NIUMAG
What is NMR
Speaking of nuclear magnetic resonance (NMR), the first reaction of many people is the examination we usually do in the hospital-magnetic resonance imaging (MRI). Due to the lack of certain scientific knowledge, many of them believe that MRI has radiation and is harmful to the body. This is called nuclear color change. It is said that precisely because of this, medical experts in the United States during the Cold War renamed magnetic resonance imaging as magnetic resonance imaging to eliminate ordinary people’s fear of nuclear.
The above mentioned are the common views of people on MRI in daily life, basically staying at the cognitive level of health care. In fact, NMR itself contains very rich content, in addition to related experimental techniques, there are systematic and complete theories. It should be said that since the early physicists such as Rabi, Purcell and Bloch discovered the NMR phenomenon, NMR has been developed from an early emerging experimental technology to a cross-physics and chemistry through the joint efforts of countless scientists around the world. , Materials, biomedicine, electronics and other multi-disciplinary cross-disciplines with important influence. In its short decades of development, the Nobel Prize has been associated with it five times, and it has been reflected in physics, chemistry and biomedicine. According to related statistics, among the scientific and technological articles published worldwide each year, the article on NMR is the most, ranking first.
All these fully show that MRI has a strong academic vitality. At the same time, as an experimental technique, its scope of application is wide: from macroscopic objects to microscopic atomic and molecular states, NMR has become an indispensable “a pair of eyes” for us to observe and study substances. In addition, in view of the good two-way interactive relationship between MRI in basic research and applied research, its development will undoubtedly have a good demonstration role for the development of other disciplines.
What does NMR mean for scientific research and technical personnel
Specifically, for a nuclear magnetic detector, NMR may be a map. For this reason, they may only need to understand the operation steps of the instrument, simple maintenance, software use, and sample preparation process. It may also require the ability to parse the map. For organic chemists, in addition to the knowledge mentioned above, they must also master the knowledge of organic chemistry such as purification of organic compounds, organic reactions, and the role of some nuclear magnetic methods to help them better explain. The structure of organic compounds and understanding the mechanism of organic chemical reactions. Only in this way can we say that this organic chemist really has mastered NMR.
Of course, the above researchers only regard NMR as a characterization method similar to infrared and ultraviolet. For an NMR instrument engineer, NMR is a sophisticated and expensive large instrument. For this reason, they need to master more about the principle of the instrument itself and the electronic knowledge of electronic devices, so that the instrument can be in a relatively good state of use and ensure that it does not go wrong. More powerful engineers, they will customize nuclear magnetic instruments and related components (such as probes) according to customer needs. Of course, for those who specifically use NMR for research in the fields of catalysis, batteries, medicine, polymers, biology, and other related fields, NMR is an important scientific research tool in their hands. Their task is to continuously develop new nuclear magnetic methods or use existing methods to study matter and discover new phenomena.
These researchers are generally called NMR spectroscopy. The NMR we talk about basically refers to NMR spectroscopy. In general, NMR spectroscopy can be divided into two major directions of theoretical and methodological research and application: the former mainly includes the development of nuclear magnetic detection technology and pulse sequence, as well as the calculation and simulation of NMR spectra and the development of related software, which involves The essence of the principle of NMR-quantum mechanics. Therefore, mastering good knowledge of quantum mechanics and mathematics is indispensable, and it is best to have a certain knowledge of computer language. The latter is to use the existing nuclear magnetic pulse sequence and technology to study the structure, reaction mechanism and dynamics of substances such as catalysts, polymers, biological macromolecules, energy storage materials, organic substances and drug molecules, and discover new experimental phenomena from them. In order to make better use of NMR, it is necessary to understand the principles of relevant pulse sequences and methods and have knowledge in the corresponding fields.
Of course, some need to develop new experimental devices (such as in-situ and combined devices) to better study substances and related reaction processes. In addition, according to the type of spectrometer used, each direction can be divided into two branches: solid nuclear magnetic and liquid nuclear magnetic.
What is the research object of NMR, can any substance be tested?
