12章核磁共振譜-1-1.ppt

1、Introduction,NMR (Nuclear magnetic resonance ) is the most powerful tool available for organic structure determination. It is used to study a wide variety of nuclei: 1H 13C 15N 19F 31P =,=,Nuclear Spin,A nucleus with an odd atomic number or an odd mass number has a nuclear spin. The spinning charged

2、 nucleus generates a magnetic field.,External Magnetic Field,When placed in an external field, spinning protons act like bar magnets.,Two Energy States,The magnetic fields of the spinning nuclei will align either with the external field, or against the field. A photon with the right amount of energy

3、 can be absorbed and cause the spinning proton to flip. =,E and Magnet Strength,Energy difference is proportional to the magnetic field strength. E = h = h B0 2 Gyromagnetic ratio, , is a constant for each nucleus (26,753 s-1gauss-1 for H). In a 14,092 gauss field, a 60 MHz photon is required to fli

4、p a proton.,If all protons absorbed the same amount of energy in a given magnetic field, not much information could be obtained. But protons are surrounded by electrons that shield them from the external field. Circulating electrons create an induced magnetic field that opposes the external magnetic

5、 field. =,Magnetic Shielding,Shielded Protons,Magnetic field strength must be increased for a shielded proton to flip at the same frequency.,=,Protons in a Molecule,Depending on their chemical environment, protons in a molecule are shielded by different amounts.,=,NMR Signals,The number of signals s

6、hows how many different kinds of protons are present. The location of the signals shows how shielded or deshielded the proton is. The intensity of the signal shows the number of protons of that type. Signal splitting shows the number of protons on adjacent atoms. =,The NMR Spectrometer,Radio Frequen

7、cy (RF) Transmitter,The NMR Graph,Tetramethylsilane,TMS is added to the sample. Since silicon is less electronegative than carbon, TMS protons are highly shielded. Signal defined as zero. Organic protons absorb downfield (to the left) of the TMS signal. =,Chemical Shift,Measured in parts per million

8、. Ratio of shift downfield from TMS (Hz) to total spectrometer frequency (Hz). Same value for 60, 100, or 300 MHz machine. Called the delta scale. =,Delta Scale,Location of Signals,More electronegative atoms deshield more and give larger shift values. Effect decreases with distance. Additional elect

9、ronegative atoms cause increase in chemical shift. =,Typical Values,Aromatic Protons, 7-8,Vinyl Protons, 5-6,Acetylenic Protons, 2.5,Aldehyde Proton, 9-10,Electronegative oxygen atom,O-H and N-H Signals,Chemical shift depends on concentration. Hydrogen bonding in concentrated solutions deshield the

10、protons, so signal is around 3.5 for N-H and 4.5 for O-H. Proton exchanges between the molecules broaden the peak. =,Carboxylic Acid Proton, 10+,Number of Signals,Equivalent hydrogens have the same chemical shift.,Intensity of Signals,The area under each peak is proportional to the number of protons

11、. Shown by integral trace.,=,How Many Hydrogens?,When the molecular formula is known, each integral rise can be assigned to a particular number of hydrogens.,=,Spin-Spin Splitting,Nonequivalent protons on adjacent carbons have magnetic fields that may align with or oppose the external field. This ma

12、gnetic coupling causes the proton to absorb slightly downfield when the external field is reinforced and slightly upfield when the external field is opposed. All possibilities exist, so signal is split. =,1,1,2-Tribromoethane,Nonequivalent protons on adjacent carbons.,Doublet: 1 Adjacent Proton,Trip

13、let: 2 Adjacent Protons,The N + 1 Rule,If a signal is split by N equivalent protons, it is split into N + 1 peaks.,Range of Magnetic Coupling,Equivalent protons do not split each other. Protons bonded to the same carbon will split each other only if they are not equivalent. Protons on adjacent carbo

14、ns normally will couple. Protons separated by four or more bonds will not couple. =,Splitting for Ethyl Groups,Splitting for Isopropyl Groups,Coupling Constants,Distance between the peaks of multiplet Measured in Hz Not dependent on strength of the external field Multiplets with the same coupling co

