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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5647_Библиотеки_им_академика_М_И_Перельмана.pdf
10.5 Magnetic Shielding 451
10.5 Magnetic Shielding
Magnetic shielding is a critical conc ept in NMR spectroscopy that describes how the
magnetic environment surrounding a nucleus affects its resonance frequency when
subjected to an external magnetic field. This phenomenon is essential for understanding how nuclear spins interact with magnetic fields, leading to variation s in
chemical shifts observed in NMR spectra. Magnetic shielding refers to the reduction
of the effective magnetic field experi enced by a nucleus due to the presence of
surrounding electrons. This reduction alters the resonance frequency of the nucleus,
affecting the chemical shift observed in NMR.
10.5.1 Mechanism of Magnetic Shielding
When an external magnetic field is applied, electrons surrounding a nucleus move in
response to this field, creating their own magnetic fields. The induced magnetic field
produced by these moving electrons can either oppose (shield) or reinforce
(deshield) the external magnetic field experienced by the nucleus.
10.5.2 Factors Affecting Magnetic Shielding
• Electron density: Higher electron density around a nucleus enhances magnetic
shielding, leading to a lower chemical shift.
• Electronegativity: Nuclei adjacent to electronegative atoms tend to be
as these
• Hybridization: The hybridization state of carbon atoms can
sp-hybridized carbons, which have greater s-character, are generally more
deshielded than sp
atoms withdraw electron density, resulting in higher chemical shifts.
influence shieldin
2
or sp3 carbons.
• Steric Effects: The spatial arrangement of atoms and steric hindrance can also
affect electron density and thus the shielding of neighboring nuclei.
deshielded,
g.
10.5.3 Applications of Magnetic Shielding
• Structure elucidation: Analyzing chemical shifts and magnetic shielding effects
helps chemists deduce molecular structures and identify functional groups.
• Molecular dynamics: Changes in shielding can provide insights into molecular
dynamics, conformational changes, and interactions within a molecule.
• Diagnostic a
pplications: I
shielding can help differentiate between various tissues based on their chemical
environments.
Example
of magnetic shielding: In a molecule such as benzene, protons attached to
the carbon atoms experience significant shielding due to the delocalized π-electron
n medical imaging (e.g., MRI), understanding magnetic
452 10 Comprehensive Insights into Nuclear Magnetic Resonance Spectroscopy
cloud. This results in relatively low chemical shifts for the protons, indicating a
highly shielded environment compared to protons adjacent to electronegative atoms
such as chlorine.
10.6 Anisotropic Shielding
Anisotropic shielding is a phenomenon in NMR spectroscopy that refers to the
direction-dependent varia tions in the magnetic shielding of a nucleus due to the
arrangement of surrounding electronic environments. This effect is particularly
significant in systems with nonuniform electron distribution, such as aromatic
compounds and molecules with multiple bonds. Anisotropic shielding occurs
when the shielding constant of a nucleus varies with the direction of the ap plied
magnetic field. This variation leads to different resonance frequencies depending on
the orientation of the molecule relative to the magnetic field.
10.6.1 Mechanism of Anisotropic Shielding
The local magnetic field experienced by a nucleus is influenced by the motion of
nearby electrons, which can create anisotropic magnetic fields around the nucleu s. In
anisotropic systems, such as those with π-bonds or aromatic rings, the electron cloud
distribution is not uniform. As a result, the magnetic environment around the nucleus
can vary in different directions, leading to directional dependence of shielding.
10.6.2 Chemical Shifts and Anisotropic Shielding
Anisotropic shielding contributes to the observed chemical shifts in NMR spectra.
The resonance frequency of a nucleus can change based on its orientation relative to
the applied magnetic field and the arrangement of surrounding electrons. For
instance, in aromatic compounds, protons on the ring may experience different
degrees of shielding depending on their position relative to the π-electron cloud.
10.6.3 Applications of Anisotropic Shielding
• Structure determination: Anisotropic shielding can provide valuable information
about the three-dimensional arrangement of atoms within a molecule. By
analyzing variations in chemical shifts based on molecular orientation, chemists
can infer structural details.
• Stereochemical analysis:
mining stereochemistry, as they can indicate the spatial relationships between
atoms and functional groups.
