nuclear magnetic resonance

[hé cí gòng zhèn]

Physical and chemical structure analysis means

open 3 entries with the same name

Collection

zero Useful+1

zero

synonym NMR (Nuclear magnetic resonance) generally refers to nuclear magnetic resonance (physical and chemical structure analysis means)

Nuclear magnetic resonance (NMR) is a process in which a non-zero spin atomic nucleus absorbs electromagnetic waves of a specific frequency under the action of an external magnetic field, resulting in energy level transitions. Nonzero spin Nucleus With magnetic moment, outside magnetic field Under the action, its spin energy level undergoes Zeeman splitting. Nuclear magnetic resonance spectroscopy yes spectroscopy A branch of resonance frequency In the RF band.

Application of Nuclear Magnetic Resonance Magnetic resonance imaging (MRI) examination has become a common imaging examination method. As a new imaging examination technology, MRI will not human health Influential, but not suitable for six groups of people Nuclear magnetic resonance examination , i.e. installation pacemaker People with or suspected of having metallic foreign bodies in the eyeball aneurysm Silver clip Ligation People, internal objects or metal prosthesis People, critical patients whose lives are in danger claustrophobia Patients, etc. Monitoring instruments and rescue equipment shall not be brought into the MRI examination room. In addition, pregnant women who are less than 3 months pregnant should not have an MRI examination.

Chinese name: nuclear magnetic resonance
Foreign name: Nuclear magnetic resonance
Application: Magnetic resonance imaging (MRI)
Genus: Physics and Chemistry

Interpretation: New medical imaging technology based on the principle of nuclear magnetic resonance
Inspection position: Brain, lumbar spine, thoracic spine, cervical spine.
See publications: Biochemical Terms and Biophysical Terms, Science Press
Time of publication: 1990^[3]

catalog

Medical Applications

Announce

edit

Magnetic resonance imaging It is a kind of utilization Principle of nuclear magnetic resonance Latest of New medical imaging technology For brain, thyroid, liver, gallbladder, spleen, kidney, pancreas adrenal gland , uterus, ovary prostate And the heart and Large blood vessel It has excellent diagnostic function. And others Supplementary Examination Compared with other means, NMR has many imaging parameters scanning speed The advantages of fast, high tissue resolution and clearer images can help doctors "see" early lesions that are difficult to detect tumour 、 heart disease and Cerebrovascular disease A sharp tool for early screening.

It is understood that because metal will interfere with the external magnetic field, patients will undergo nuclear Magnetic resonance examination All metal objects on the body must be removed before. Cannot wear such as watch, metal necklace, false teeth, metal button, metal intrauterine device And other magnetic articles Nuclear magnetic resonance examination 。 In addition, wear pacemaker , with Paramagnetic metal Implants , such as metal clip, bracket, steel plate and screw, etc Magnetic resonance imaging Check. conduct Epigastrium (such as liver, pancreas, kidney, adrenal gland, etc.) An empty stomach is required before MRI examination, but sufficient water can be drunk during the examination to make the boundaries between stomach, liver and spleen clearer.

MRI is the most effective image for brain, spinal cord and other diseases diagnostic method , which can not only be found early tumour 、 cerebral infarction 、 cerebral hemorrhage 、 brain abscess 、 Cerebral cysticercosis Symptoms and Congenital cerebrovascular malformation , can also determine Hydrocephalus The type and reason of. The first gynecological disease endangering the life and health of Chinese women—— mammary cancer Through accurate MRI screening, early breast cancer lesions can be found; For“ hypertension 、 Hyperlipidemia 、 hyperglycemia ”Etc Three high population Through the nuclear magnetic examination of the head and heart, we can find high risk diseases such as heart disease and cerebral infarction at an early stage before the red light warning is sent out. In addition, MRI can also be used to examine the abdomen and pelvis, such as liver, gallbladder, pancreas, uterus, etc., and angiography of major blood vessels in the abdomen and extremities can confirm the diagnostic authenticity Pseudoaneurysm ， Dissecting aneurysm And various pathological changes of blood vessels of limbs. Nuclear magnetic resonance (MRI) is very precise in the diagnosis of various joint tissue lesions, and is very sensitive to bone marrow and aseptic necrosis of bone.

Instrument development

Announce

edit

CW NMR

Figure 1 Schematic diagram of nuclear magnetic resonance instrument

Today's Nuclear magnetic resonance instrument There are two forms of continuous wave (CW) and pulse Fourier transform (PFT). Continuous wave nuclear magnetic resonance instrument is mainly composed of magnet , RF transmitter detector , amplifier and Recorder And other components (see Figure 1). Magnets are used to generate magnetic fields. There are three main types: permanent magnets and electromagnets[ Magnetic induction Up to 24000 Gs (2.4 T)], superconducting magnet [magnetic induction strength up to 190000 Gs (19 T)].

