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10.10 Intensities of Resonance Signals 461
10.10.4.2.4 Integration of Signals
Unlike proton NMR, direct integration of carbon signals to determine the number of
equivalent carbon atoms is less common. Instead, relative intensities of signals are
often used to infer the number of equivalent carbon environments in the molecule
10.2).
(Fig.
10.10.4.2.5 Decoupling Techniques
Broadband decoupling technique is commonly used in
13
C NMR to simplify spectra
by removing proton-coupling effects. In this method, all proton signals are
decoupled, allowing carbon signals to appear as singlets, which makes interpretation
easier.
13
10.10.4.2.6 Applications of
• Structura l elucidation:
C NMR
13
C NMR is crucial for identifying and characterizing the
carbon skeleton of organic compounds, helping resear chers deduce molecular
structures.
• Quantitative analysis: While direct integration is less common,
13
C NMR can still
be used for quantitative analyses of carbon-containing compounds in mixtures.
• Functional group
identification:
By analyzing chemical shif ts, researchers can
identify functional groups and infer their effects on the surrounding carbon atoms.
Fig. 10.2 Schematic representation of the
illustrates the
the carbon atoms in the molecule. The labeled molecular structure at the top correlates with the
corresponding peaks in the spectrum: C1 (green arrow): The carbonyl carbon (C=O) of the ester
group, which appears at a chemical shift of approximately 175–180 ppm, indicating a deshielded
environment due to the electronegative oxygen atoms. C2 (red arrow): The methoxy carbon (OCH₃)
attached to the ester oxygen shows a peak around 50–60 ppm. C3 (blue arrow): The methylene
carbon (CH₂) adjacent to the ester carbonyl group resonates between 30 and 40 ppm. C4 (pink
arrow): The terminal methyl group (CH₃) attached to the methylene group appears at a lower
chemical shift, around 10–20 ppm, due to a relatively shielded environment. The peak at 0 ppm
correspon
the spectrum
13
C NMR spectrum of methyl propionate, highlighting the chemical shift positions for
ds
to the internal reference standard, TMS (tetramethylsilane), which is used to calibrate
13
C NMR spectrum of methyl propionate. The figure
462 10 Comprehensive Insights into Nuclear Magnetic Resonance Spectroscopy
10.10.4.2.7 Limitations
• Sensitivity:
natural abundance of
13
C NMR is generally less sensitive than 1 H NMR due to the lower
13
C (about 1.1% of carbon) compared to 1 H. This requires
higher sample concentrations or longer acquisition times.
• Overlapp ing signals: In complex molecules, overlapping signals can complicate
the interpretation of spectra.
10.10.4.2.8 Example of Carbon-13 NMR Analysis
13
In the
C NMR spectrum of acetic acid (CH₃COOH):
• The methyl carbon (–CH₃) appears as a signal around 20 ppm.
• The carbonyl carbon (C=O) appears as a signal around 175 ppm.
• The carboxyl carbon (–COOH) might show up at a similar chemical shift as the
carbonyl, depending on the environment.
10.11 Types of NMR Spectroscopy
NMR spectroscopy encompasses a variety of techniques used to study the nuclear
magnetic properties of atomic nuclei. Different types of NMR spectroscopy are
employed for various applications, each offering unique insights into different
aspects of molecules and materials. The following are some common types of
NMR spectroscopy.
10.11.1 1D NMR Spectroscopy
1D NMR spectroscopy is one of the most fundamental and widely used techniques
in NMR analysis. It provides detailed information about the chemical environment
of nuclei (such as protons or carbons) within a molecule by analyzing how they
interact with an applied magnetic field. In 1D NMR spectroscopy, the data is
presented as a plot of signal intensity versus chemical shift in parts per million
(ppm), with each peak corresponding to a specific nucleus within the molecule.