The effects of anisotropic shielding are useful in deter-
10.7 Isotropic Shielding 453
• Advanced NMR techniques: Techniques such as 2D NMR (e.g., COSY and
NOESY) can exploit anisotropic shielding to obtain information about molecular
dynamics and interactions.
10.6.4 Examples of Anisotropic Shielding
• Aromatic compounds: In benzene, protons on different carbon atoms experience
different degrees of shielding due to the delocalized π-electron cloud. For example, protons ortho to a substituent may be deshielded compared to protons para to
the same substituent.
• Alkenes and alkynes: In alkenes, protons attached to carbon atoms involved in
double bonds may experience anisotropic shieldin g due to the electron-rich
π-bonding. The orientation of the double bond relative to the appli ed magnetic
field can affect the chemical shifts observed for these protons.
10.7 Isotropic Shielding
Isotropic shielding is a concept in NMR spectroscopy that refers to the uniform
magnetic shielding of a nucleus regardless of the orientation of the applied magnetic
field. This phenomenon is characteristic of certain molecular environments where
the electron distribution around the nucleus is symmetrical, leading to consistent
resonance frequencies in all directions. Isotropic shielding occurs when the shielding
constant of a nucleus remains the same regardless of the direction of the external
magnetic field. In this scenario, the effective magnetic field experienced by the
nucleus is consistent in all orientations.
10.7.1 Mechanism of Isotropic Shielding
In systems exhibiting isotropic shielding, the electronic environment surrounding a
nucleus is homogeneously distributed. As a result, the induced magnetic fields from
surrounding electrons do not vary with direction. This uniformity means that the
local magnetic field experienced by the nucleus does not depend on the orientation of
the molecule in the magnetic field.
10.7.2 Chemical Shifts and Isotropic Shielding
The δ observed in NMR spectra for isotropically shielded nuclei reflects the uniform
shielding effect. The resonance frequency of the nucleus will be consistent across
different orientations, resulting in a single, well-defined chemical shift value.
454 10 Comprehensive Insights into Nuclear Magnetic Resonance Spectroscopy
10.7.3 Examples of Isotropic Shielding
• Alkane protons: Protons in saturated hydrocarbons (alkanes) typically exhibit
isotropic shielding. The symmetrical distribution of electron density around these
protons results in similar shielding effects regardless of molecular orientation.
• Tetramethylsilane (TMS): TMS, commonly used as a refere nce standa rd in NMR
spectroscopy, displays isotropic shielding for its protons due to its symmet rical
structure. This property makes it an ideal reference point for chemical shifts.
10.7.4 Applications of Isotropic Shielding
• Simplified interpretation of spectra: The uniformity of isotropic shielding
simplifies the interpretation of NMR spectra, as the chemical shifts are consistent
and do not vary with molecular orientation.
• Reference standards: Isotropic shielding provides a reliable basis for comparing
chemical shifts in different compounds. TMS is a common reference because its
isotropic shielding simplifies calibration.
10.8 Diamagnetic Shielding
Diamagnetic shielding is a specific type of magnetic shielding observed in NMR
spectroscopy, resulting from the presence of electrons that create an induced magnetic field opposing the applied magnetic field when a sample is placed in a magnetic
field. This phenomenon is significant in understanding how electronic environments
influence the chemical shifts of nuclei in NMR spectra. Diamagnetic shielding refers
to the effect whereby electrons in a molecule respond to an external magnetic field
by generating an induced magnetic field that opposes the applied field. This effect
reduces the effective magnetic field experienced by a nucleus, leading to shifts in
resonance frequency.
10.8.1 Mechanism of Diamagnetic Shielding
When an external magnetic field is applied, the electrons surrounding a nucleus
move in response to the field, resulting in the generation of their own magnetic fields.
According to Lenz’s law, the induced magnetic field created by the motion of
electrons opposes the change in the magnetic field, leading to a reduction in the
effective magnetic field at the nucleus. This results in shielding, causing the nucleus
to resonate at a lower frequency than it would without the influence of the electron
cloud.