The resolution of nuclear magnetic resonance spectrometer is usually expressed by frequency (also called "meganumber"), which is defined as excitation under the magnetic field of the instrument hydrogen atom Required electromagnetic wave Frequency. Such as one magnetic field intensity In 9.4T superconducting nuclear magnetic resonance, the excitation frequency of hydrogen atom is 400MHz, so the instrument is "400M". The instrument with high frequency has good resolution, high sensitivity, and simple and easy analysis. Available on magnet Scanning coil Use it to ensure that the magnetic field generated by the magnet is uniform and can continuously and accurately change within a narrow range. RF transmitter is used to generate fixed frequency electromagnetic radiation wave detector and amplifier to detect and amplify resonance signal. The recorder plots the resonance signal into a resonance spectrum.

CW-NMR is cheap and easy to operate, but its sensitivity is poor. Therefore, it is necessary to Sample size Large, and can only be measured such as ¹ H/¹ F/³ ¹ P Natural abundance Very high nuclei, low for such as ¹ ³ C abundance The nucleus of the atom can not be measured.

PFT-NMR

In the mid-1970s, the pulsed Fourier nuclear magnetic resonance instrument appeared, which enabled the study of ¹ ³ C nuclear magnetic resonance to be carried out rapidly.^[1]

The pulse Fourier transform NMR instrument is different from the continuous wave instrument in that it adds pulse program controller and Data acquisition and processing system , use a strong and short pulse (1~50 μ s) to excite all the nuclei to be tested at the same time, and open the receiving system in time when the pulse terminates to collect Free induction attenuation Signal（ FID ）, the nucleus to be excited passes Relaxation process return Equilibrium state Then the next pulse will be excited. The FID signal obtained is a time-domain function, which is the superposition of signals of several frequencies Fourier transform Change to frequency domain Function can be recognized by people. PFT-NMR often samples many times during testing, and then Fourier transforms the total FID signal obtained to improve sensitivity and Signal-to-noise ratio (The signal to noise ratio is increased by n ^ 0.5 times after n times of accumulation).

PFT-NMR has high sensitivity and can be used for low abundance nuclei. The test time is short (one to several seconds per scan). It can also measure the Relaxation time So it is possible to measure the reaction dynamics by nuclear magnetic resonance.^[2]

Fundamentals

Announce

edit

Nuclear magnetic resonance is mainly composed of Nucleus Caused by the spin motion of. Different atomic nuclei have different spin motions. They can use nuclear Spin quantum number I. Spin quantum number and the sum of atomic mass number Atomic number There are certain relationships between them, which can be roughly divided into three situations, as shown in the table below.

classification	Mass number	Atomic number	Spin quantum number (I)	NMR signal
①	even numbers	even numbers	zero	nothing
②	even numbers	Odd number	1,2,3,... (I is an integer)	yes
③	Odd number	odd or even	0.5,1.5,2.5,... (I is a half integer)	yes

The nucleus with I value of zero can be regarded as a non spin sphere, and the nucleus with I value of 1/2 can be regarded as a spin sphere with uniform charge distribution. The nucleus with I greater than 1/2 can be regarded as a spin ellipsoid with non-uniform charge distribution.^[1]

When the spin nucleus is in the external magnetic field with the intensity of H ₀, in addition to spin, it will also move around H ₀, which is very similar to the motion of the gyroscope, called the larmor precession. The angular velocity ω ₀ of spin nucleus precession is proportional to the external magnetic field strength H ₀, and the proportional constant is the magnetic ratio γ, which is the characteristic constant of each atomic nucleus.

ω₀=2 πν₀=γ H ₀, ν₀ is the precession frequency.

Fig. 2 Schematic diagram of two orientations of 1H spin nucleus in external magnetic field (4 sheets)

The movement of the atomic nucleus in the absence of an external magnetic field is shown in Figure 2. The orientation of the microscopic magnetic moment in the external magnetic field is quantized (directional quantization). The atomic nucleus with a spin quantum number I can only have 2I+1 orientations under the action of an external magnetic field, and each orientation can be represented by a spin magnetic quantum number m.

m=I,I-1,I-2,…,-I+1,-I。

Each orientation of the nucleus represents an energy state of the nucleus in the magnetic field. The nucleus with I=1/2 has only two orientations under the action of the external magnetic field, corresponding to m=1/2 or - 1/2, and the energy difference between the two states is Δ E.

ΔE=γhH₀/2π=hν₀。

When the spin nucleus in the external magnetic field receives the electromagnetic wave whose energy is exactly equal to the energy level difference of different orientations, the spin nucleus in the low energy state absorbs the electromagnetic wave and transitions to the high energy state. Since the energy of the radio frequency electromagnetic wave E=h ν, the transition occurs under the condition that ν=ν ₀=γ H ₀/2 π, which is called nuclear magnetic resonance.

There are two methods to detect the type of atomic nucleus by nuclear magnetic resonance: one is to fix the magnetic field intensity H ₀, gradually change the frequency ν of the irradiated electromagnetic wave for scanning, and when ν matches H ₀, nuclear magnetic resonance will occur; The second is to fix the electromagnetic wave frequency ν, and then gradually change H ₀ from low field to high field. When H ₀ matches ν, NMR occurs. The second method is called sweeping field method, which is adopted by all kinds of instruments.

