NMR Spectrum Discussion Examples | How to Interpret Chemical Shifts, Integration Ratios, and Splitting

What Is an NMR Spectrum?

NMR is an analytical method that uses nuclear magnetic resonance.
Certain atomic nuclei placed in a magnetic field absorb radio waves and change their energy states.
Because the position of this absorption changes depending on the electronic environment surrounding the nucleus, it is possible to investigate the chemical environments of atoms within a molecule.

¹H NMR and ¹³C NMR are commonly used in organic chemistry.
¹H NMR makes it relatively easy to determine the types, numbers, and neighboring relationships of hydrogen atoms and is therefore extremely useful for structural estimation.
¹³C NMR can be used to confirm the carbon skeleton and carbon environments around functional groups.

Main Items to Include in the Results

In NMR spectrum results, organize the nucleus measured, measurement solvent, reference substance, chemical shifts of the major signals, integration ratios, splitting patterns, coupling constants, and assigned hydrogens or carbons.
In synthesis experiments, compare the spectra of the starting material and product and confirm whether signals originating from the starting material have disappeared and whether signals corresponding to the product have appeared.

Main Items to Include in the Results

• Nucleus measured
• Measurement solvent
• Reference substance
• Chemical shift
• Integration ratio
• Splitting pattern
• Coupling constant
• Peak assignment
• Solvent peak
• Water peak
• Impurity peaks
• Comparison with the starting-material spectrum
• Comparison with the product spectrum
• Comparison with literature values
• Correspondence with the structural formula
• Sources of error and points for improvement

Reference Experimental Values and Analysis Examples for NMR Spectra

Here, reference experimental values for estimating the structures of organic compounds using chemical shifts, integration values, splitting patterns, and coupling constants obtained from ¹H NMR spectra are organized.

In an NMR spectrum, the chemical shift changes depending on the chemical environment of the hydrogen atom.
The ratio of the numbers of hydrogens can be read from the integration values, and the number of neighboring hydrogens can be estimated from the splitting pattern.
By combining this information, partial structures and the presence of functional groups in a molecule can be discussed.

Reference Experimental Conditions

Main Items Examined in NMR Analysis

Approximate Chemical-Shift Ranges

In ¹H NMR, peak positions change depending on the environment surrounding the hydrogen.
Representative approximate chemical-shift ranges are shown below.

Basics of Splitting Patterns

When there are n equivalent hydrogens on an adjacent carbon, a peak is approximately split into n+1 lines.

Example 1H NMR Measurement of Ethanol

Reference ¹H NMR data for ethanol, CH3CH2OH, are shown below.

The integration ratio is 3:2:1, corresponding to the numbers of hydrogens in the CH3, CH2, and OH groups of ethanol.
Because CH3 and CH2 split each other, a combination of a triplet and a quartet is observed.

Example of Determining the Number of Hydrogens From Integration Ratios

NMR integration values represent the ratio of the numbers of hydrogens corresponding to each peak.
For example, if the integration values for a compound are 3.00, 2.00, and 1.00, the ratio is 3:2:1.

However, integration values represent ratios rather than absolute numbers of hydrogens, so they are judged in combination with the molecular formula and other analytical results.

Example 1H NMR Measurement of Ethyl Acetate

A reference spectrum for ethyl acetate, CH3COOCH2CH3, is shown below.

The triplet at 1.26 ppm and quartet at 4.12 ppm are a characteristic combination for an ethyl group.
The singlet at 2.05 ppm corresponds to the CH3 of an acetyl group with no neighboring hydrogens.

How to Recognize an Ethyl Group

In an ethyl group, CH3CH2-, the CH3 tends to appear as a triplet and the CH2 as a quartet, with an integration ratio of 3:2.

Example 1H NMR Measurement of Ethylbenzene

Reference data for ethylbenzene, C6H5CH2CH3, are shown below.

