同核J-偶合(Homonuclear J-Coupling)
多重峰出现的规则: 1. 某一原子核与N个相邻的核相互偶合将给出(n+1)重峰. 2. 等价组合具有相同的共振频率.其强度与等价组合数有关. 3. 磁等价的核之间偶合作用不出现在谱图中. 4. 偶合具有相加性. 例如: observed spin coupled spin intensity
JCH JCH
H C
p-pulse on H
H C
这相当于使用一系列1800脉冲快速照射氢核。 pH pH
C-H
+J/2
C-H
-J/2
C-H
+J/2
pH
C-H
-J/2
pH
C-H
+J/2
pH
C-H
-J/2
Fig. 4-2.5 The proton-decoupled 13C spectrum of 1-propanol
H-12C H-13C H-13C x100
105 Hz
proton-coupled spectra (nondecoupled spectra)
Quartet, J=127 Hz
Proton-coupled spectra for large molecules are often difficult to interpret. The multiplets from different C commonly overlap because the 13C-H coupling constants are frequently larger than the chemical differences of the C in the spectrum. 原子核间的偶合导致谱图 的复杂化(―精细裂分”)， 灵敏度下降。 Fig. 4-2.4 Ethyl phenylacetate. (a) The proton-coupled 13C spectrum. (b) The proton-decoupled 13C spectrum
羰基
芳烃、杂芳环
13C
220
200 180
160 140
120 100
80
60
40
20
0
B. Calculation of 13C Chemical Shifts
m-xylene, the base value for the C in a benzene ring is 128.5 ppm. ipso ortho meta para CH3 8.9 0.7 -0.1 -2.9
NMR: 原子核间的相互作用
分子中的原子并不是孤立存在,它不仅在相互间发生作用也同周围环境发生作用, 从而导致相同的原子核却有不同的核磁共振频率.
化学位移
自旋-自旋偶合
Larmor
E B0
频率
e.g. B0=11.7 T,
(1H)=500 MHz (13C)=125 MHz 化学位移 ~ B0 kHz 自旋-自旋偶合 Hz-kHz
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C-13 spectra
C-13 spectra can be determine the number of nonequivalent carbons and to identify the types of C (CH3, CH2, aromatic, C=O) that may be present in a compound. C-13 NMR provides direct information about the carbon skeleton of a molecule.
Spin-Spin Coupling
JHH JCH
H H H C
异核 J-coupling
C
C
同核 J-coupling
Coupling Constants: nJ is a constant. Homonuclear coupling, 3J(1H-1H) = 8Hz; heteronuclear coupling, 1J(13C-1H) = 156Hz; The electrons in the intervening bonds between the two nuclei transfer spin information from one nucleus to another by means of interaction between the nuclear and electronic spins.
Ha C Cห้องสมุดไป่ตู้Hb
A B
B B A A
HA JAB b
1 1 1 1
HA
HB JAB
HB
a
同核J-偶合(Homonuclear J-Coupling)
Ha Hb
B,C是磁等价的核
C
JAB=JAC
C
Hc
B,C
A
A
A
B
C
异核J-偶合(Heteronuclear J-Coupling)
自旋-自旋偶合引起共振线的分裂而形成多重峰.多重峰实际 代表了相互作用的原子核彼此间能够出现的空间取向组合.
The decoupling technique obliterates all interactions between 13C-H; therefore, only singlets are observed in a decoupled 13C NMR spectrum. The decoupler simultaneously irradiates a second, tunable radiofrequency which causes the protons to become saturated, and they undergo rapid upward and downward transitions, among all their possible spin states. These rapid transitions decouple any spin-spin interactions between 13C-H being observed. In effect, all spin interactions are averaged to zero by the rapid changes. The 13C ―senses‖ only one average spin state for the attached Hs rather than two or more distinct spin states.
去偶(Decoupling)
氢对碳的偶合作用可以通过对氢施加一个脉冲消除。此一技术称为去偶。对氢核 的饱和照射，促使氢核的自旋状态快速的变换，临近的碳核无法感觉到氢核的自 旋状态的取向而只感受到氢核两种取想的平均效果。具体的说，对氢核的饱和照 射使碳核原来的两条共振线w-J/2和w+J/2合并平均而得到[(-J/2)+(+J/2)]/2=。
Electronegativity, hybridization, and anisotropy all affect 13C chemical shifts. The electronegative element produces a large downfield shift since the electronegativity atom is directly attached to the 13C atom.
去偶(Decoupling)
氢去偶除简化碳谱还因为有核的Overhauser效应而增加信噪比。
C-H
C-H2
decoupled
*CH3-CH2coupled
Nuclear Overhauser Enhancement (NOE)
The intensities of many of the C resonances in a protondecoupled 13C spectrum increase significantly above those observed in a proton-coupled experiment. 核Overhauser效应（NOE） — Overhauser等人及以后 的研究人员发现，当核外电子自旋或相邻磁性核的核 自旋发生共振并达到饱和时，偶极－偶极相互作用(弛 豫)引起核自旋态分布变化，使得待观察的核的信号强 度增加。其空间作用距离<5~6Å。 C with H directly attached are enhanced the most, and the enhancement increases (but not always linearly) as more Hs are attached.
去偶(Decoupling)
如果峰数不多，偶合的方式仍可分析出。但当很多锋出现时，偶合方 式的分析就不是那么容易。 1J(13C,1H) ~ 125 - 150Hz 直接偶合: nJ(13C,1H) ~ 1 - 10Hz 长程偶合:
*CH3-CH2-
未去偶
氢去偶
Proton-Decoupled 13C Spectra
Hc B
C
A
A
异核J-偶合(Heteronuclear J-Coupling)
*C *CH
*CH2 H2 H1 C C C H1 C H1
*CH3 H3
H2
异核J-偶合(Heteronuclear J-Coupling)
由于一些核的自然丰度并非如此100%。因此谱图中可能出 现偶合分裂的峰和无偶合的峰。氯仿中的氢谱是一个典型 的例子。
Fig. 4-2.2 A 13C correlation chart for carbonyl and nitrile functional groups