Saturation and relaxation of nuclear magnetic resonance

If the high-energy nucleus cannot return to the low-energy state, then as the transition continues, the low-energy nucleus will continue to decrease until zero, and the synchronous NMR signal will also gradually weaken until disappear, the above phenomenon is called saturation.

In fact, a nucleus can change from a high-energy state to a low-energy state in a non radiative way. This process is called relaxation. There are two ways of relaxation. The nucleus in the high energy state transfers energy to the surrounding molecules through alternating magnetic fields, that is, the system releases energy to the environment and returns to the low energy state itself. This process is called spin lattice relaxation. The rate is expressed in 1/T ₁, which is called the spin lattice relaxation time. Spin lattice relaxation reduces the total energy of magnetic nuclei, also known as longitudinal relaxation. The process of two nuclei at a certain distance with the same precession frequency and different precession orientation interacting, exchanging energy and changing the precession direction is called spin spin relaxation. Its rate is expressed in 1/T ₂, which is called spin spin relaxation time. Spin spin relaxation does not reduce the total energy of magnetic nuclei, also known as transverse relaxation.^[1]

Abundance and sensitivity of nuclear magnetic resonance

The I value of natural rich ¹² C is zero, and there is no NMR signal. ¹ I value of ³ C is 1/2, and there is NMR signal. Generally speaking, the carbon spectrum is ¹ ³ C nuclear magnetic resonance spectrum. Since the spin quantum numbers of ¹ ³ C and ¹ H are the same, the nuclear magnetic resonance principle of ¹ ³ C is the same as that of ¹ H. However, the γ value of ¹ ³ C core is only about 1/4 of ¹ H core, and the detection sensitivity is proportional to γ ³. Therefore, even the ¹ ³ C core with 100% abundance is only 1/64 of ¹ H core, and the abundance of ¹ ³ C is only 1.1%. Therefore, its detection sensitivity is only about 1/6000 of ¹ H core, and detection of ¹ ³ C is more technically difficult than detection of ¹ H. The following table shows the natural abundance and relative sensitivity of several nuclei with 1/2 spin quantum number.

Natural Abundance of Several Spin Nuclei
Primordial nucleus	Natural abundance/%
1H	ninety-nine point nine eight
13C	one point one zero eight
15N	zero point three six five
19F	one hundred
31P	one hundred

^[1]

chemical shift

Announce

edit

Theoretically, there should be only one H ₀ corresponding to ν in the same atomic nucleus, but in fact, when the chemical environment (distribution of surrounding electrons) of the same atomic nucleus in the molecule is different, even if the irradiation frequency is the same, resonance will occur in different magnetic fields at the same time to produce absorption peaks, which is called chemical shift. The nuclear magnetic resonance spectrum of ethyl acetate showed that the 8 hydrogen in ethyl acetate showed absorption peaks under three different magnetic fields because they were in three different chemical environments, a, b, and c.

origin

Under the action of external magnetic field, the electrons around the atomic nucleus will move circularly, producing an induced magnetic field (Lenz's Law) opposite to the direction of the external magnetic field, so the effective magnetic field H that the atomic nucleus actually feels is equal to the external magnetic field H ₀ minus the induced magnetic field H ᵢ.

H=H₀(1-σ)=H₀-Hᵢ。

This phenomenon is called shielding effect, also called diamagnetic effect. σ is shielding constant. Since the magnetic induction line is closed, the induced magnetic field and the external magnetic field in some areas will also have the same direction. In fact, the effective magnetic field H felt by the atomic nucleus should be equal to the external magnetic field H ₀ plus the induced magnetic field H ᵢ. This effect is called deshielding effect and paramagnetic effect. To sum up, the actual conditions for nuclear NMR are

ν=γH/2π=γH₀(1-σ)/2π=γ(H₀-Hᵢ)/2π。

Under the irradiation of electromagnetic waves of the same frequency, the same atomic nuclei in different chemical environments are subject to different shielding effects, so their external magnetic fields required for nuclear magnetic resonance are also different, that is, chemical shifts occur.

Definitional expression

The chemical shift is directly related to the shielding constant, which is about one millionth of an order of magnitude. It is very difficult to accurately determine its value. At present, the relative numerical representation method is adopted, that is, a reference material is selected, and the position of the resonance absorption peak of the reference material is taken as the zero point. The chemical shift values of other absorption peaks are determined according to the distance between the positions of these absorption peaks and the zero point. It is specified that the chemical displacement is expressed by δ, which is defined as

δ=(σₛ-σ)/σₛ×10⁶，

Where σ ₛ is the shielding constant of the standard. The left side of the coordinate axis is the low field, and the right side is the high field. Under the irradiation of electromagnetic waves with the same frequency, the larger the shielding constant, the stronger the external magnetic field required for resonance, and the smaller the chemical shift.