10.9 Paramagnetic Shielding 455
10.8.2 Chemical Shifts and Diamagnetic Shielding
The presence of diamagnetic shielding affects the observed δ of a nucleus. The
induced magnetic field results in a lower effective magnetic field at the nucleus. The
shielding effect results in a chemical shift that is dependent on the electronic
environment, with more shielded nuclei exhibiting smaller chemical shifts (lower
ppm values) compared to less shielded (or deshielded) nuclei .
10.8.3 Examples of Diamagnetic Shielding
• Aromatic compounds: In aromatic compounds, the delocalized π-electron cloud
contributes to diamagnetic shielding effects. Protons on aromatic rings are often
more shielded due to the presence of these electrons.
• Alkanes: Protons in saturated hydrocarbons (alkanes) typically experience signif-
icant diamagnetic shielding due to the uniform distribution of electron density
around them.
10.8.4 Applications of Diamagnetic Shielding
• Molecular structure determination: Anal yzing chemical shifts resulting from
diamagnetic shielding provides insights into the electronic environments of
specific nuclei wi thin a molecule, aiding in structural determination.
• Functional group
shielding help identify functional groups and understand their influence on the
overall electronic environment.
analysis:
Variations in chemical shifts due to diamagnetic
10.9 Paramagnetic Shielding
Paramagnetic shielding is a phenomenon observed in NMR spectroscopy that occurs
due to the presence of unpaired electrons in a paramagnetic species. This effect can
significantly influence the resonance frequency and chemical shifts of nearby nuclei,
providing insights into molecular structure and dynamics. Paramagnetic shielding
refers to the modification of the magnetic environment experienced by nuclei in the
presence of unpaired electrons. Unlike diamagnetic shielding, where electron pairs
create a uniform magnetic field, paramagnetic shielding involves unpaired electrons
that contribute to a more complex and often stronger magnetic interaction with the
applied field.
456 10 Comprehensive Insights into Nuclear Magnetic Resonance Spectroscopy
10.9.1 Mechanism of Paramagnetic Shielding
When an external magnetic field is applied, unpaired electrons in a paramagnetic
species (such as free radicals or transition metal complexes) align with the magnetic
field. This alignment creates an additional magnetic field that interacts with nearby
nuclei. The effective magnetic field experienced by these nuclei is altered by the
presence of unpaired electrons, resulting in a deshielding effect. This means that the
chemical shifts of nearby nuclei are generally higher (downfield) compared to those
in diamagnetic environments.
10.9.2 Chemical Shifts and Paramagnetic Shielding
The resonance frequency of a nucleus in the vicinity of unpaired electrons will be
influenced by the magnetic fields generated by these electrons. In the presence of
unpaired electrons, the effective magnetic field experienced by the nuclei decreases,
leading to higher chemical shifts (more deshielded) compared to what would be
observed in a purely diamagnetic environment.
10.9.3 Examples of Paramagnetic Shielding
• Free radicals: In free radical compounds (e.g., alkyl radicals), the unpaired
electrons contribute to significant paramagnetic shielding effects, causing nearby
nuclei to exhibit pronounced chemical shifts.
• Transiti on metal complexes: Paramagnetic transition metal complexes often
exhibit unique NMR spectra due to the influence of unpaired d-electrons,
resulting in altered chemical shifts for nuclei within the complex or in close
proximity.
10.9.4 Applications of Paramagnetic Shielding
• Structura l analysis: Analyzing the chemical shifts influenced by paramagnetic
shielding helps chemists deduce structural information about paramagnetic species and their interactions with surrounding nuclei.
• Dynamic studies: Param
dynamics and interactions, as changes in the electronic environment can lead to
variations in chemical shifts.
agnetic shiel
ding can provide insights into molecular
10.9.5 Comparison with Other Shielding Types
• Paramagnetic vs. diamagnetic shielding: Paramagnetic shielding occurs due to
unpaired electrons, leading to deshielding and higher chemical shifts, while
10.10 Intensities of Resonance Signals 457
diamagnetic shielding is caused by paired electrons, resulting in shielding and
lower chemical shifts.
• Anisotropic vs. isotropic shielding: Paramagnetic shielding is often anisotropic,
meaning that the chemical shift may vary depending on the orientation of the
molecule in the magnetic field. In contrast, isotropic shielding provides consi stent
chemical shifts regardless of orientation.
