C-13 easily get saturated on higher frequency and population difference may be almost zero . it make interpretation difficult
lets say for example in some quaternary carbon, we have to increase the scans upto or more than 1500 scans. In such case wouldnt higher frequency 13C would be helpful.
13C and 1H have different gyromagnetic ratios. Thus, at the same magnetic field strength 13C and 1H have different lamor frequencies.
http://en.wikipedia.org/wiki/Gyromagnetic_ratio
E.g. at 16.4 Tesla proton resonance frequency is 700Mhz while 13C resonates at ~175Mhz.
Signal to noise is generally proportional to magnetic field strength to the power 3/2 (B0^3/2), which means higher field should give better spectra. Also in terms of resolution which is direct proportional to the magnetic field.
But the relaxation properties of different nuclei are dependent on magnetic field as well. So in some cases a lower magnetic field strength might therefore give superior spectra due to slower relaxation. This is for example observed for the carbonyl groups of proteins. This should not be such a big problems for small molecules though.
Helge Meyer is correct.
The relationship between field and frequency is shown mathematically by
dE= hv = h (Bo - Be)y,
where:
dE is the energy difference between spin states
h is Planck's constant
v is the frequency of the B1 field
Be is a small magnetic field generated by the circulation of electrons of the molecule
Bo is the strength of the external homogeneous magnetic field
y (gyromagnetic ratio) is a constant which is a property of the particular nucleus
The gyromagnetic ratio for C13 nucleus is 67.262 units whereas for 1H is 267.513 (i.e. around 4 times). So, thats why if 1Hnmr is done at 400MHz, then 13Cnmr will be done at 100 MHz.
Thank you Helge Meyer, Ahamed Salaf and Mohit Tripathi. I believe I understand it now.
We have 400 MHz NMR machine. For small molecules at the same frequency sometimes we do not get the desired 13C spectra although we know from proton that it is the same compound. In such cases, for example working with the spiro carbons or quartenary carbons. Increasing the number of scan helps. And although while processing 1H shows the frequency to be 400 MHz, processed 13C shows 100 MHz. I believe if I have understood it correctly. And if the problem persists even after increasing the number of scans, I will have to try it with 600 MHz or 700 MHz NMR.
Carbon spectra have much lower signal to noise than proton spectra. That is normal.
Be aware that if you are going from a 400 Mhz to a 700 Mhz spectrometer you will "only" gain a factor of ~2.3 in sensitivity. This might just be enough to see what you want. But if you do not see any signal at 400 Mhz, do not expect to much from changing to a higher field.
You might also consider to increase the concentration of your compund by using 5mm instead of 10mm tubes. This gives a slight increase in signal to noise as well. I would guess a factor of 1.5 in sensitivity...
Ideally, you would need to have access to a spectrometer with cryogenically cooled carbon probe. This would provide the best sensitivity. (~4 times higher signal to noise)
A parameter called the gyromagnetic ratio ( or the magnetogyric ratio) determines the frequency at which a nucleus resonates. Since each nmr active nucleus has a different gyromagnetic ratio is it has its own characteristic frequency. The gyromagnetic ratio for carbon is about one quarter of that for hydrogen. It also happens to be much less sensitive than hydrogen so we need to aquuire for longer to achieve the same signal to noise ratio
With regard to your supplementary question concerning quaternary carbons - you cannot solve this problem by changing the frequency. Since you wish to observe 13C you must operate at the characteristic frequency for that nucleus. Quaternary carbons give weaker signals because of their slow relaxation rates. In simple terms relaxation rate depends on the number of attached hydrogens. You can deal with this in a number of ways - acquire for longer, use a longer relaxation delay or add a relaxing agent like Cr(acac)
As John says, using a PARR (PAramagnetic Relaxation Reagent) is very important in 13C NMR. Put some Cr(acac)3 into your solution and use a relaxation delay of 10s. You should get good spectra after 256/512/1024 scans. And don't forget, spectral editing does not help for quaternary carbon atoms. You must use GASPE and Cr(acac)3 to isolate these. Never use Cr(acac)3 when spectral editing with DEPT, because you will lose particular carbon atoms.
Also, for simple 13C spectra you must use the Inverse Gated sequence to eliminate NOE and get quantitative spectra. This is important stuff.
Paramagnetic Relaxation Reagents have compatibility issues with the analytes and they may spoil a precious sample that requires further analysis. Moreover they often shorten the T2 leading to broader lines and lower S/N. Although the sensitivity issue of 13C cannot be resolved without isotope enrichment (a difficult task). I would suggest that you should use a different sequence, called uniform driven equilibrium Fourier transform (UDEFT), returns the carbon magnetization with a high accuracy along its equilibrium position after each transient is completed. Thus, the sequence allows the use of relaxation delays (RD), which are much shorter than the carbon T1 of the molecule, thereby speeding up the acquisition process of 1D 13C{1H} spectra (even for slow relaxing nuclei).
This pulse sequence generates good S/N even for the C=O of camphor having T1 ~90s in a reasonable time frame. It is a part of the standard parameter set provided on Bruker machines nowadays. I have been using it and it works well. The reference and the link is provided below:
Kindly let me know if it works for you
Piotto M1, Bourdonneau M, Elbayed K, Wieruszeski JM, Lippens G., Magn Reson Chem. 2006 Oct;44(10):943-7.
Article New DEFT sequences for the acquisition of one-dimensional ca...
It happens due to gyromagnetic ratio of particular molecule, which is the ratio of its magnetic moment to its angular momentum. Each NMR active nucleus has its own distinctive frequency because its gyromagnetic ratio is varied. Carbon has a gyromagnetic ratio of about one-quarter that of hydrogen. So, 13C NMR always done at lower frequency as compared to 1H NMR.
is carbon 13 NMR of a compound done at higher frequency gives abnormal repetitive signals
You cannot observe carbon at "higher frequency". The convention is to refer to an instrument by the frequency for hydrogen. So on a 400 Mhz instrument hydrogen would be observed at a frequency of 400 Mhz and carbon at 100 MHz. You cannot observe carbon at any other frequency than this. So "moving to higher frequency" would be pointless. You would not see any carbon signal at all at any other frequency than 100 Mhz on this instrument.
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