The most commonly used reference material is tetramethyl silicon (CH ∨) ₄ Si, or TMS for short. TMS was selected as the standard because the four methyl groups in the first TMS are symmetrically distributed, all hydrogen is in the same chemical environment, and it has only one sharp absorption peak; The shielding effect of the second TMS is very high. The absorption peak appears in the high field, and the position of the absorption peak is in the region where the proton of general organic matter does not absorb. The δ value of the absorption peak of the standard is zero, the δ value on the left is positive, and the δ value on the right is negative.

The proton signals of most organic compounds occur at 0~10, and some proton signals occur at less than 0. For example, the chemical shift of the protons in the torxene ring can even reach -2.99 due to the influence of the magnetic anisotropy of its outer aromatic ring.

During the determination, the standard and sample can be put together to prepare a solution, which is called the internal standard method, or the standard can be sealed with capillary tube and put into the sample solution for determination, which is called the external standard method. In addition, the solvent peak can also be used to determine the chemical shifts of each peak of the sample to be measured.

influence factor

The chemical shift depends on the shielding constant, and the shielding constant depends on the density of the surrounding electron cloud of the atomic nucleus. Therefore, various factors that affect the density of the electron cloud have an impact on the chemical shift, and electronegativity and anisotropy have the greatest impact.

(1) Electronegativity

The stronger the electronegativity of the atom (or group) near the target nucleus, that is, the stronger the electron absorption ability, the lower the density of the electron cloud around the target nucleus, the weaker the shielding effect, and the larger the chemical shift. Here are some examples:

Example 1
Electronegativity	C　2.6	N　3.0	O　3.5
δ	C—C H ₃(0.77～1.88)	N—C H ₃(2.12～3.10)	O—C H ₃(3.24～4.02)

Example 2
Electronegativity	Cl　3.1	Br　2.9	I　2.6
δ	C H ₃—Cl(3.05) C H ₂—Cl₂(5.30) C H —Cl₃(7.27)	C H ₃—Br(2.68)	C H ₃—I(2.16)

The influence of electronegativity on chemical shift is through chemical bond, and its effect belongs to local shielding effect.

(2) Anisotropy

When the arrangement of the electron cloud of some groups in the molecule is not spherical symmetry, it produces an anisotropic magnetic field on the adjacent atomic nucleus, thus shielding the atomic nucleus at some spatial positions and de shielding the atomic nucleus at some spatial positions. This phenomenon is called anisotropic effect.

(3) Conjugate effect

The parallel p orbitals in the molecule will blend with each other, so that the motion range of the electrons in the molecule will expand to the whole blend system. When the hydrogen on the benzene ring is replaced by the electron donor group, due to the p - π conjugation, the electron cloud density of the benzene ring increases, and the proton peak shifts to the high field, while the electron acceptor group is on the contrary. The conjugation effect has a similar effect on double bond and other systems.

(4) Others

In addition to the above, hydrogen bond, van der Waals force and solvent also affect the chemical shift. The effect of hydrogen bond on the chemical shift of hydroxyl proton is related to the strength of hydrogen bond and the nature of electron donor. In most cases, hydrogen bond produces a screening effect, which makes the δ value of ¹ H move to the low field.

When the distance between the substituent and the resonant nucleus is less than the van der Waals radius, the substituent and the electron cloud around the resonant nucleus repel each other, which reduces the density of the electron cloud around the resonant nucleus, significantly reduces the shielding effect, and the absorption peak moves to the low field.

The use of different solvents may also change the chemical shift value, and the solvent effect of active hydrogen is obvious. The three effects of hydrogen bond, van der Waals force and solvent are very useful in analyzing NMR spectra.

Proton chemical shift

Announce

edit

Since the chemical shifts of different types of protons are different, the chemical shift value is important for distinguishing various types of protons, and determining the proton type is very meaningful for clarifying the molecular structure. The following table lists the chemical shifts of some characteristic protons Boldface H is the proton to be studied.

Chemical shifts of characteristic protons
Proton type	chemical shift	Proton type	chemical shift
RC H three	zero point nine	ArO H	4.5-4.7 (intramolecular association 10.5~16)
R2C H two	one point three	ArO H	4.5-4.7 (intramolecular association 10.5~16)
R3C H	one point five	R2C=CR—O H	15~19 (intramolecular association)
	zero point two two	RCH2O H	3.4～4
R2C=C H two	4.5～5.9	ROC H three	3.5～4
R2C=CR H	five point three	RC H O	9~10
R2C=CR—C H three	one point seven	RCOCR2— H	2～2.7
RC≡C H	7～3.5	H CR2COO H	2～2.6
ArCR2— H	2.2～3	R2C H COOR	2～2.2
RC H 2F	4～4.5	RCOOC H three	3.7～4
RC H 2Cl	3~4	RC≡CCOC H three	2~3
RC H 2Br	3.5～4	RN H 2 or R2N H	0.5~5 (the peak is not sharp, often in the shape of steamed bread)
RC H 2I	3.2~4	RN H 2 or R2N H
RO H	0.5~5.5 (temperature, solvent . The concentration changes greatly)	RCONR H Or ArCONR H	5～9.4

