁共振。,(10.7),(10.8),1.4,核跃迁与电磁辐射,(,核磁共振,NMR),(1),核有自旋,(,磁性核,),(2),外磁场,能级裂分,;,(3),照射频率与外磁场的比值,0,/,H,0,=,/(2),共振条件,1.5,样品的制备：,试样浓度,：,5-10%,；需要纯样品,15-30 mg,；,傅立叶变换核磁共振波谱仪需要纯样品,1 mg,；,标样浓度,（四甲基硅烷,TMS,）,：,1%,；,溶剂,：,1,H,谱 四氯化碳，二硫化碳；,氘代溶剂,：氯仿，丙酮、苯、二甲基亚砜的氘代物；,In 1949&50,it was observed that ethanol gave 3 signals.Not due to experimental error.,20,000,000 Hz,20,000,050 Hz,20,000,085 Hz,2.Measurement,2.1 The Chemical Shift,Circulation of electrons around nuclei generate local magnetic fields(which,usually,oppose,B,appl,),B,0,=,B,appl,s,B,appl,=B,appl,(1,s,),B,o,=locally observed,B,0,B,appl,=applied,B,0,s,=,shielding constant,These variations in frequency(,)are due to different magnetic environments.,Explanation:The electrons surrounding the H nucleus provide,shielding,of applied magnetic field so that:-,Chemical Shift,on the left side of TMS +,on the right side of TMS -,由此，化学位移成为一个无因次的数，并以多少个,ppm,来表示。,Standard:TMS,四甲基硅烷,(1)Only one peak on NMR spectrum,(2)High electronic density of H in TMS.Almost all the H peaks of organic compounds appear on the left of the TMS peak,(3)TMS is volatile compound(,bp,26.5,o,C),very easy to remove.It dissolves in most organic solvents,(4)of TMS is set as 0,2.2 Factors affecting chemical shift,(,1),诱导效应,(a)Depends on adjacent group,For protons on carbon attached to an electronegative atom or group X,the chemical shift increases with the,electronegativity,of X.This is due to the inductive effect on the shielding of the protons and is apparent in the methyl halides.,电负性大的取代基（,吸电子基团,），可使邻近氢核的电子云密度减少（去屏蔽效应），导致该质子的共振信号向,低场,移动，化学位移,?,移；,电负性小的取代基（,推电子基团,），可使邻近氢核的电子云密度增加（屏蔽效应），导致该质子的共振信号向,高场,移动，化学位移,?,移。,Depends on atom attached,Fig 1 NMR Chemical Shifts and Splitting Patterns,Compound CH,3,X,CH,3,F,CH,3,OH,CH,3,Cl,CH,3,Br,CH,3,I,CH,3,C-,3,CH,4,(CH,3,),4,Si,Element X,F,O,Cl,Br,I,C,H,Si,Electronegativity,of X,4.0,3.5,3.1,2.8,2.7,2.5,2.1,1.8,Chemical shift,ppm,4.26,3.40,3.05,2.68,2.16,0.9,0.23,0.0,ppm,(b)Depends on adjacent group,The inductive(,deshielding,)effect of a substituent on a proton decreases as the separation between the proton and substituent is increased.,CH,3,-CH,2,-CH,2,-OH,0.92 1.57 3.58,试比较下面化合物分子中,H,a,H,b,H,c,值的大小。,b a c,电负性较大的原子，可减小,H,原子受到的屏蔽作用，引起,H,原子向低场移动。向低场移动的程度正比于原子的电负,性和该原子与,H,之间的距离。,/,ppm,/,ppm,chemical shift,Depends in hybridization:,(2)Depends on anisotropy(,各向异性）,(a),芳烃,以苯环为例，在外加磁场,B,0,条件下，苯环,电子的电子流系统产生的磁的各向异性效应如图,苯环中由,电子诱导环流产生的磁场,显然，在苯环平面的上下方，因环电流形成的第二磁场方向相反，将使该处氢核共振信号移向高磁场处，化学位移值减小，故为屏蔽区。而其它方向，如苯环周围，则因两者方向正好一致，将使氢核共振信号移向低磁场处，因此化学位移值增大，故为去屏蔽区。,屏蔽区位于苯环的上下方，而苯环平面为去屏蔽区，故苯环上,1,H,核的,=7.27ppm,Electrons circulate in the plane of the bond:,deshields,protons,(b),烯烃,3.55ppm,3.75ppm,氢处于双键电子屏蔽区内,氢处于双键电子屏蔽区外,烯氢和醛氢的磁各向异性效应,胡薄荷酮,Pulegone,a,b,(a)1.77ppm,(b)1.96ppm,甲基与羰基相隔较远,甲基处于羰基的去屏蔽区,烯氢和醛氢的磁各向异性效应,Electrons circulate around the axis of the bond:,Shields protons,(c),炔烃,（,d,）,C-C,单键的磁各向异性效应,(-CH,3,)(-CH,2,),(-CH10 weak coupling(primary spectra),/J 4,端甲基,C,=13-14,C,CH,CH2,CH3,邻碳上取代基增多,C,越大,化学位移规律：烯烃,C,=100-150,（,成对出现）,端,碳,=,CH2,110,；,邻碳上取代基增多,C,越大：,化学位移规律：,炔烃,C,=65-90,化学位移表,2,chemical shift table,三、偶合与弛豫,13,C-,13,C,偶合的几率很小（,13,C,天然丰度,1.1%,）；,13,C-,1,H,偶合；偶合常数,1,J,CH,：,100-250 Hz,；,峰裂分；谱图复杂；,去偶方法：,(1),质子噪声去偶或宽带去偶,(,proton noise decoupling or,boradband,decoupling,),：,采用宽频带照射，使氢质子饱和；,去偶使峰合并，强度增加,(2),质子偏共振去偶：,识别碳原子类型；,弛豫：,13,C,的弛豫比,1,H,慢，可达数分钟；采用,PFT-NMR,可测定，,碳谱与氢谱的对比谱图去偶作用对比,谱图去偶作用对比,谱图去偶作用对比,四、,13,C NMR,谱图,13,C NMR,谱图,2,13,C NMR,谱图,3,20,30,40,50,60,q,d,q,s,t,例,3.,化合物,C,12,H,26,，,根据,13,C NMR,谱图推断其结构。,13,C NMR,谱图,4,C,4,H,10,O,6:1:2:1,C,8,H,14,O,4,6:4:4,C,4,H,7,O,2,Cl,C,3,H,6,O,3,7.8 peak can exchange with D,2,O,C,11,H,13,O,2,N,5:2:6H,