15、nstants may come from adjacent groups of protons that split each other. =,Values for Coupling Constants,Complex Splitting,Signals may be split by adjacent protons, different from each other, with different coupling constants. Example: Ha of styrene which is split by an adjacent H trans to it (J = 17

16、 Hz) and an adjacent H cis to it (J = 11 Hz). =,Splitting Tree,=,Spectrum for Styrene,Stereochemical Nonequivalence,Usually, two protons on the same C are equivalent and do not split each other. If the replacement of each of the protons of a -CH2 group with an imaginary “Z” gives stereoisomers, then

17、 the protons are non-equivalent and will split each other. =,Some Nonequivalent Protons,=,Time Dependence,Molecules are tumbling relative to the magnetic field, so NMR is an averaged spectrum of all the orientations. Axial and equatorial protons on cyclohexane interconvert so rapidly that they give

18、a single signal. Proton transfers for OH and NH may occur so quickly that the proton is not split by adjacent protons in the molecule. =,Hydroxyl Proton,=,Ultrapure samples of ethanol show splitting. Ethanol with a small amount of acidic or basic impurities will not show splitting.,N-H Proton,Modera

19、te rate of exchange. Peak may be broad.,=,Identifying the O-H or N-H Peak,Chemical shift will depend on concentration and solvent. To verify that a particular peak is due to O-H or N-H, shake the sample with D2O Deuterium will exchange with the O-H or N-H protons. On a second NMR spectrum the peak w

20、ill be absent, or much less intense. =,Carbon-13,12C has no magnetic spin. 13C has a magnetic spin, but is only 1% of the carbon in a sample. The gyromagnetic ratio of 13C is one-fourth of that of 1H. Signals are weak, getting lost in noise. Hundreds of spectra are taken, averaged. =,Fourier Transfo

21、rm NMR,Nuclei in a magnetic field are given a radio-frequency pulse close to their resonance frequency. The nuclei absorb energy and process (spin) like little tops. A complex signal is produced, then decays as the nuclei lose energy. Free induction decay is converted to spectrum. =,Hydrogen and Car

22、bon Chemical Shifts,=,Combined 13C and 1H Spectra,Differences in 13C Technique,Resonance frequency is one-fourth, 15.1 MHz instead of 60 MHz. Peak areas are not proportional to number of carbons. Carbon atoms with more hydrogens absorb more strongly. =,Spin-Spin Splitting,It is unlikely that a 13C w

23、ould be adjacent to another 13C, so splitting by carbon is negligible. 13C will magnetically couple with attached protons and adjacent protons. These complex splitting patterns are difficult to interpret. =,Proton Spin Decoupling,To simplify the spectrum, protons are continuously irradiated with “no

24、ise,” so they are rapidly flipping. The carbon nuclei see an average of all the possible proton spin states. Thus, each different kind of carbon gives a single, unsplit peak. =,Off-Resonance Decoupling,13C nuclei are split only by the protons attached directly to them. The N + 1 rule applies: a carb

25、on with N number of protons gives a signal with N + 1 peaks. =,Interpreting 13C NMR,The number of different signals indicates the number of different kinds of carbon. The location (chemical shift) indicates the type of functional group. The peak area indicates the numbers of carbons (if integrated).

26、 The splitting pattern of off-resonance decoupled spectrum indicates the number of protons attached to the carbon. =,Two 13C NMR Spectra,第十二章 核磁共振譜,核磁共振技術(shù)是珀塞爾（Purcell）和布洛齊（Bloch）始創(chuàng)于 1946年，至今已有近六十年的歷史。自1950年應(yīng)用于測定有機(jī) 化合物的結(jié)構(gòu)以來，經(jīng)過幾十年的研究和實(shí)踐，發(fā)展十分迅速， 現(xiàn)已成為測定有機(jī)化合物結(jié)構(gòu)不可缺少的重要手段。 從原則上說，凡是自旋量子數(shù)不等于零的原子核，都可發(fā)生 核磁共振。但