^[1]

alkane

Fig. 3 1H-NMR Spectra of C6D11H at Different Temperatures (3 sheets)

methane Chemical shift value of hydrogen is 0.23, others are on Alkane In, the first order proton appears at the high field δ ≈ 0.91, the second order proton moves to the low field at δ ≈ 1.33, and the third order proton moves to the lower field at δ ≈ 1.5. For example:

alkane	C H four	C H 3—C H three		C H 3—C H 2—C H three			(C H 3)3C H
δ	zero point two three	zero point eight six	zero point eight six	zero point nine one	one point three three	zero point nine one	zero point eight six	one point five zero

The methyl peak generally has obvious characteristics, methylene Peak sum Methylene The peak has no obvious characteristics, and often presents a very complex peak shape, which is difficult to identify. When others are introduced into the molecule functional group The chemical shifts of methyl, methylene and methylene will change, but δ Values rarely exceed the range of 0.7 to 4.5.

Naphthenic hydrocarbon Can be different conformation When the unsubstituted naphthenic hydrocarbon is in a certain conformation Single bond Of anisotropy Shielding effect, different hydrogen δ Values vary slightly. For example, in cyclohexane Of Chair conformation Medium, due to the Flat volt key Hydrogen is in the C (2) - C (3) bond and C (5) - C (6) bond Deshielding Zone, while on C-I Vertical key Hydrogen is not in the unshielded zone, (Fig. Anisotropy of cyclohexane Shielding effect ）。 Therefore, the chemical shift of flat bond hydrogen is slightly 0.2~0.5 higher than that of vertical bond hydrogen. When the conformation is fixed at low temperature (- 100 ℃), it can be clearly seen that two absorption peak One represents the standing bond hydrogen, and the other represents the flat bond hydrogen. However, at room temperature, due to the rapid conversion of the conformation (Fig. conversion of cyclohexane conformation), only one absorption peak is generally seen (see Figure 3).

Other unsubstituted naphthenes also have only one absorption peak at room temperature. Cyclopropane δ value of is 0.22, Cyclobutane The δ value of is 1.96, and that of other naphthenes is about 1.5. In substituted cycloalkanes, different hydrogens on the ring have different chemical shifts, and their spectra sometimes have complex peak shapes, which are difficult to identify.^[1]

olefin

The alkene hydrogen is connected to the double bonded carbon, because Carbon carbon double bond Anisotropy of Heterosexual effect , alkene hydrogen and simplicity alkane Hydrogen ratio δ The average value moves 3~4 to the low field ethylene The chemical shift of hydrogen is about 5.25, which is different from Aryl group The chemical shifts of the conjugated substituted alkenes vary from 4.5 to 6.5, and the δ value will increase when they are conjugated with the aryl group. Vinyl It also affects the chemical shifts of methyl, methylene and methylene. For example:

chemical compound	C H four	C H 3—CH=CH2	C H 3—C H three		C H 3—C H 2—CH=CH2		(CH3)2C H two	(CH3)2C H —CH=CH2
δ	zero point two three	one point seven one	zero point eight six	zero point eight six	one	two	one point three three	one point seven three

From the above data, it can be seen that the hydrogen δ value of vinyl on the same carbon is about 1.59~2.14, which varies greatly. There is vinyl hydrogen on the adjacent carbon, δ The value changes slightly.^[1]

Alkyne

Alkynyl hydrogen is related to Triple bond For the hydrogen connected to carbon, due to the shielding effect of the alkyne bond, the chemical shift of the alkyne hydrogen moves to the high field. Generally, there is an absorption peak at δ=1.7~3-5. For example, HC ∨ C H （1.80），RC≡C H（ 1.73～1.88），ArC≡C H （2.71～3-37），—CH=CH-C≡C H （2.60～3.10），—C≡C—C≡CH（1.75～2.42）, CH3-C≡C-C≡C-C≡C H （1.87）。 HC ∨ C - If connected to an atom without hydrogen, the alkyne shows a sharp single peak. The alkynyl group has an impact on the chemical shift of methyl and methylene, and the hydrogen chemical shift on the carbon directly connected to the alkynyl group has the greatest impact, with a δ value of about 1.8~2.8.^[1]

aromatic hydrocarbon

Due to π electron The chemical shift of aromatic hydrogen moves to the low field due to the shielding effect of circulation, and the δ of hydrogen on benzene is 7.27. The proton on naphthalene is more affected by the two aromatic rings. The δ value of α proton is 7.81, and that of β proton is 7.46. Generally, the δ value of the proton on the aromatic ring is in the range of 6.3~8.5, and the δ value of the heterocyclic aromatic proton is in the range of 6.0~9.0.^[1]

Halohydrocarbon

Because of halogen Electronegativity Strong, so that the shielding of protons on the directly connected carbon and adjacent carbon is reduced, and the chemical shift of protons moves to the direction of low field. The impact is F, Cl , Br, I in descending order. Directly connected with halogen carbon atom The chemical shift of protons on carbon is generally between δ=2.16 and 4.4, and the influence on protons on adjacent carbon is reduced, between δ=1.25 and 1.55, separated by one carbon Atomic time , the influence is smaller, δ=1.03~1.08.^[1]

carboxylic acid derivatives

Nuclear magnetic resonance hydrogen spectrum of ethyl acetate

The chemical shift δ H of the proton RCOOCH2R on the alkyl group in the ester is 3.7 ～ 4. amide The chemical shift of the proton RCONHR on the medium nitrogen is generally between δ=5 and 9.4, and often cannot give a sharp peak.