Because an aromatic peak corresponding to 5H is present near 7 ppm, a monosubstituted benzene ring is considered to be present.
In addition, the triplet at 1.23 ppm and quartet at 2.65 ppm suggest an ethyl group attached to the aromatic ring.

Example Calculation of Coupling Constant J

The coupling constant J can be determined from the spacing between split peaks.
Consider a 400 MHz NMR spectrum in which the spacing between adjacent peaks of a triplet is 0.018 ppm.

J (Hz) = Peak spacing (ppm) × Measurement frequency (MHz)

J = 0.018 × 400 = 7.2 Hz

If the coupling constants of the CH3 and CH2 groups in an ethyl group are both approximately 7 Hz, it becomes easier to associate the peaks as originating from the same neighboring relationship.

Examples of Coupling-Constant Comparisons

Example 1H NMR Measurement of Unknown Sample X

Suppose the following peaks were obtained in the ¹H NMR spectrum of unknown sample X.

This spectrum contains a 3H triplet and a 2H quartet, suggesting the presence of an ethyl group.
In addition, the 3H singlet at 2.05 ppm corresponds to a methyl group adjacent to a carbonyl, such as CH3CO-.
Combining these findings suggests that unknown sample X is highly likely to be ethyl acetate.

Comparison of Unknown Sample X With Candidate Compounds

Unknown sample X most closely matches ethyl acetate in the combination of integration ratio, splitting, and chemical shift.

Confirmation of an OH Peak by Addition of Heavy Water

Exchangeable hydrogens such as OH and NH may disappear or become weaker when D2O is added.

A peak that disappears after addition of D2O is highly likely to be an exchangeable hydrogen such as OH or NH.

Examples of Solvent Peaks and Impurity Peaks

In NMR, solvent peaks and impurity peaks that do not originate from the sample may also be observed.
Care must be taken not to mistake these for peaks of the target compound.

For example, when CDCl3 is used as the solvent, care must be taken not to mistake the peak near 7.26 ppm for an aromatic hydrogen.

Example of Peak Overlap

When multiple hydrogens have similar chemical shifts, their peaks may appear to overlap.
In such cases, interpretation of integration values and splitting becomes difficult.

Reference Data for 13C NMR

In ¹³C NMR, chemical shifts vary greatly depending on the type of carbon.
Combining ¹³C NMR with ¹H NMR increases the reliability of structural estimation.

Reference 13C NMR Example for Ethyl Acetate

If an ethyl group and an acetyl group are suggested by ¹H NMR and an ester carbonyl carbon is observed near 171 ppm in ¹³C NMR, the structure of ethyl acetate is more strongly supported.

Example of How to Write the Results

In the ¹H NMR spectrum of ethanol, a 3H triplet at 1.18 ppm, a 2H quartet at 3.65 ppm, and a 1H broad peak at 2.10 ppm were observed.
The integration ratio was 3:2:1, corresponding to the numbers of hydrogens in CH3, CH2, and OH.
In addition, because CH3 appeared as a triplet and CH2 as a quartet, the presence of a CH3CH2- ethyl portion was suggested.

For ethyl acetate, a 3H triplet at 1.26 ppm, a 2H quartet at 4.12 ppm, and a 3H singlet at 2.05 ppm were observed.
The peaks at 1.26 ppm and 4.12 ppm had an integration ratio of 3:2 and corresponded to an ethyl group in which the signals split each other.
Furthermore, the CH2 peak at 4.12 ppm was considered to appear downfield because the CH2 is adjacent to oxygen.

The 3H singlet at 2.05 ppm corresponds to CH3CO-, which has no neighboring hydrogens.
This peak appears farther downfield than an ordinary alkyl CH3 because of the influence of the carbonyl group.
Based on these results, unknown sample X is highly likely to be ethyl acetate containing an ethyl group and an acetyl group.

Points for Connecting the Results to the Discussion

In a discussion of NMR spectra, it is important not to examine chemical shifts, integration ratios, and splitting patterns separately, but to combine them to estimate the structure.