10.10 Intensities of Resonance Signals
The intensity of resonance signals in NMR spectroscopy is an important aspect that
provides valuable infor mation about the number of nuclei contributing to a specific
signal, the molecular environment, and the dynamics of the sample. The intensity of
a resonance signal in an NMR spectrum is proportional to the number of identical
nuclei contributing to that signal. Each signal corresponds to a specific chemical
environment, and its intensity reflects the abundance of those nuclei within the
molecule.
10.10.1 Factors Influencing Signal Intensities
Several factors affect the intensity of resonance signals in NMR, which are provided
in the following sections.
10.10.1.1 Number of Nuclei
The most direct influence on signal intensity is the number of equivalent nuclei
contributing to the signal. For example, if a molecule has four equivalent protons, the
corresponding resonance signal will be four times more intense than that of a single
proton.
10.10.1.2 Relaxation Processes
Relaxation processes, including T1 and T2 relaxation, can impact signal intensity.
Longer relaxation times can lead to increased signal intensity, as they allow more
time for the spins to return to equilibrium before the next pulse is applied.
• T1 relaxation: Longer T1 values allow for greater signal intensity, as the spins can
return to a higher energy state before being excited again.
• T2 relaxation:
to the loss of phase coherence among spins.
r T2 values can lead to broader and less intense signals due
Shorte
10.10.1.3 Concentration of the Sample
The concentration of the sample plays a significant role in determining signal
intensity. Higher concentrations generally lead to stronger signals due to the
increased number of contributing nuclei.
458 10 Comprehensive Insights into Nuclear Magnetic Resonance Spectroscopy
10.10.1.4 Experimental Conditions
Parameters such as the strength of the magnetic field, temperature, and pulse
sequences can affect signal intensities. For instance, using a higher magnetic field
strength can improve sensitivity and signal intensity.
10.10.2 Integration of Signals
In NMR spectra, the area under each resonance peak is proportional to the number of
nuclei contributing to that signal. Integration is a technique used to quantify the
relative number of equivalent nuclei. By integrating the area of each signal, chemists
can determine the relative ratios of different types of nuclei in the sample, providing
insights into molecular composition and structure.
10.10.3 Applications of Signal Intensity Analysis
• Quantitative analysis: Signal intensities can be used for quantitative analysis,
allowing researchers to determine concentrations of components in mixtures or
the composition of complex mixtures.
• Structura l elucidation: Analyzing the relative intensities of signals aids in deduc-
ing molecular structure and confirming the presence of specific functional groups.
10.10.4 Types of Signal Intensities
10.10.4.1 1 H NMR
1
H NMR spectroscopy is a powerful analytical technique used to determine the
structure of organic compounds by examining the magnetic properties of hydroge n
nuclei (protons) in a magnetic field. It provides detailed information about the
chemical environment of protons in a molecule, enabling researchers to deduce
structural and functional characteristics.
10.10.4.1.1 Basic Principle
• Nuclear magnetic resonance: When placed in an external magnetic field, certain
nuclei (such as protons) can absorb radiofrequency energy and transition between
different spin states. The frequency at which this occurs is determined by the
strength of the magnetic field and the electronic environment surrounding the
nucleus.
•
Chemical shift: The
to a reference compound, usually TMS. The chemical shift reflects the electronic
environment around the protons and provides insight into the molecular structure.
position of the resonance signal is measured in ppm relative
10.10 Intensities of Resonance Signals 459
10.10.4.1.2 Chemical Shift Ranges
The chemical shifts for protons typically fall within the range of 0–12 ppm. Here are
some common chemical shift values for various functional groups:
• Alkyl protons (–CH₃, –CH₂): 0.5–2.5 ppm
• Allylic protons: 1.5–3.0 ppm
• Protons adjacent to electronegative atoms (–OH, –NH): 0.5–5.0 ppm
• Aromatic protons: 6.0–8.5 ppm
• Vinyl protons (–CH=CH₂): 4.5–6.5 ppm
10.10.4.1.3 Signal Splitting (Spin–Spin Coupling)
The presence of neighboring protons can cause splitting of resonance signals, known
as spin–spin coupling. The number of peaks in a signal corresponds to the number of
neighboring protons ( n ) plus one (n + 1).