10.11.1.1 Basic Principles of 1D NMR
• Resonance: In 1D NMR, nuclei such as 1 H or
pulse in the presence of a strong external magnetic field. This causes the nuclei to
absorb energy and resonate at a specific frequency.
• Chemical shift:
The position
of the NMR signal (measured in ppm) is called the
chemical shift, which is influenced by the electronic environment of the nucleus.
Each distinct chemical environment in the molecule gives rise to a unique
chemical shift.
• Signal splitting (spin–spin coupling): The interaction between nearby (nonequiv-
alent) nuclei leads to signal splitting, also known as spin–spin coupling. This
13
C are excited by a radiofrequency
10.11 Types of NMR Spectroscopy 463
interaction results in multiple peaks for a single signal, providing information
about the number of neighboring nuclei.
• Integration: The area under each peak in the spectrum is proportional to the
number of equivalent nuclei (e.g., protons or
carbons)
contributing to that signal.
10.11.1.2 Types of Nuclei Analyzed in 1D NMR
1D NMR experiments can be conducted on various nuclei, with the most common
being:
1
•
H NMR: Analyzes hydrogen atoms and is the most widely used NMR technique
for studying organic compounds.
13
C NMR: Analyzes carbon atoms, providing insight into the carbon backbone of
•
organic molecules.
• Other nuclei: NMR can also be used to study less common nuclei such
(phosphorus),
19
F (fluorine), and 2 H (deuterium), depending on the sample.
as
31
10.11.1.3 Key Features of 1D NMR Spectroscopy
10.11.1.3.1 Chemical Shift
The position of an NMR peak is influenced by the electronic environment
surrounding the nucleus. For example:
1
•
H NMR: Proton chemical shifts range from 0 to 12 ppm, with aliphatic protons
appearing around 0.5–2 ppm and aromatic protons appearing around 6–8 ppm.
13
C NMR: Carbon chemical shifts range from 0 to 220 ppm, with aliphatic
•
carbons around 0–50 ppm and carbonyl carbons around 150–220 ppm.
P
10.11.1.3.2 Signal Multiplicity (Spin–Spin Coupling)
Signal splitting occurs when a nucleus interacts with neighboring nuclei. For
example, in
1
H NMR, a proton with one neighboring proton gives a doublet (splits
into two peaks). A proton with two neighboring protons gives a triplet (splits into
three peaks) . This coupling pattern helps identify how protons are arranged within
the molecule.
10.11.1.3.3 Integration
In 1D NMR, the integration of the peaks (area under each peak) corresponds to the
relative number of equivalent nuclei (e.g., protons or carbons). This is particularly
useful in proton NMR for determining the ratio of different types of hydrogen atoms
in the molecule.
10.11.1.4 Common Experiments in 1D NMR
•1 H NMR: Provides information about the hydrogen atoms in the molecule. It is
highly sensitive and gives insights into the local environment, including chemical
shifts, coupling constants, and signal integration.
464 10 Comprehensive Insights into Nuclear Magnetic Resonance Spectroscopy
13
•
C NMR: Focuses on the carbon atoms in the molecule. While less sensitive than
1
H NMR due to the low natural abundance of
tion about the carbon skeleton of the molecule.
• Decoupled
13
C NMR: Proton-decoupling is often used in
13
C, it provides detailed informa-
13
C NMR to remove the
coupling between carbon and hydrogen atoms, simplifying the spectrum and
allowing all carbon signals to appear as singlets.
10.11.1.5 Applications of 1D NMR
• Structura l elucidation : 1D NMR spectroscopy is widely used to determine the
structure of organic molecules by providing information about the chemical
environment, connectivity, and number of nuclei in the molecule.
• Quantitative analysis: 1D NMR can be used for quantification by analyzing the
integration of peaks, especially in
1
H NMR, to determine the ratios of different
proton types in a sample.
• Quality control: 1D NMR is commonly used in industries such as
pharmaceuticals and chemicals for quality control by comparing the spectra of
samples with reference standards.