• Can functional groups and the surrounding environment be estimated from chemical shifts?
• Can the ratio of the numbers of hydrogens be correctly read from integration values?
• Can the number of neighboring hydrogens be estimated from splitting patterns?
• Can a combination of a triplet and quartet be explained as an ethyl group?
• Can the reason for a singlet be explained in relation to the absence of neighboring hydrogens?
• Can the downfield shift caused by oxygen, carbonyl groups, and aromatic rings be explained?
• Can it be explained that exchangeable hydrogens such as OH and NH may become broad or disappear after addition of D2O?
• Can solvent peaks and impurity peaks be distinguished from peaks of the target compound?
• Can the structure be judged in combination with other analytical results such as ¹³C NMR, IR, and MS?

Example Discussion

In this experiment, the structure of unknown sample X was discussed using its ¹H NMR spectrum.
In unknown sample X, a 3H triplet at 1.26 ppm, a 2H quartet at 4.12 ppm, and a 3H singlet at 2.05 ppm were observed.
The integration ratio was 3:2:3 and was considered to correspond to a total of eight hydrogens.

The triplet at 1.26 ppm and quartet at 4.12 ppm form a typical splitting pattern for an ethyl group.
CH3 becomes a triplet because of the two neighboring hydrogens of CH2, while CH2 becomes a quartet because of the three neighboring hydrogens of CH3.
In addition, because the CH2 at 4.12 ppm is farther downfield than an ordinary alkyl CH2, it is considered to be adjacent to an oxygen atom.

The 3H singlet at 2.05 ppm corresponds to a methyl group with no neighboring hydrogens.
Because its chemical shift is near 2 ppm, this methyl group is highly likely to be adjacent to a carbonyl group.
Therefore, unknown sample X can be estimated to contain the partial structures CH3CO- and -OCH2CH3.
Combining these structures suggests that unknown sample X is highly likely to be ethyl acetate.

Furthermore, if a carbonyl carbon is observed near 171 ppm and an OCH2 carbon near 60 ppm in ¹³C NMR, the presence of an ester structure is supported.
On the other hand, because no aromatic-hydrogen peak is observed near 7 ppm, the sample is not considered to be an aromatic compound such as ethylbenzene.

Possible sources of error include peak overlap, integration-range settings, solvent peaks, impurity peaks, and broadening of exchangeable hydrogens.
For example, when CDCl3 is used, a residual CHCl3 peak appears near 7.26 ppm, so care must be taken not to mistake it for an aromatic peak.
In NMR analysis, it is important to evaluate chemical shifts, integration ratios, and splitting patterns comprehensively and, when necessary, combine them with results from IR, MS, and other methods.

Summary

In NMR spectra, the surrounding environment of hydrogens can be estimated from chemical shifts, the ratio of the numbers of hydrogens can be read from integration ratios, and the number of neighboring hydrogens can be judged from splitting patterns.

This reference example dealt with chemical shifts, integration, splitting, coupling constants, D2O addition, solvent peaks, impurity peaks, and correspondence with ¹³C NMR using ethanol, ethyl acetate, ethylbenzene, and unknown sample X as examples.
In a report, rather than explaining each peak separately, it is useful to discuss the entire structure from the relationships among the peaks.

What Is Chemical Shift?

Chemical shift is a value representing the position at which a signal appears in an NMR spectrum.
It is expressed in ppm.
Chemical shift is affected by the electron density surrounding the nucleus and by nearby functional groups, aromatic rings, double bonds, electronegative atoms, and other factors.

Hydrogens in environments with high electron density generally tend to appear upfield, that is, at smaller ppm values.
In contrast, hydrogens affected by oxygen, nitrogen, halogens, carbonyl groups, aromatic rings, and similar groups tend to appear downfield, that is, at larger ppm values.

How to Read Chemical Shifts

In ¹H NMR, the type of hydrogen can be roughly estimated from the chemical-shift range.
Hydrogens in alkyl groups tend to appear at relatively small ppm values, while hydrogens adjacent to oxygen or nitrogen, hydrogens on aromatic rings, aldehyde hydrogens, and carboxylic-acid hydrogens appear farther downfield.

