New Vistron Biozone, you know the common problems in the use of nuclear magnetic resonance

1. What can NMR do?

  NMR (Nuclear magnetic resonance spectroscopy) is to study the absorption of radio frequency radiation by atomic nuclei. It is one of the most powerful tools for qualitative analysis of the composition and structure of various organic and inorganic substances, and sometimes it can also be used for quantitative analysis.

  Nuclear magnetic resonance (NMR) is an important means to identify the structure of organic compounds. Generally, groups are identified according to Chemical shift; Determine the group connection relationship based on the number of coupling splitting peaks and coupling constants; Determine the proton ratio of each functional group based on the integrated area of each H peak. Nuclear magnetic resonance spectroscopy can be used in the study of Chemical kinetics, such as intramolecular rotation, chemical exchange, etc. Nuclear magnetic resonance is also used to study the mechanism of polymerization reactions and the sequence structure of polymers. Two dimensional nuclear magnetic resonance spectroscopy can already analyze the spatial structure of protein molecules with smaller molecular weights.

Nuclear magnetic resonance spectroscopy is one of the "four famous spectra" in the minds of organic chemists, including ultraviolet spectroscopy, infrared spectroscopy, and mass spectrometry. H-spectrum and C-spectrum are widely used nuclear magnetic resonance spectra, with more commonly used nuclear magnetic resonance spectra such as F, P, and N.

2. How to analyze Proton nuclear magnetic resonance?

  Generally, the isolated methyl group and its type are determined first, and the hydrogen distribution is calculated based on the integral height of the peak area of the isolated methyl group; The second is to analyze the low field resonance absorption peak (such as aldehyde hydrogen, carbonyl hydrogen, etc.). Because these hydrogen are easy to identify, the attribution is determined according to the Chemical shift; Finally, the high-level coupling part of the spectrum is analyzed, and Protein secondary structure information such as substitution position, Structural isomer and Stereoisomerism is inferred according to the coupling constant, peak splitting and peak type.

3. How to parse nuclear magnetic resonance carbon spectrum?

Generally, first check whether the number of lines on the fully decoupled carbon spectrum is the same as the number of carbon contained in the Molecular formula? The same number indicates that the chemical environment of each carbon is different, and the molecules have no symmetry; Different (fewer) numbers indicate that the chemical environment with carbon is the same, and the molecules have symmetry; Then, the number of hydrogen coupled with carbon is determined from the partial resonance spectrum; Finally, the attribution of carbon is determined by the Chemical shift of each carbon.

4. How to combine carbon spectroscopy with hydrogen spectroscopy?

The C and H spectra can complement each other. The H-spectrum cannot determine functional groups without hydrogen, such as carbonyl and cyano groups; For organic compounds with high carbon content, such as steroidal compounds, they are often indistinguishable due to the similar chemical environment of alkanes and hydrogen, which is the weakness of the hydrogen spectrum; And carbon spectroscopy compensates for the shortcomings of hydrogen spectroscopy, which can provide information on various carbon functional groups, almost every carbon nucleus can be distinguished, and can provide rich carbon skeleton information. However, the disadvantage of ordinary carbon spectra is that the peak height is often not directly proportional to the carbon number, while the integral height of the hydrogen peak area is directly proportional to the hydrogen number, so the two can complement each other.

5. Can all elements in the periodic table be measured by nuclear magnetic resonance spectroscopy?

No. First, the spin Quantum number of the measured atomic nucleus should not be zero; Secondly, the spin Quantum number should preferably be 1/2 (the nucleus with spin Quantum number greater than 1 has an electric quadrupole moment, and the peak is very complex); Finally, the natural abundance of the measured element (or its isotope) is relatively high (natural abundance is low, sensitivity is too low, and signal cannot be measured).

6. How to better display active hydrogen in the H-spectrum?

The hydrogen connected with O, S and N is active hydrogen. If you want to see active hydrogen, you must choose Deuterated chloroform or DMSO as the solvent. The peak position of active hydrogen in DMSO is lower than that in CDCl3. Due to the influence of hydrogen bond, concentration, temperature and other factors, the Chemical shift value of active hydrogen will change within a certain range, and sometimes the intramolecular hydrogen bond will make the peak shape sharp.

7. How to do heavy water exchange?

To determine active hydrogen, heavy water exchange is required. The method is: after measuring the hydrogen spectrum of the sample, drop a few drops of heavy water into the sample tube (it is not advisable to add too much, usually 1-2 drops), shake it, and then measure the hydrogen spectrum. The active hydrogen in the spectrum will disappear. The amino hydrogen exchange between aldehyde hydrogen and amide is very slow, and it needs to be left for a long time before spectrum measurement or heated with a hair dryer, and then left for a while before detection. At this point, it will be found that the water peak signal in the spectrum is enhanced, and the HDO peak in CDCl3 is at the position of 4.8ppm. In addition, methanol and trifluoroacetic acid have heavy water Exchange interaction, and no active hydrogen peak can be seen.

8. What are the different approaches for analyzing the spectra of synthesized compounds, extracted compounds from plants, and unknown compounds?

The results of synthesizing compounds are known, and by comparing spectra and structures, it is possible to determine whether the compound is consistent with the predetermined structure. For the spectra of compounds extracted from plants, the first step is to determine which type of compound it is, and then compare it with known literature data to see if it is a known substance. If this data is not available in the literature, continue to measure DEPT spectra and two-dimensional spectra to deduce the structure. For a completely unknown compound, in addition to measuring nuclear magnetic resonance, it is also necessary to combine mass spectrometry, infrared, ultraviolet, and elemental analysis to infer the structure step by step.

9. Why do we use deuterium reagents to prepare samples? How to choose deuterium reagents?

Because the hydrogen in the solvent will also peak during the test, the amount of solvent is far greater than the amount of sample, and the solvent summit masks the sample peak, so deuterium is used to replace the hydrogen in the solvent. The frequency of deuterium's Formant is very different from that of hydrogen, and there will be no deuterium peak in the hydrogen spectrum, reducing the interference of solvent. The solvent peak appearing in the spectrum is the peak of incomplete substitution of deuterium for residual hydrogen. In addition, deuterium peaks need to be used for field locking during testing.

Since there are not many kinds of deuterated solvents, solvents with similar polarity should be selected according to the polarity of samples. The polarity of deuterated solvents is arranged from small to large as follows: benzene, chloroform, acetonitrile, acetone, dimethyl sulfoxide, pyridine, methanol and water. Pay attention to the Chemical shift of the solvent peak, and it is better not to block the sample peak.

The nuclear magnetic resonance spectra that can be provided by Xinweichuang Biological Co., Ltd. include: Proton nuclear magnetic resonance, carbon spectrum, fluorine spectrum, phosphorus spectrum, N-15 (N-15) spectrum, N-14 spectrum, etc. In the determination of Proton nuclear magnetic resonance and nuclear magnetic resonance carbon spectrum, New Vision Biotech also has many modern measurement technologies available for more in-depth testing of samples, such as multi pulse spectrum, multi-dimensional two-dimensional spectrum, etc., to facilitate the derivation of molecular structure and even geometric isomerism of compounds.