10.11.1.6 Limitations of 1D NMR
• Complex molecules: In very complex molecules, 1D NMR spectra can be chal-
lenging to interpret due to overlapping signals and complicated coupling patterns.
• Low sensitivity: Nuclei such as
NMR less sensitive compared to
13
C have low natural abundance, making
1
H NMR. This often requires higher sample
13
concentrations or longer acquisition times.
C
10.11.1.7 Example of 1D NMR Analysis
Ethanol (CH₃CH₂OH): In 1 H NMR, the –CH₃ produce a triplet around 1.2 ppm,
indicating coupling with the adjacent –CH₂ group. The methylene protons (–CH₂)
produce a quartet around 3.7 ppm, indicating coupling with the –CH₃ group. The
hydroxyl proton (–OH) often appears as a singlet, typically around 1–5 ppm,
depending on the solvent and hydrogen bonding. In
13
C NMR, the carbon in the
methyl group (–CH₃) appears around 20 ppm. The carbon in the methylene group (–
CH₂) appears around 60 ppm.
10.11.2 2D NMR Spectroscopy
2D NMR spectroscopy is an advanced form of NMR that provides much more
detailed information about molecular structure compared to 1D NMR. In 2D NMR,
data is presented in two dimensions, typically plotting frequency against frequency.
The key advantage of 2D NMR is its ability to reveal correlations between nuclei,
allowing for the identification of atom-to-atom connectivity within a molecule. This
makes it an essential tool for the structural elucidation of complex molecules,
including large biomolecules such as proteins and nucleic acids.
10.11 Types of NMR Spectroscopy 465
10.11.2.1 Principle of 2D NMR
In 2D NMR spectroscopy, the experiment is performed in two phases:
• Preparation phase: A series of pulses is applied to the sample to manipulate the
nuclear spins.
• Evolution and detection phases: During these
phases, the
spins evolve under the
influence of magnetic interactions, and the resulting signals are detected and
recorded. The data is then processed and represented as a 2D plot where
interactions between nuclei are visualized.
The spectrum shows two frequency axes, one for each interacting nucleus, which
helps determine correlations between different nuclei.
10.11.2.2 Types of 2D NMR Spectroscopy
Several types of 2D NMR experiments exist, with each one designed to provide
specific structural information. Some of the most commonly used techniques are
provided in the following sections.
10.11.2.2.1 COSY
• Purpose: COSY is used to detect correlations between
1
H that are coupled
through one or two bonds (through-bond interactions).
• Application: COSY helps determine the connectivity of protons in a molecule,
revealing which protons are coupled to each other via J-coupling.
• Example: In a simple molecule such as ethanol (CH₃CH₂OH), COSY can show
correlations between the methylene protons (CH₂) and the methyl protons (CH₃),
indicating that they are close in the structure and are coupled through two bonds.
10.11.2.2.2 Heteronuclear Single Quantum Coherence (HSQC)
• Purpose: HSQC is used to correlate
1
H with heteronuclei such as
13
C or
15
N,
which are directly bonded to them (thr ough-bond interactions).
• Application: This technique is wi
protons and carbons in organic compounds and biomolecules. HSQC is particularly useful in
13
C and
15
N NMR because it enhances sensitivity by observing
dely used
to map the connectivity between
proton signals.
• Example: In a small organic molecule, HSQC can be used to correlate proton
signals with the carbon atoms they are attached to, giving detailed information
about the carbon framework.
10.11.2.2.3 Heteronuclear Multiple Bond Correlation (HMBC)
• Purpose: HMBC detects correlations between protons and heteronuclei (such as
13
C or
15
N) that are separated by two, three, or even four bonds (long-range
couplings).
• Application: HMBC
is crucial for identifying relationships between atoms that are
not directly bonded, providing connectivity across larger distances in the
molecule.