27、到目前為止，有實(shí)用價(jià)值的實(shí)際上只有H1，叫氫 譜，常用1HNMR表示；13C叫碳譜，常用13CNMR表示。,12.1 基本原理：,原子核發(fā)生自旋運(yùn)動?？捎米孕孔訑?shù)ms來表示,為,，,。,+,ms=+1/2,其取向與外磁場方向平行，為低能級（ 低能態(tài)）;,ms=-1/2,其取向方向與外磁場方向相反，為高能級（高能 態(tài)）。,E=h,高能態(tài),低能態(tài),氫原子在外加磁場子中的取向,r為旋磁比，一個核常數(shù)，h為Planck常數(shù)。,核磁共振：當(dāng)E射= h射 = E 時(shí)，質(zhì)子吸收電磁輻射的 能量，從低能級躍起遷至高能級，這種現(xiàn)象 即稱為核磁共振。,射 =,掃場和掃頻,用恒定頻率的電磁輻射進(jìn)行照射樣品,改變外

28、加磁碭強(qiáng)度引起共振,叫掃場 。,用恒定外加磁碭強(qiáng)度，改變電磁輻射的頻率進(jìn)行照射樣品引起共振，叫掃頻.,12.2、屏蔽效應(yīng)和化學(xué)位移,一化學(xué)位移 氫質(zhì)子（1H）用掃場的方法產(chǎn)生的核磁共振，理論上都在同一磁場 強(qiáng)度（Ho）下吸收，只產(chǎn)生一個吸收信號。實(shí)際上，分子中各種不同 環(huán)境下的氫，在不同Ho下發(fā)生核磁共振，給出不同的吸收信號。例如， 對乙醇進(jìn)行掃場則出現(xiàn)三種吸收信號，在譜圖上就是三個吸收峰。 如圖：,這種由于氫原子在分子中的化學(xué)環(huán)境不同，因而在不同磁場強(qiáng)度下產(chǎn) 生吸收峰,峰與峰之間的差距稱為化學(xué)位移。,二屏蔽效應(yīng)化學(xué)位移產(chǎn)生的原因,有機(jī)物分子中不同類型質(zhì)子的周圍的電子云密度不一樣，在加 磁場作

29、用下，引起電子環(huán)流，電子環(huán)流圍繞質(zhì)子產(chǎn)生一個感應(yīng)磁 場（H），這個感應(yīng)磁場使質(zhì)子所感受到的磁場強(qiáng)度減弱了，即 實(shí)際上作用于質(zhì)子的磁場強(qiáng)度比Ho要小。 這種由于電子產(chǎn)生的感應(yīng)磁場對外加磁場的抵消作用稱為屏蔽 效應(yīng)。,三化學(xué)位移值,化學(xué)位移值的大小，可采用一個標(biāo)準(zhǔn)化合物為原點(diǎn)，測出峰與 原點(diǎn)的距離，就是該峰的化學(xué)位移值，一般采用四甲基硅烷為標(biāo) 準(zhǔn)物（代號為TMS）。 化學(xué)位移是依賴于磁場強(qiáng)度的。不同頻率的儀器測出的化學(xué)位 移值是不同的， 為了使在不同頻率的核磁共振儀上測得的化學(xué)位移值相同（不 依賴于測定時(shí)的條件），通常用來表示，的定義為：,標(biāo)準(zhǔn)化合物TMS的值為0。,12.3影響化學(xué)位移的因素,（

30、1）誘導(dǎo)效應(yīng) 1值隨著鄰近原子或原子團(tuán)的電負(fù)性的增加而增加。如： CH3H CH3Br CH3Cl CH3NO2 值 0.23 2.69 3.06 4.29,2 值隨著H原子與電負(fù)性基團(tuán)距離的增大而減小。如： CH3CH2CH2Cl 值 1.06 1.81 3.47,3 烷烴中H的值按伯、仲、叔次序依次增加。如： CH3H RCH2H R2CHH R3CH 值 0.2 1.10.1 1.30.1 1.50.1,（2） 電子環(huán)流效應(yīng),烯烴、醛、芳環(huán)等中，電子在外加磁場作用下產(chǎn)生環(huán)流，使氫原子周圍產(chǎn)生感應(yīng)磁場，其方向與外加磁場相同，即增加了外加磁場，所以在外加磁場還沒有達(dá)到Ho時(shí)，就發(fā)生能級的躍遷