carbonyl Or protons on α - carbon near the nitrogen base have similar chemical shifts=2-3, for example, CH3COCl δH=2.67,CH3COOCH3 δH=2.03， RCH2COOCH3 δH=2.13,CH3CONH2 δH= 2.08，RCH2CONH2 δH=2.23，CH3CN δH=1.98，RCH2CN δH=2.30。^[1]

Other information

See the following for the characteristics of NMR spectrum of alcohols. The chemical shift of ether α - H is about 3.54.

The δ value of NMR of phenol hydroxy hydrogen is very unstable, and is greatly affected by temperature, concentration and solvent, so only its approximate range can be listed. Generally, the δ value of phenol hydroxy hydrogen is in the range of 4~8, and the δ value of phenol hydroxy hydrogen with intramolecular association is in the range of 10.5~16.

The chemical shift of carboxylic acid H is between 2 and 2.6. In carboxylic acid carboxyl Due to the electron absorption effect of two oxygen atoms, the shielding of the protons is greatly reduced, and the chemical shift is in the low field. R2CHCOOH δH=10～12。

In amine, protons on nitrogen are generally not easy to identify. Due to different hydrogen bond degrees, they change greatly. Sometimes the chemical shifts of N-H and C-H protons are very close, so they are not easy to identify. Generally, α - H δ H=2.7 ～ 3.1, β - H δ=1.1 ～ 1.71. N-H δ=0.5 ～ 5, and the approximate range of δ values of RNH2 and R2NH is 0.4 ～ 3.5, Ar The δ value of NH2, ArzNH and ArNHR ranges from 2.9 to 4.8.^[1]

Carbon chemical shift

Announce

edit

¹ The chemical shift of ³ C is also based on tetramethyl silicon. Compared with ¹ H chemical shift, ¹ ³ C chemical shift is affected by more factors, but the electron shielding around the spin nucleus is still one of the important factors. Carbon atom is the skeleton of organic molecules. The structure of the molecule itself and the interaction between carbon nuclei within the molecule mostly affect the chemical displacement, including the hybridization mode, intramolecular and intermolecular hydrogen bonds, various electronic effects, conformation, configuration, the type of solvent during measurement, the concentration of solution, and the acidity and alkalinity of the system. Now there are some approximate methods to calculate δ, which can make qualitative or semi quantitative estimates of δ of some compounds, but more perfect theories need further research. The following table summarizes the chemical shifts of C in some groups based on a large number of experimental data, in which the boldface "C" is the carbon studied.

Chemical shifts of some characteristic carbons
Type of carbon	chemical shift	Type of carbon	chemical shift
Methane carbon	-2.68	α - carbon of ether (tertiary)	70~85
Straight chain alkane	0~70	Alpha carbon of ether (secondary)	60~75
Grade IV carbon	35~70	Alpha carbon of ether (first grade)	40~70
Tertiary carbon	30~60	Alpha carbon of ether (methyl carbon)	40~60
Secondary carbon	25~45	R C OOH，R C OOR	160~185
Primary carbon	0~30	R C OCl，R C ONH₂	160~180
C H₂=CH₂	one hundred and twenty-three point three	Carbonyl carbon of imide	165~180
Olefinic carbon	100~150	Carbonyl carbon of anhydride	150~175
C H≡CH	seventy-one point nine	Carbonyl carbon substituted for urea	150~175
carbyne	65~90	α - carbon of amine (tertiary)	65~75
Cyclocarbon of cyclopropane	-2.8	α - carbon of amine (secondary)	50~70
( C H₂)ₙ　（4~7）	22~27	α - carbon of amine (primary)	40~60
Benzene ring carbon	one hundred and twenty-eight point five	α - carbon of amine (methyl carbon)	20~45
Aromatic carbon in aromatics and substituted aromatics	120~160	Cyanocarbon	110~126
Heterocyclic aromatic carbon	115~140	Isocyano carbon	155~165
Aldehyde carbon	175~205	R₂ C =N-OH	145~165
C=C- C HO	175~195	RN C O	118~132
Carbonyl carbon of α - halogenated aldehyde	170~190	α - carbon of thioether (tertiary)	55~70
Carbonyl carbon of R ₂ C=O (including cycloketones)	200~220	α - carbon of sulfide (secondary)	40~55
Carbonyl carbon of unsaturated ketones and aromatic ketones	180~210	α - carbon of thioether (first grade)	25~45
Carbonyl carbon of α - haloketone	160~200	α - carbon (methyl carbon) of thioether	10~30

Coupling constant

Announce

edit

The phenomenon of mutual shielding or de shielding effect between atomic nuclei is called coupling. When nuclear magnetic resonance is generated, the probability of an atomic nucleus being in their respective spin states is usually equal. Each spin state has different shielding effects on the surrounding atomic nuclei. Different atomic nuclei are combined with different spins, resulting in a coupling split peak in the nuclear magnetic resonance spectrum.