466 10 Comprehensive Insights into Nuclear Magnetic Resonance Spectroscopy
• Example: In complex organic molecules or natural products, HMBC can identify
relationships between carbons and protons across multiple bonds, helping deduce
longer-range structural features.
10.11.2.2.4 Nuclear Overhauser Effect Spectroscopy (NOESY)
• Purpose: NOESY is used to detect correlations between protons that are spatially
close to each other (through-space interactions) rather than coupled through
bonds.
• Application: This technique is essential for determining the 3D structure of
molecules because it reveals interactions between protons that are near each
other in space but may not be bonded.
• Example: NOESY is widely used in biomolecular NMR to determine the 3D
structures of proteins and nucleic acids by identifying spatial proximity between
hydrogen atoms.
10.11.2.2.5 Total Correlation Spectroscopy (TOCSY)
• Purpose: TOCSY detects all protons in a spin system that are connected through a
chain of coupling interactions, providing a complete picture of the connectivity in
a molecular fragment.
• Application: TOCSY is useful in identifying coupled protons within the same
spin system, such as in amino acids or sugar molecules.
• Example: In sugar molecules, TOCSY can trace the connectivity of all the protons
in a single sugar unit, helping in the structural analysis of carbohydrates.
10.11.2.3 Key Features of 2D NMR
• Diagonal peaks: These peaks appear along the diagonal of the 2D spectrum and
correspond to the signa ls that would appear in a standard 1D NMR experiment for
each nucleus.
• Cross-peaks : These peaks are the most
informative
part of a 2D NMR spectrum
and represent correlations between different nuclei. Cross-peaks provide information about either through-bond or through-space interactions, depending on the
type of 2D NMR experiment.
• Coupling patterns: 2D NMR spectra reveal complex coupling patterns that are
not always obvious in 1D NMR. These patterns help determine which nuclei are
interacting, either through direct bonding (J-coupling) or NOE.
10.11.2.4 Applications of 2D NMR
• Structura l elucidation: 2D NMR is widely used to determine the complete
structure of organic compounds, including complex natural products,
pharmaceuticals, and biomolecules such as proteins and nucleic acids.
• Conformational analys
is: NOESY and other 2D techniques are used to study the
3D conformations of molecules, especially in the case of biomolecules such as
proteins and peptides.
10.11 Types of NMR Spectroscopy 467
• Chemical and pharmaceutical research: 2D NMR is an essential tool for
identifying the structures of novel compounds, studying molecular interactions,
and performing quality control in the pharmaceutical industry.
• Metabolomics: In metabolomics, 2D NMR is used to analyze complex mixtures
of metabolites in biological samples, helping to identify molecular markers for
diseases or metabolic processes.
10.11.2.5 Advantages of 2D NMR
• Detailed structural information: 2D NMR provides much richer information
about molecular structure than 1D NMR, revealing how different nuclei are
connected or spatially related.
• Complex molecules: It is particularly useful for analyzing complex molecules
where 1D spectra may be too crowded or complicated to interpret.
• Through-space interactions: NOESY and related techniques allow for the study
of spatial proximity between atoms, which is essential for understanding the 3D
structure of molecules.
10.11.2.6 Limitations of 2D NMR
• Time-consuming: 2D NMR experiments generally take longer to acquire than 1D
NMR, especially for large or complex molecules.
• Sensitivity: While 2D NMR provides detailed information, it often requires higher
sample concentrations compared to 1D NMR due to lower sensitivity.
• Complexity of data analysis: The interpretation of 2D NMR spectra can be more
challenging and requires expertise, particularly when dealing with complex
molecules with many interacting nuclei.
10.11.2.7 Example of 2D NMR Analysis
• Ethanol (CH₃CH₂OH): In a COSY spectrum of ethanol, cross-peaks would be
observed between the methylene protons (CH₂) and the methyl protons (CH₃),
indicating J-coupling between these two groups.