• Singlet: No neighboring protons (e.g., –CH₃ in TMS)
• Doublet : One neighboring proton (e.g., –CH₃ next to –CH₂)
• Triplet: Two neighboring protons (e.g., –CH₂ next to –CH₃)
• Multiplet: More complex splitting patterns due to multiple neighboring protons.
10.10.4.1.4 Integration
of Signal
s
The area under each signal in the NMR spectrum is proportional to the number of
equivalent protons contributing to that signal. This integration allows researchers to
determine the relative ratios of different types of proto ns in the molecule.
10.10.4.1.5 Applications of Proton NMR
• Structura l elucidation:
1
H NMR is widely used to identify and characterize
organic compounds by providing information about the a rrangement of atoms
and functional groups.
• Quantitative analysis: By measuring the integration of signals, researchers can
quantify the concentration of different components in a mixture.
• Dynamics and interactions: Proton NMR can provide insights into molecular
dynamics, conformational changes, and interactions between different parts of a
molecule or between different molecules.
10.10.4.1.6 Limitations
• Overlapp ing signals: In complex molecules or mixtures, overlapping signals can
complicate interpretation.
• Sensitivity to environment: Proton NMR is sensitive to environmental changes,
such as pH or solvent effects, which can influence chemical shifts and signal
intensities.
10.10.4.1.7 Example of Proton NMR Analysis
1
H NMR spectrum of ethanol (C₂H₅OH):
In the
460 10 Comprehensive Insights into Nuclear Magnetic Resonance Spectroscopy
• The methyl protons (–CH₃) appear as a triplet due to coupling with the adjacent
methylene protons (–CH₂), with a chemical shift around 1.2 ppm.
• The methylene protons (–CH₂) appear as a quartet due to coupling with the three
methyl protons, with a chemical shift around 3.7 ppm.
• The hydroxyl proton (–OH) appears as a singlet, typically around 1–5 ppm,
depending on the hydrogen bonding environment.
10.10.4.2
13
C NMR spectroscopy is a vital analytical technique used to study the structure and
dynamics of organic compounds by analyzing the magnetic proper ties of carbon- 13
nuclei in a magnetic field. While
13
C NMR
1
H NMR focuses on hydrogen atoms,
13
C NMR
provides valuable insights into the carbon framework of molecules. In carbon-13
NMR, the intensity of a signal corresponding to a carbon atom is also proportional to
the number of identical carbon atoms in the molecular structure .
10.10.4.2.1 Basic Principle
• Nuclear magnetic resonance: Like proton NMR,
principle that certain nuclei (
13
C) can absorb radiofrequency energy when placed
13
C NMR operates on the
in a magnetic field, transitioning between different energy levels.
• Chemical shift: The position of the resonance signal is measured in ppm relative
to a reference compound, usually
The chemical shift reflects the electronic
TMS.
environment around the carbon atom and provides insights into the molecular
structure.
10.10.4.2.2 Chemical Shift Ranges
The chemical shifts for carbon atoms typically fall within the range of 0–220 ppm.
Here are some common chemical shift values for various functional groups:
3
• Aliphatic carbons (e.g., sp
• Alkenes (sp
• Aromatics (sp
2
hybridized): 100–150 ppm
2
hybridized): 110–160 ppm
hybridized): 0–50 ppm
• Alkynes (sp hybridized): 60–100 ppm
• Carbonyl s (C=O): 150–220 ppm (e.g., aldehydes, ketones, esters)
10.10.4.2.3 Signal Multiplicity
13
C NMR, carbon signals may appear as singlets, doublets, triplets, or multiplets
In
based on the number of neighboring protons (n) and their coupling:
• Singlet: No adjacent protons (e.g., a quaternary carbon)
• Doublet : One neighboring proton
• Triplet: Two neighboring protons
• Multiplet: M
However,
coupling with protons compared to protons in
ore complex s
carbon signals are often observed as singlets due to relatively weak
plitting due to multiple adjacent protons
1
H NMR.