First of all, Nuclear Magnetic Resonance (NMR) is one of the most useful instrumental test methods for material characterization, and it is called the “four spectra” together with UV absorption spectroscopy, IR absorption spectroscopy and mass spectrometry.
It is one of the most powerful tools for qualitative analysis of the composition and structure of various organic and inorganic substances, and also for quantitative analysis.
At present, NMR has identified more than 100,000 compounds in combination with other instruments such as infrared and mass spectrometer.
1. Principle
Nuclear magnetic resonance (NMR) spectra originate from the jump between energy levels of atomic nuclei.
Only certain atomic nuclei placed in strong magnetic fields undergo energy level splitting, and when the absorbed radiation energy is equal to the nuclear energy level difference, and energy level jump occurs and an NMR signal is generated.
The NMR spectrum is obtained by irradiating a sample with electromagnetic waves of a certain frequency, which causes the nuclei in a specific chemical structure environment to make a resonance jump, and recording the position and intensity of the signal when the resonance occurs in the irradiation scan.
The position of the resonance signal on the NMR spectrum reflects the local structure of the sample molecule (e.g., functional groups, molecular conformation, etc.), and the intensity of the signal is often related to the amount of the nucleus in the sample.
| NMR | IR | UV | |
| Intrinsic | Molecular absorption spectrum | Molecular absorption spectrum | Molecular absorption spectrum |
| Wavelength range | 1-1000μm | 0.75-1000μm | 200-800nm |
| Signal source | Nucleus energy levels of Atomic | Vibrational energy levels of molecules | Electronic energy levels of molecules |
2. Features
Nuclear magnetic resonance (NMR) is precise, accurate, and penetrates deeply into the substance without destroying the sample under test.
Nuclear magnetic resonance (NMR) is precise, accurate, and penetrates deeply into the substance without destroying the sample under test.
3. Classification
3.1 Continuous wave nuclear magnetic resonance spectrometer (CW-NMR) RF waves generated by the RF oscillator irradiate the sample sequentially by frequency magnitude, and the frequency spectrum is obtained.
3.1 Continuous wave nuclear magnetic resonance spectrometer (CW-NMR) RF waves generated by the RF oscillator irradiate the sample sequentially by frequency magnitude, and the frequency spectrum is obtained.
3.3 The continuous wave NMR spectrometer consists of six parts: magnetic field, probe, RF emission unit, RF, magnetic field scanning unit, [k1] [WU2] RF detection unit, and data processing instrument control.
Magnets are used to generate magnetic fields, of which there are three main types.
| Type | Permanent magnets | Electromagnets | Superconducting magnets |
| Frequency | 60MHz | 100MHz | 200MHz or more |
Instruments with large frequencies have the advantage of good resolution, high sensitivity, and graphs for easy analysis.
| Continuous-wave NMR instrument | PFT-NMR Spectroscopy |
|---|---|
| Single-frequency transmit, single-frequency receive | Strong pulse irradiation Free induction decay (FID) signal, Fourier transform NMR spectrogram by computer |
| Long scanning time, less information per unit time, weak signal | Low spectral background noise, high measurement speed, faster automatic determination and resolution of spectral lines and the corresponding relaxation times |
| The number of accumulations is limited, and the sensitivity is still not high | High sensitivity and resolution, fast analysis speed |
| Wide spectral lines, poor resolution, not much information obtained | Solid-state high-resolution NMR, using magic angle rotation and other techniques, directly yields well resolved narrow spectral lines. |
| Used for dynamic processes, transient processes and reaction kinetics studies; measure weak resonance signals such as 13C, 14N, etc. |
4. Scope of application
Measurement object elements:
NMR spectra can be classified according to the measurement object: 1H-NMR spectrum (the measurement object is hydrogen nuclei), 13C-NMR spectrum and fluorine spectrum, phosphorus spectrum, nitrogen spectrum, etc.
The characteristic structures of different elements in a compound are determined from the spectra. Organic compounds and polymer materials are mainly composed of carbon and hydrogen, so 1H and 13C spectra are most widely used in the study of material structure and properties.
Testable properties:
In addition to its use in medical imaging examinations, it is most often used in analytical chemistry and in structural studies of organic molecules and materials characterization.
Structural identification of organic compounds:
Generally, the identification of groups is based on chemical shifts. The number of coupled splitting peaks and coupling constants determine the association of groups. And the proton ratio of each group is determined from the integrated area of each H-peak.
NMR spectroscopy can be used for chemical kinetic studies, such as intramolecular rotation, chemical exchange, etc., because they all affect the conditions of the chemical environment outside the nucleus, and thus should be reflected in the spectra.
NMR imaging of polymeric materials:
NMR imaging techniques have been successfully used to detect defects or damage inside materials, to study extruded or foamed materials, adhesive action, pore size distribution in pore-like materials, etc. It can be used to improve processing conditions and enhance the quality of products.
Analysis of multi-component materials:
When the material has more components, the NMR parameters of each component exist independently, and the compatibility between polymers is studied.
When the identities between two polymers are good, the chilling time of the blends should be the same, but when the compatibility is poor, it is different.
The solid-state NMR technique is used to determine the chilling time of polymer blends to determine their compatibility and to understand the structural stability and excellent performance of the material.
In addition, it is also used in the study of polymers to investigate polymerization reaction mechanisms, polymer sequence structures, qualitative identification of unknown polymers, mechanical and physical property analysis, etc.
5. Troubleshooting
6.1 Can all elements in the periodic table be measured by NMR spectroscopy?
Answer: No.
First, the spin quantum number of the nucleus to be measured should not be zero.
Second, the spin quantum number should preferably be 1/2 (nuclei with spin quantum numbers greater than 1 have electric quadrupole moments and complex peaks).
Third, the natural abundance of the element (or its isotopes) to be measured is relatively high (low natural abundance, too low sensitivity, no signal can be measured).
6.2 What is the difference in thinking about resolving the spectrum of a synthetic compound, the spectrum of a compound extracted from a plant, and the spectrum of an unknown compound?
The results of synthetic compounds are known, and it is enough to use the spectra to compare with the structures to know whether the compounds are consistent with the intended structures.
For a spectrum of a compound extracted from a plant, you should first look at which class of compound it is, then compare it with known literature data to see if it is a known substance, and if this data is not in the literature then proceed to measure the DEPT spectrum and 2D spectrum to launch the structure.
For an all-unknown compound, in addition to NMR measurement, mass spectrometry, IR, UV and elemental analysis should be combined to deduce the structure step by step.
For more troubleshootings, please click here.