However, chemical shifts change depending on surrounding structure, solvent, concentration, and hydrogen bonding.
Therefore, the structure should not be determined from chemical shifts alone, but judged together with integration ratios and splitting patterns.

What Is an Integration Ratio?

An integration ratio is a value representing the ratio of the areas of individual NMR signals.
In ¹H NMR, the integration value is, in principle, proportional to the number of hydrogen atoms corresponding to that signal.
Therefore, examining the integration ratio makes it possible to estimate how many hydrogens of each type are present.

For example, if the signal integration ratio of a compound is 3:2:1, the signals may correspond to three, two, and one hydrogen, respectively.
However, integration values do not always become exact integers and may be affected by baseline correction, peak overlap, concentration, relaxation time, and other factors.

How to Read Integration Ratios

When reading an integration ratio, first check the relative area of each signal.
Next, compare it with the number of hydrogens expected from the structural formula.
For example, a methyl group often corresponds to 3H, a methylene group to 2H, and a methine group to 1H.

If integration values are displayed as 3.00, 2.02, and 1.01, they can be treated as a ratio of 3:2:1.
However, exchangeable hydrogens such as OH and NH may not give accurate integration values.
Integration values may also deviate greatly when peaks overlap.

What Is Splitting?

Splitting in NMR is the phenomenon in which one hydrogen signal appears divided into multiple lines.
It is caused by spin-spin coupling with neighboring hydrogen atoms.
The number of nearby hydrogens can be estimated from the splitting pattern.

As a basic concept, when a hydrogen has n equivalent neighboring hydrogens, its signal is often split into n+1 lines.
For example, if there are three equivalent neighboring hydrogens, the signal splits into four lines; if there are two, it splits into three; and if there is one, it splits into two.

Discussion of the n+1 Rule

The n+1 rule is a basic rule for predicting the number of split lines from the number of equivalent neighboring hydrogens.
If there are n equivalent neighboring hydrogens, the observed signal is split into n+1 lines.
For example, in an ethyl group, CH3 has two neighboring hydrogens in CH2 and therefore tends to become a triplet, while CH2 has three neighboring hydrogens in CH3 and therefore tends to become a quartet.

However, the n+1 rule is an approximation that is effective in simple cases.
If several types of nonequivalent neighboring hydrogens are present, complex multiplets may occur.
In addition, splitting may be difficult to observe for readily exchangeable hydrogens such as OH and NH.

How to Interpret Singlets, Doublets, Triplets, and Quartets

A singlet appears when no coupling with neighboring hydrogens is observed.
It may be seen for OCH3 in a methoxy group or for an isolated methyl group.
A doublet tends to appear when there is one equivalent neighboring hydrogen, a triplet when there are two, and a quartet when there are three.

Not only the number of split lines but also the integration ratio and chemical shift are checked together.
For example, if a 3H triplet and a 2H quartet are observed at the same time, this strongly indicates the presence of an ethyl group.

What Is a Coupling Constant?

The coupling constant J is a value representing the spacing between split peaks.
Its unit is Hz.
Signals involved in the same coupling relationship often show the same J value, providing a clue to which hydrogens are coupled with one another.

For example, the CH3 triplet and CH2 quartet of an ethyl group have matching J values because they are coupled to each other.
In alkenes, coupling constants may differ depending on whether the arrangement is cis or trans.

Equivalent and Nonequivalent Hydrogens

In NMR, hydrogens in the same chemical environment are equivalent and appear together as the same signal.
In contrast, hydrogens in different chemical environments are nonequivalent and are observed as separate signals.
In highly symmetrical molecules, multiple hydrogens may be equivalent, resulting in fewer signals.

Conversely, in molecules with low symmetry, hydrogens become nonequivalent and the number of signals increases.
The number of signals provides a clue to how many types of hydrogen environments are present in the molecule.