31、，因而它們的很大（= 4.512）。,乙炔也有電子環(huán)流,但炔氫的位置不同，處在屏蔽區(qū)（處在感應(yīng)磁場與外加磁場對抗區(qū)），所以炔氫的值較小。,12. 4 自旋偶合裂分,一峰的裂分：應(yīng)用高分辨率的核磁共振儀時(shí)，得到等性質(zhì)子的吸收峰可能不是一個單峰而是一組峰的信息。 二自旋偶合 ：裂分是因?yàn)橄噜弮蓚€碳上質(zhì)子之間的自旋偶合（自旋干擾）而產(chǎn)生的，我們把這種由于鄰近不等性質(zhì)子自旋的相互作用（干擾）而分裂成幾重峰的現(xiàn)象稱為自旋偶合。1. 兩個相鄰氫的偶合例1：1,1,2-三氯乙烷的核磁共振譜,例2：3-戊酮的核磁共振譜,自旋偶合的產(chǎn)生（以溴乙烷為例）,Ha在外磁場中自旋，產(chǎn)生兩種方向的感應(yīng)小磁場H,H作用于H

32、b周圍時(shí)，使得Hb的實(shí)受磁場有兩種情況：,這樣就使得Hb的信號分裂為二重峰。,當(dāng)兩個Ha的自旋磁場作用于H b時(shí)，H b的實(shí)受磁場為：,2個Ha對Hb偶合作用，使Hb的信號裂分為三重峰，其面積比為1：2：1。,同上，三個H b對H a的偶合作用可使H a 分裂為四重峰，其面積比為 1 ：3 ：3 ：1，如右圖所示：,2. 偶合常數(shù):偶合使得吸收信號裂分為多重峰，多重峰中相鄰兩個峰之間的距離稱為偶合常數(shù)（J），單位為赫（Hz）。J的數(shù)值大小表示兩個質(zhì)子間相互偶合（干擾）的大小。,Jab =Jba,3.自旋偶合的限度（條件）,1磁等性質(zhì)子之間不發(fā)生偶合。 2兩個磁不等性質(zhì)子相隔三個鍵以上時(shí)，則不發(fā)

33、生偶合。,3同碳上的磁不等性質(zhì)子可偶合裂分。,如某個質(zhì)子周圍有n個等價(jià)質(zhì)子，則其信號裂分為n+1重峰,BrCH2CH2CH2Cl :,4. (n+1) 規(guī)律：,條件：只適合于叁鍵偶合，即適合于相鄰碳原子上的質(zhì)子； 而且適合于等價(jià)質(zhì)子,(n+1)(n+1) = 33 = 9 重峰,一張譜圖可以向我們提供關(guān)于有機(jī)分子結(jié)構(gòu)的如下信息：,1. 由吸收峰的組數(shù)，可以判斷有幾種不同類型的H核；,2. 由峰的強(qiáng)度（峰面積或積分曲線高度），可以判斷 各類H的相對數(shù)目；,3.由峰的裂分?jǐn)?shù)目，可以判斷相鄰H核的數(shù)目；,4. 由峰的化學(xué)位移（值），可以判斷各類型H所屬 的化學(xué)結(jié)構(gòu)；,5. 由裂分峰的外型或偶合常數(shù)，可以判斷哪種類型H 是相鄰的。,12.5、核磁共振譜的解析及應(yīng)用,a. 區(qū)分出雜質(zhì)峰、溶劑峰、活潑氫峰等；,不飽和度大于4時(shí)，應(yīng)考慮含苯環(huán)或吡啶（三個雙鍵和一個環(huán)）。,b. 計(jì)算不飽和度（unsaturation number),一級譜解析的一般程序,U.N.=1+n