The measure of spin coupling is called the coupling constant, which is represented by the symbol J. The value of J indicates the strength of the coupling action. The upper left of J is often marked with a number, which represents the number of bonds separated between two coupling atomic nuclei, and the lower right of J is marked with other information. In essence, the coupling constant is the difference between the nuclear magnetic resonance energy levels of two nuclei of the same kind after spin splitting, which can be reflected by the position difference of resonance absorption, which is the distance between the splitting peaks in the spectrum.

J=νΔδ。

The coupling constant is related to the relative position of two interacting atomic nuclei. It will decrease quickly with the increase of the number of separated bonds. In general, coupling splitting can occur when two atomic nuclei are separated by less than or equal to three single bonds, and the coupling constant tends to zero when more than three single bonds are separated. For example, in butanone, there are three single bonds between H (a) and H (b), so coupling splitting can occur between them, while there are more than three single bonds between H (a) and H (b) or between H (b) and H (c), and the coupling between them is extremely weak, that is, the coupling constant tends to zero. However, two nuclei containing double or triple bonds in the middle are exceptional, and they can be remotely coupled.

Low resolution NMR spectra of acetaldehyde (2 sheets)

The two maps are PMR maps of acetaldehyde (CH3CHO) made by low resolution NMR and high resolution NMR respectively. Comparing these two maps, it can be found that acetaldehyde has only two single peaks in the map made by low resolution nuclear magnetic resonance. In the high-resolution spectrum, two groups of peaks are obtained, which are double peaks and quadruple peaks.

The coupling split peak is different from the chemical shift peak. It will expand and contract with the change of irradiation frequency. When the two peaks are not easy to distinguish, they can be distinguished by changing the irradiation frequency.^[1]

Magnetic equivalence

Announce

edit

When the types of two atoms in the molecule are the same as the chemical environment, they are called chemical equivalence. Chemically equivalent hydrogen must have the same chemical shift. For example, two H's in CH ₂ Cl ₂ are chemically equivalent, and their chemical shifts are also the same. Conversely, hydrogen with the same chemical shift may not be chemically equivalent. It is very important to judge whether hydrogen is chemically equivalent in spectrum identification. The general basis for judging is that if hydrogen in molecules can be exchanged through symmetry operation or rapid mechanism, they are chemically equivalent.

Protons that can be interchanged through rotational symmetry are called homotopic protons, which are chemically equivalent in any environment; Protons that can be interchanged by mirror symmetry are called enantiotopic protons. A group of chemical shift equivalence nuclei, if their coupling constants to any nuclei outside the group are the same, then this group of nuclei is called magnetic equivalence or magnetic identity. Obviously, magnetic equivalence must be chemical equivalence, while chemical equivalence is not necessarily magnetic equivalence. When judging whether the hydrogen in the molecule is chemically equivalent, pay attention to the following situations:

① The two H's on CH ₂ connected to an asymmetric carbon atom are chemically unequivalent. This effect of asymmetric carbon atoms can extend further to hydrogen.

② In olefins, if one carbon on the double bond is connected with two identical groups, and the other carbon is connected with two hydrogen, the two hydrogen are chemically equivalent. When the rotation of the single bond is blocked, the chemical equivalence of the two hydrogen connected with the single bond with some double bond properties can also be judged by the same method.

③ Hydrogen that is not chemically equivalent under some conditions may be chemically equivalent under other conditions. For example, when the molecular conformation of CH ₂ on cyclohexane is fixed, the two hydrogens are chemically equivalent. When the conformation is rapidly converted, the two hydrogens are chemically equivalent. Only chemically nonequivalent protons can show self spin coupling.^[1]

Peak area and integral curve

Announce

edit

In the nuclear magnetic resonance spectrum, the area under the resonance peak is proportional to the number of hydrogen atoms that generate the peak, so the ratio of the peak area is the ratio of the number of different types of hydrogen atoms. If the number of hydrogen atoms in the whole molecule is known, the specific number of magnetic equivalent hydrogen in each group can be calculated from the proportion relationship of the peak area. The nuclear magnetic resonance instrument uses an electronic integrator to measure the peak area, and the spectrum from low field to high field is represented by a continuous step integration curve. The total height of the integral curve is proportional to the total number of hydrogen atoms in the molecule, and the height of the step curve of each peak is proportional to the area of the peak, that is, proportional to the number of hydrogen atoms generating the absorption peak. The relative integral value of each peak area can also be directly displayed with numbers on the spectrum. If the peak area containing one hydrogen is specified as 1, the number on the spectrum is consistent with the number of hydrogen atoms.^[1]

Analysis of atlas

Announce

edit

The NMR hydrogen spectrum provides information such as integral curve, chemical shift, peak shape and coupling constant. The analysis of the spectrum is to reasonably analyze these information and correctly deduce the structure of the compound corresponding to the spectrum. The following steps are usually used.