10.11.3 3D and 4D NMR Spectroscopy
As NMR techniques have evolved, 3D and 4D NMR spectroscop y have been
developed to address the complexity of larger biomolecules such as proteins and
nucleic acids. These higher-dimensional techniques allow researchers to resolve
crowded NMR spectra and study intricate molecular structures with greater precision. In 3D and 4D NMR, additional dimensions represen t different types of
interactions, p roviding more comprehensive information on molecular structure
and dynamics.
10.11.3.1 3D NMR Spectroscopy
3D NMR spectroscopy extends the principles of 2D NMR by introducing a third
dimension. This extra dimension helps resolve overlapping signals that occur in 2D
468 10 Comprehensive Insights into Nuclear Magnetic Resonance Spectroscopy
spectra, making it particularly useful for studying large biomolecules such as
proteins, peptides, and nucleic acids.
10.11.3.1.1 Principle of 3D NMR
In 3D NMR, a third frequency axis is added by using a series of pulse sequences that
correlate interactions between multiple nuclei across three different dimensions. The
most common application of 3D NMR is in protein structure determination, where
each dimension often corresponds to a different type of nucleus or interaction (e.g.,
proton, nitrogen, and carbon correlations).
1
• First dimension: Typically represents the chemical shifts of protons (
• Second dimension: Often corresponds to chemical shifts of heteronuclei, such as
15
N or 13C.
H).
• Third dimension: Correlates either spatial interactions or further heteronuclear
couplings, adding depth to the analysis.
10.11.3.1.2 Key Techniques in 3D NMR
• HNCO: Involves correlations between amide protons (
the adjacent carbonyl carbon (
13
C) of the protein backbone. It provides informa-
1
H), nitrogen-15 (
15
N), and
tion on sequential connectivity between amino acids.
• NOESY-HSQC: NOESY with HSQC to identify spatial proximity between
protons while correlating them with heteronuclei (
13
C or
15
N). This is crucial
for 3D structure determin ation.
10.11.3.1.3 Applications of 3D NMR
• Protein structure elucidation: 3D NMR is essential for studying large proteins
and peptides, where 2D NMR spectra are often too complex due to signal overlap.
It helps in determining which amino acids are connected and how they are
arranged in space.
• Molecular dynamics: 3D NMR
can also provi
de information about molecular
motions and conformational changes in biomolecules.
• Nucleic acid studies: 3D NMR techniques are also applied for the study of RNA
and DNA structures, giving insights into their folding and interactions with
proteins or ligands.
10.11.3.2 4D NMR Spectroscopy
4D NMR spectroscopy adds yet another dimension to further resolve highly
congested spectra, making it possible to analyze even larger biomolecules with
greater accuracy.
10.11.3
NMR, a fourth frequency dimension is introduced, often involving multiple
In 4D
rinciple of 4D NMR
.2.1 P
types of nuclei or multiple types of interactions simultaneously. The added
dimensions help to separate overlapping peaks and distingu ish between signals
10.11 Types of NMR Spectroscopy 469
that are very close in frequency, which is critical when studying larger proteins or
complexes.
• First and second dimensions: Often represent
bond/through-space interactions.
• Third dimension: Typically involves a heteronucleus such as
1
H chemical shifts or through-
15
N or 13C.
• Fourth dimension: Involves additional correlations such as interacti ons between
heteronuclei (e.g., carbon–carbon or carbon–nitrogen couplings).
10.11.3.2.2 Key Techniques in 4D NMR
• 4D NOESY-HSQC: Combines through-space NOE interactions with
heteronuclear correlation, adding an extra dimension of separation between
protons and heteronuclei such as
13
C and
15
N. This technique is particularly
useful in studying large proteins where 3D NMR spectra become crowded.
• 4D HNCOCA: Provides correlations between amide protons, nitrogen, and two
adjacent carbon atoms (Cα and carbonyl carbon) along the protein backbone,
offering detailed sequential connectivity information.
