According to the basic principle discussed earlier, under a certain irradiation frequency, nuclear magnetic resonance can only occur under a certain magnetic induction intensity. For example, the irradiation frequency is 60 MHz, and the magnetic induction intensity is14.092gs (14.092x10-4t),100mhz-23.486gs (23.486x10-4t). However, experiments show that when the chemical environment of 1H in the molecule is different (the chemical environment refers to the movement of the extranuclear electrons of 1H and those of other nuclei adjacent to 1H), even at the same irradiation frequency, the absorption peak will be displayed under different * * * vibrating magnetic fields. The figure below shows the NMR vibration spectrum of ethyl acetate. The spectrum shows that eight hydrogens in ethyl acetate have absorption peaks under three different vibrating magnetic fields because they are in three different chemical environments: A, B and C. Because of the different chemical environments in the molecule, the same kind of nuclei have absorption peaks under different vibrating magnetic induction intensities, which is called chemical shift. How is the chemical shift produced? The magnetic core in a molecule is not completely naked, and protons are surrounded by valence electrons. These electrons circulate under the action of external magnetic field, which will produce an induced magnetic field, which should be opposite to the external magnetic field (Lenz's law). Therefore, the effective magnetic induction intensity actually felt by protons should be the external magnetic field induction intensity minus the induced magnetic field intensity. that is
Be effective = B0 (1-σ) = B0-B0σ = B0-B induction.
This effect of external electrons on the nucleus is called shielding effect, also called diamagnetic shielding effect. It is called the shielding constant. Compared with protons with less shielding, protons with more shielding feel less about the external magnetic field and will only be absorbed by the higher external magnetic field B0. Because the magnetic field lines are closed, the induced magnetic field is consistent with the external magnetic field in some areas, and the effective magnetic field actually felt by protons in these areas should be induced by the external magnetic field B0 plus the induced magnetic field B. This effect is called the unmasking effect. Also known as paramagnetic demagnetization effect. Protons affected by unmasking effect can undergo vibration absorption under the action of low external magnetic field B0. To sum up, proton nuclear magnetic resonance should actually meet the following requirements:
V injection capacity =γB effective /2π
Due to the different shielding effects of protons in different chemical environments under the irradiation of electromagnetic radiation waves with the same frequency, the external magnetic field B0 required by them to generate nuclear magnetic resonance is also different, that is, chemical shift occurs.
Local shielding effect and remote shielding effect are the main factors affecting the chemical shift of1h. The shielding effect of electron cloud density of bonding electrons outside the nucleus on the nucleus is called local shielding effect. The shielding effect of extranuclear electrons of other atoms and groups in the molecule on the studied nucleus is called remote shielding effect. The remote shielding effect is anisotropic. The difference of chemical shift is about 10 parts per million, so it is difficult to determine its value accurately. At present, the relative numerical representation method is adopted, that is, a reference material is selected, and the position of the vibration absorption peak of the reference material is taken as the zero point, and the chemical shift values of other absorption peaks are determined according to the distance between these absorption peak positions and the zero point. The most commonly used standard material is tetramethyl silicon (CH3)4Si, or TMS for short. TMS is chosen as the standard because the four methyl groups in TMS are symmetrically distributed, so all hydrogen is in the same chemical environment, and they only have a sharp absorption peak. In addition, the shielding effect of TMS is very high, and the absorption of * * * vibration occurs in high field, and the absorption peak is located in the area where protons in general organic matter do not absorb. The specified chemical shift is expressed by δ, and the δ value of tetramethyl silicon absorption peak is zero, and the δ value on the right side of the peak is negative and the δ value on the left side is positive. When measuring, the standard substance and sample can be put together to make a solution, which is called internal standard method. The standard substance can also be sealed by capillary tube and put into the sample solution for determination, which is called external standard method. In addition, the solvent peak can also be used to determine the chemical shift of each peak of the sample to be measured.
Because the induced magnetic field is proportional to B0 of the external magnetic field, the chemical displacement caused by shielding is also proportional to B0 of the external magnetic field. In actual measurement, in order to avoid the change of chemical displacement caused by using nuclear magnetic resonance vibrators with different magnetic induction intensity, δ is generally expressed as a relative value and is defined as
δ = (ν sample-ν standard)/ν instrument×106④
In formula (4), the ν sample and the ν standard respectively represent the * * * vibration frequencies of the sample and the standard compound, and the ν instrument is the frequency selected for operating the instrument. Proton signals of most organic compounds appear at 0 ~ 10, where zero is a high field and 10 is a low field. It should be noted that there are also some proton signals that appear in places less than 0. For example, the proton on the androstene ring is influenced by the magnetic anisotropy of its outer aromatic ring, which can even reach -2.99. In addition, the value of chemical shift is the same in different trillion instruments. The chemical shift depends on the density of the electron cloud outside the nucleus, so various factors affecting the density of the electron cloud have an influence on the chemical shift, among which electronegativity and anisotropy are the most influential.
(1) electronegativity (induced effect)
The influence of electronegativity on chemical shift can be summarized as follows: the atom (or group) with large electronegativity has strong electron-absorbing ability, the electron-absorbing group near 1H nucleus makes the proton peak move to the low field (left), and the electron-donating group turns off to make the proton peak move to the high field (right). This is because the electron-withdrawing group reduces the electron cloud density around the hydrogen nucleus and the shielding effect is also reduced, so the chemical potential of protons moves to the low field. Giving electron groups increases the electron cloud density around the hydrogen nucleus, and the shielding effect also increases, so the chemical shift of protons moves to the high field. Here are some examples.
Example 1: electronegativity C2.6n3.0o3.5δ c-CH3 (0.77 ~1.88) n-CH3 (2.12 ~ 3.10) o-CH3 (3.24 ~ 4.
CH2—Cl2(5.30)
The electronegativity of ch-cl3 (7.27) CH3-br (2.68) CH3-I (2.16) affects the chemical shift through chemical bonds, and its shielding effect belongs to local shielding effect.
⑵ Anisotropic effect
When the electron cloud arrangement of some groups in the molecule is not spherically symmetric, an anisotropic magnetic field is generated to the adjacent 1H nuclei, so that some nuclei in some spatial positions are shielded, while others are not. This phenomenon is called anisotropic effect.
In addition to electronegativity and anisotropy, hydrogen bonding, solvent effect and van der Waals effect also have effects on chemical shift. The influence of hydrogen bond on the chemical shift of hydroxyl proton is related to the strength of hydrogen bond and the properties of hydrogen bond electron donor. In most cases, hydrogen bonding produces shielding effect, which makes δ value 1H move to low field. Sometimes using different solvents for the same sample will also change the chemical shift value, which is called solvent effect. The solvent effect of active hydrogen is obvious.
When the distance between the substituent and the * * * vibrating nucleus is less than the van der Waals radius, the substituent and the electron cloud around the * * * vibrating nucleus repel each other, which leads to the decrease of the electron cloud density around the * * * vibrating nucleus, the shielding effect of protons is significantly reduced, and the proton peak moves to the low field, which is called van der Waals effect. The influence of hydrogen bond, solvent effect and van der Waals effect is very useful in NMR analysis.
(3)*** yoke effect
If the hydrogen on the benzene ring is replaced by the electron-pushing group, the electron cloud density of the benzene ring increases and the proton peak moves to the high field due to the P-π*** yoke. However, when there are electron-withdrawing substituents, the situation is just the opposite. It has a similar effect on systems such as double bonds.