① Identify impurity peaks. In ¹ H-NMR spectra, impurity peaks unrelated to compounds often appear, and they should be marked before analyzing the spectra.

The most common impurity peak is the solvent peak. The solvent peak will be generated by the solvent not removed from the sample and the non deuterium solvent mixed in the deuterium solvent for determination. To facilitate their identification, the following table lists the chemical shifts of the most commonly used solvents.

Chemical shift of common solvents
Common solvents	chemical shift	Common solvents	chemical shift
cyclohexane	one point four zero	acetone	two point zero five
benzene	seven point two zero	acetic acid	two point zero five 8.50（COOH）*
chloroform	seven point two seven	Tetrahydrofuran	3.60（α） 1.75（β）
acetonitrile	one point nine five	Dioxane	three point five five
1,2-Dichloroethane	three point six nine	Dimethyl sulfoxide	two point five zero
water	four point seven	N. N-Dimethylformamide	two point seven seven two point nine five 7.5（CHO）*
methanol	three point three five 4.8*	Silica gel impurity	one point two seven
Ether	one point one six three point three six	pyridine	8.50（α） 6.98（β） 7.35（γ）
*The value varies with the measurement conditions.

During ¹ H-NMR measurement, the rotating sample tube will produce an uneven magnetic field, resulting in symmetrical small peaks on both sides of the main peak. This pair of small peaks is called the rotating side peak. The distance between the rotating side peak and the main peak changes with the change of the rotating speed of the sample tube, which can be eliminated by adjusting the appropriate instrument.

¹ ³ C and ¹ H can couple and produce split peaks, which are called ¹ ³ C isotope side peaks. Since the natural abundance of ¹ ³ C is only 1.1%, the ¹ ³ C isotope edge peak can only appear when the sample concentration is very high or the spectrum is enlarged.

② Calculate the corresponding hydrogen atom number of each peak according to the integral curve. If the hydrogen atom number has been directly marked in the spectrum, this step can be saved.

③ Their attribution is determined according to the chemical shifts of the peaks.

④ The relationship between groups is determined according to the shape of the peak and the coupling constant.

⑤ Active hydrogen is identified by heavy water exchange. Because the ionized active hydrogen on - OH, - NH ₂, - COOH can exchange with D ₂ O. The signal of active hydrogen disappears, so comparing the spectra before and after heavy water exchange can basically distinguish whether there is active hydrogen in the molecule.

⑥ Synthesize various analyses, infer the molecular structure and check the conclusions.^[1]

Simplification of atlas

Announce

edit

The primary atlas is relatively simple and can be analyzed directly according to the above aspects, but the sequence of analysis can be flexibly mastered according to the actual situation. The spectral lines of advanced atlas are generally complex and difficult to analyze directly. For convenience, it is better to simplify the atlas with reasonable methods before analysis. Please refer to relevant monographs for common methods of simplifying atlas.^[1]

Decoupling

Announce

edit

Fig. 4 13C Spectra of Acetone

¹ ³ C's nuclear magnetic resonance principle is the same as ¹ H's nuclear magnetic resonance principle, so ¹ ³ C and the directly connected hydrogen nucleus will also have coupling effect. Because most organic molecules have hydrocarbon bonds, so the split spectrum lines overlap each other, and the spectrum becomes complex and difficult to identify. Only through the decoupling process, can the spectrum become clear and recognizable. The most commonly used decoupling method is hydrogen (noise) decoupling method, which uses the double irradiation method. The power of the irradiation field includes the resonance frequency of all hydrogen in various chemical environments, so it can eliminate the coupling of ¹ ³ C with all hydrogen nuclei, so that ordinary organic compounds containing only C, H, O, N can be in ¹ ³ C-NMR spectra, ¹ ³ C signals become unimodal, that is, all the unequal ¹ ³ C cores have their own independent signals.

Fig. 4 shows the ¹ ³ C spectrum of acetone. (a) Is the coupling spectrum, and (b) is the hydrogen decoupling spectrum. In the coupling spectrum, the carbonyl carbon (δ=206.7) is coupled with six hydrogens by two bonds, splitting into seven peaks, and the α carbon (δ=30.7) is coupled with three hydrogens by one bond, splitting into four peaks. In the hydrogen decoupling spectrum, the splitting peaks of carbonyl carbon and α - carbon become single peaks. Acetone has two identical alpha carbons and one carbonyl carbon, and the peak intensity of alpha carbon is greater than that of carbonyl carbon. Hydrogen decoupling spectrum is commonly referred to as carbon spectrum, also known as broadband decoupling spectrum, which is represented by ¹ ³ C {H}. There are many other ways to get married. For interested readers, please refer to the relevant monographs.^[1]

Novice on the road

Growth task Getting Started with Editing Edit Rule Edit by myself

I have questions

Content query Online Service Official post bar Feedback

Complaints and suggestions

Report bad information Failed to appeal through entries Complaint of infringement information Blocking query and unblocking

Jinggong Network Anbei No. 1100000200001