10.11.3.2.3 Applications of 4D NMR
• Large protein structure determination: 4D NMR is primarily used for determin-
ing the structures of large proteins (e.g., >30 kDa) that cannot be fully resolved
with 2D or 3D NMR. This is especially useful in structural biology, where
proteins or protein–ligand complexes are too large for lower-dimensional NMR
methods.
• Studying protein–ligand interactions: 4D NMR is used to study how proteins
interact with other molecules, such as drugs, inhibitors, or nucleic acids,
providing detailed insights into binding sites and molecular dynamics.
• High-re solution analysis: The extra dimensions in 4D NMR enable very fine
resolution of complex spectra, helping to distinguish between nuclei that are
difficult to resolve using lower-dimensional methods.
10.11.3.3 Advantages of 3D and 4D NMR
• Resolution of crowded spectra: By adding extra dimensions, 3D and 4D NMR
allow for the resolution of overlapping signals that are often seen in 1D or 2D
spectra, particularly in large molecules.
• Detailed structural information: These techniques provide a wealth of informa-
tion about molecular structure, including through-bond and through-space
interactions, which is crucial for understanding large biomolecules.
• 3D structure eluci dation: Particularly for proteins, peptides, and nucleic acids, 3D
and 4D NMR are invaluable tools for determining the 3D arrangement of atoms
and the folding patterns of these molecules.
10.11.3.4 Limitations of 3D and 4D NMR
• Complexity: The data generated by 3D and 4D NMR are highly complex and
require advanced software and significant expertise to interpret.
470 10 Comprehensive Insights into Nuclear Magnetic Resonance Spectroscopy
• Time and resource intensive: 3D and 4D NMR experiments take considerably
longer to acquire and process compared to 1D or 2D NMR, and they require high
sample concentrations.
• Sensitivity: Higher-dimensional NMR techniques often have lower sensitivity,
particularly for heteronuclei such as
13
15
C or
N, requiring longer acquisition times
or more concentrated samp les.
10.11.3.5 Example of 3D and 4D NMR Applications in Protein Analysis
In the study of a large protein, 3D NMR can be used to assign individual resonances
to specific amino acids and to determine the connectivity of the protein backbone
(e.g., via HNCO or NOESY-HSQC experiments). 4D NMR would then be
employed to resolve ambiguities in the assignment, provi ding clear, high-resolution
data for regions of the protein that may involve complex interactions or overlapping
signals in 3D spectra.
10.11.4 Solid-State NMR Spectroscopy
Solid-state NMR spectroscopy (SSNMR) is a powerful technique used to study
materials that are in solid form, as opposed to the liquid-phase samp les typically
analyzed by solution-state NMR. Solid-state NMR is particularly valuable for
investigating the molecular structure, dynamics, and interactions of materials such
as polymers, biomolecules, inorganic compounds, and pharmaceuticals in their
native solid state.
10.11.4.1 Principle of Solid-State NMR
In solid materials, molecular motions are much more restricted compared to liquids,
leading to broad and complex NMR signals due to interactions such as dipolar
couplings, chemical shift anisotropy (CSA), and quadrupolar interactions. These
interactions cause large inhomogeneous line broadening in the NMR spectra of
solids. To overcome this, several specialized techniques are used in solid-state
NMR, including:
• Magic angle spinning (MAS): By rapidly spinning the sample at a specific angle
(54.74°) relative to the magnetic field, line broadening caused by anisotropic
interactions can be averaged out, resulting in sharper NMR signals.
• Cross-polarization
(such as
1
H.
13
C,
• High-power decoupling : Hel
(CP): Use
15
N) by transferring polarization from more abundant nuclei such as
d to enhance the sensitivity of low-sensitivity nuclei
ps to suppress dipolar couplings between nuclei,
improving resolution and allowing the observation of clearer spectra.