h********g
发帖数: 143 | 1 各位大牛，
小弟又来求助了。
我最近做了C-NMR的时候，出现了很奇怪的Doublet。而product的H-NMR和MS都是纯的。
这个doublet是怎么回事呢？会是C-N coupling么？
这两个分子式2,5-dibromoterephthalonitrile和2-amino-4-bromobenzonitrile，结构
式见下图。
C-NMR见下图。两个doublet分别在138 ppm和118 ppm的地方。
谢谢各位大牛咯~ |
t*******y
发帖数: 2432 | 2 C-Br coupling
similar to C-Cl coupling as you see in CDCl3 |
h********g
发帖数: 143 | 3 Thank you very much!
Are you talking about the triplet of CDCl3 in C-NMR?
Actually, I don't think it is due to C-CL coupling.
I searched the triplet of CDCl3 in C-NMR and found this:
http://u-of-o-nmr-facility.blogspot.com/2007/09/isotope-shifts-
The CDCl3 signal is a 1:1:1 triplet due to the J coupling to the deuteron
which is a spin I=1 nucleus having three energy levels. The chemical shift
difference between the CHCl3 and CDCl3 isotopomers is called an isotope
shift.
Question 1: Since chlorine has two NMR active isotopes (35Cl and 37Cl) why
don't we observe J coupling between the 13C and the 35/37Cl like we do for
the 13C and 2H?
Answer 1: The relaxation between the energy levels of the chlorine isotopes
is very fast and the 13C "sees" each Cl at an average energy. 2H on the
other hand relaxes slowly between the energy levels and the 13C "sees" all
three energy states of the deuteron.
【在 t*******y 的大作中提到】
: C-Br coupling
: similar to C-Cl coupling as you see in CDCl3
|
h********g
发帖数: 143 | 4 Also， the 13C-79Br coupling constant is about 135 Hz.
But the coupling constant in my spectra are 26 Hz and 16 Hz.
【在 t*******y 的大作中提到】
: C-Br coupling
: similar to C-Cl coupling as you see in CDCl3
|
t*******y
发帖数: 2432 | 5 sorry about the CDCl3 mistake
the 13C-79Br coupling constant is dependent on the number of bromines
present
see this paper
MAGNETIC RESONANCE IN CHEMISTRY, VOL. 35, 141-143 (1997)
they obtained a 13C-79Br coupling constant at around 48 for CH3Br
and lit. data for CBr4, CHBr3, and CH2Br2 are around 151, 127, and 95.
【在 h********g 的大作中提到】
: Also， the 13C-79Br coupling constant is about 135 Hz.
: But the coupling constant in my spectra are 26 Hz and 16 Hz.
|
h********g
发帖数: 143 | 6 Thank you very much!
I also find a article. JOURNAL OF MAGNETIC RESONANCE 37, 17-29 (1980)
It mentioned that C-Br Coupling constant in bromobenzene is about 125 Hz.
【在 t*******y 的大作中提到】
: sorry about the CDCl3 mistake
: the 13C-79Br coupling constant is dependent on the number of bromines
: present
: see this paper
: MAGNETIC RESONANCE IN CHEMISTRY, VOL. 35, 141-143 (1997)
: they obtained a 13C-79Br coupling constant at around 48 for CH3Br
: and lit. data for CBr4, CHBr3, and CH2Br2 are around 151, 127, and 95.
|
l******3
发帖数: 272 | 7 我觉得这就是两个峰，而不是J coupling造成的分裂。79Br和81Br的spin I＝3／2的核
，假如是分裂的话应该分裂成四重峰吧。另外，在300MHz核磁，13C和br79的共振频率
都是75MHz，我不知道有没有可能看到79Br的信号。 |
h********g
发帖数: 143 | 8 非常感谢！
不过我我还是有点不明白，为什么在300MHz核磁，13C和br79的共振频率都是75MHz？
这是怎么算出来的呢？
再次谢过！
【在 l******3 的大作中提到】
: 我觉得这就是两个峰，而不是J coupling造成的分裂。79Br和81Br的spin I＝3／2的核
: ，假如是分裂的话应该分裂成四重峰吧。另外，在300MHz核磁，13C和br79的共振频率
: 都是75MHz，我不知道有没有可能看到79Br的信号。
|
t******t
发帖数: 3045 | 9 same gyromagnetic ratio (different by ~ 0.4%) for 13C and 79Br
【在 h********g 的大作中提到】
: 非常感谢！
: 不过我我还是有点不明白，为什么在300MHz核磁，13C和br79的共振频率都是75MHz？
: 这是怎么算出来的呢？
: 再次谢过！
|
t******t
发帖数: 3045 | 10 They are different by ~4000 ppm, not likely to come up in the same spectrum
【在 l******3 的大作中提到】
: 我觉得这就是两个峰，而不是J coupling造成的分裂。79Br和81Br的spin I＝3／2的核
: ，假如是分裂的话应该分裂成四重峰吧。另外，在300MHz核磁，13C和br79的共振频率
: 都是75MHz，我不知道有没有可能看到79Br的信号。
|
|
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h********g
发帖数: 143 | 11 受教了，谢谢啦~~~
spectrum
【在 t******t 的大作中提到】
: They are different by ~4000 ppm, not likely to come up in the same spectrum
|
q****g
发帖数: 19 | 12 I do not know why there are doublets or two peaks, either. But there are two
things for sure.
1. Those doublets are not due to C-Br J-coupling. For quadrupolar nuclei,
deuterium is probably the only common one that you can see J couplings. For
others, you won't observe them due to fast quadrupolar relaxation of them.
Google "self decoupling" for more information.
2. The JMR paper measures the J coupling of C-Br by an indirect way. That
does not mean you can observe them by 1D spectrum. |
t******t
发帖数: 3045 | 13 看不出是不是coupling那就换个不同频率的仪器再做一次。。。 |
h********g
发帖数: 143 | 14 Thank you very much!
That helps me a lot. I will do a 2D spectrum to explore it further.
two
For
【在 q****g 的大作中提到】
: I do not know why there are doublets or two peaks, either. But there are two
: things for sure.
: 1. Those doublets are not due to C-Br J-coupling. For quadrupolar nuclei,
: deuterium is probably the only common one that you can see J couplings. For
: others, you won't observe them due to fast quadrupolar relaxation of them.
: Google "self decoupling" for more information.
: 2. The JMR paper measures the J coupling of C-Br by an indirect way. That
: does not mean you can observe them by 1D spectrum.
|
h********g
发帖数: 143 | 15 哈，好主意，我去试试！
【在 t******t 的大作中提到】
: 看不出是不是coupling那就换个不同频率的仪器再做一次。。。
|