It should be said that current NMR instruments can detect substances in any state. The most common are liquids and solids. Of course, liquid crystals, colloids and even gases between liquids and solids can be directly detected. But the premise is that the nuclear spin quantum number of the detected material element’s nucleus is not zero (most elements in the periodic table of the element meet this condition), which involves NMR research object-nuclear spin. Of course, this is only theoretical. Whether it can be tested on an instrument and whether an NMR spectrum with a good signal-to-noise ratio can be obtained. It also needs to pay attention to whether the Rama frequency of the nuclei of the tested element is within the specified frequency range of the instrument Inside, whether the natural abundance and relative sensitivity of the nucleus are high can be obtained from the NMR periodic table on the Internet. In addition, some instruments such as liquid NMR also require that the measured substance should not contain paramagnetic substances as much as possible.
What important information can we get from NMR?
To put it simply, an NMR experiment mainly obtains basic information by analyzing data such as peak position, peak shape, peak intensity, and relaxation time. This information can be obtained from the two aspects of “static” and “dynamic” mentioned earlier. For example, from the perspective of “static”, whether it is liquid or solid nuclear magnetic, the peak position generally refers to the chemical shift, which is an important basis for qualitative confirmation of the composition and type of substance. It is just that in liquid nuclear magnetic field, we generally see the information of the functional groups of organic substances. At the same time, combined with the two-dimensional spectrum, we can get the structure and spatial conformation information of the entire organic substance. In addition, for liquid nuclear magnetic, the peak shape should be concerned with the number of peak splits and the distance between the splits, from which the value of the J coupling constant can be obtained, which is also helpful for the judgment of the material structure. In addition, from the peak splitting situation, we can also indirectly know whether the magnetic field shimming is good or bad.
The information obtained by the peak position and peak shape in solid-state NMR is more abundant, and different information can be obtained by combining different disciplines and research directions. Here, in addition to the peak shape, the peak shape needs to pay attention to the linear shape and peak width of the peak. From the data of these peak shapes, NMR interaction parameters such as chemical shift anisotropy, dipole-dipole coupling constant, quadrupole coupling constant, etc. can be obtained indirectly to obtain substances such as bond length, bond angle, and spatial distribution of chemical bonds Important structural information. Of course, the difference between these chemical shifts and the peak shape is also due to the different bonding mode of the atoms and the chemical environment (for example, the surface and bulk phases, the free material and the adsorbed material, and the different spatial arrangements of the atoms of different crystal types). These nuclei The amount of NMR interaction caused by different Hamiltonian.
In addition, in order to prove the reasonableness of the spectrum judgment, it is necessary to use multi-dimensional nuclear magnetic and nuclear magnetic experiments of a variety of related nuclei, computational simulation of nuclear magnetic spectrum and other related characterization techniques to further confirm. As for the peak intensity, whether it is liquid or solid nuclear magnetic, it can be used to qualitatively compare the relative amount of a certain component or to quantitatively calculate the content of a certain component or substance by adding a certain amount of standard material.
From the perspective of “dynamic”, NMR is a very useful and important technical means to study molecular dynamics. Through variable temperature experiments, two-dimensional exchange experiments, cross-polarization and other methods to study the spectral line shape, peak width and peak intensity changes and nuclear magnetic relaxation time, etc., NMR can study molecules on the order of picoseconds to tens of seconds Movement, such as molecular vibration, rotation, diffusion, and chemical exchange. Here are two more classic examples. For example, in liquid NMR, it is found that the speed of the chemical exchange rate often affects the change in line shape. In solid NMR, the movement of molecules can be studied by analyzing the line change of 2H spectrum at different temperatures Happening.
Through further study of these molecular movements, we can not only obtain physical parameters such as activation energy, reaction rate and diffusion coefficient, but also re-understand the relevant properties of substances and some reaction processes from a macro perspective, such as the crystal form transformation of drug molecules , The glass transition process of polymers and the movement of some gas and liquid molecules in porous materials. So as to help us better design materials with unique properties.
As for why we can use the change of NMR spectrum to detect the movement process of molecules, the fundamental reason is that the movement process of these molecules affects the nuclear spin Hamiltonian of the nucleus, thus causing changes in the spectrum such as line shape, peak width and peak intensity. . The specific degree of influence depends on the relative size of the time scale of these motions and the NMR interaction. It’s like how fast a camera can capture some details of movement, it depends on the relationship between shooting speed and movement speed