Discussion of Aromatic Hydrogens

Aromatic hydrogens generally appear near 6–8 ppm.
Because of the ring-current effect of the benzene ring, hydrogens on aromatic rings shift downfield.
Their chemical shifts and splitting patterns change depending on the types and positions of substituents.

If an aromatic ring has substituents, the signals in the aromatic region may become complex.
The number of signals and splitting patterns differ among monosubstituted benzene, disubstituted benzene, and symmetrical compounds.
If IR suggests an aromatic ring and NMR shows signals at 6–8 ppm, these findings support the presence of an aromatic ring.

NMR Discussion of Alkyl Groups

Hydrogens in alkyl groups generally tend to appear near 0.5–2 ppm.
The chemical shifts and splitting patterns of methyl, methylene, and methine groups change depending on surrounding functional groups and the number of neighboring hydrogens.
If they are close to oxygen, nitrogen, or a carbonyl group, they shift downfield.

Integration ratios and splitting patterns are particularly important in discussing alkyl groups.
A 3H signal may correspond to a methyl group, a 2H signal to a methylene group, and a 1H signal to a methine group.

Discussion of Hydrogens Adjacent to O or N

Hydrogens on carbons adjacent to electronegative atoms such as oxygen and nitrogen tend to shift downfield.
For example, O-CH or O-CH2 in alcohols, ethers, and esters and N-CH in amines may appear near 3–5 ppm.

This is because electronegative atoms attract electrons and reduce the shielding surrounding nearby hydrogens.
If C-O or N-H is confirmed by IR and an alkyl signal appears downfield in NMR, it can be assigned to a hydrogen near the functional group.

Discussion of OH and NH Protons

OH and NH hydrogens are readily affected by concentration, solvent, temperature, hydrogen bonding, and exchange reactions, so their chemical shifts vary easily.
Their peaks may also become broad or their splitting may become difficult to observe.
When D2O is added, OH and NH signals may disappear or weaken, which can be used to confirm exchangeable protons.

Integration values for OH and NH are sometimes difficult to obtain accurately.
Therefore, when dealing with OH and NH signals, the chemical shift and integration ratio should not be treated too rigidly, and they should be discussed together with other structural information.

Discussion of Aldehyde Hydrogens

Aldehyde hydrogens generally appear far downfield near 9–10 ppm.
Because they are strongly affected electronically by the carbonyl group, they produce signals at very characteristic positions.
They provide an important clue for confirming the presence of an aldehyde by NMR.

If C=O absorption is present in the IR spectrum and a signal corresponding to 1H is observed near 9–10 ppm in NMR, the possibility of an aldehyde structure becomes higher.
However, the result must be judged together with the integration value and structural formula so that it is not confused with a carboxylic acid or other downfield proton.

Discussion of Carboxylic-Acid Hydrogens

The OH proton of a carboxylic acid may appear as a very broad signal far downfield.
It generally appears near 10–12 ppm and may sometimes occur over an even broader range.
Because it is strongly affected by hydrogen bonding and exchange, the peak may become broad or its position may change depending on the measurement conditions.

For confirmation of a carboxylic acid, broad O-H absorption and C=O absorption in IR are also important in addition to the downfield NMR signal.
Combining multiple analytical results rather than relying only on NMR increases reliability.

Discussion of Solvent Peaks

Deuterated solvents are used for NMR measurements.
However, even deuterated solvents may contain a small amount of residual solvent containing ordinary hydrogen, producing residual solvent peaks.
For example, residual CHCl3 is observed in CDCl3.
It is important not to mistake this for a peak of the target compound.

The position of the solvent peak differs depending on the solvent used.
In a report, the measurement solvent should be clearly stated and solvent-derived peaks excluded from structural assignments.
Water peaks may also appear in the solvent.

Discussion of Water Peaks and Impurity Peaks

NMR spectra may contain peaks originating from water in the sample or solvent.
The position of the water peak may change depending on the solvent, concentration, and temperature.
Impurity peaks originating from unreacted starting materials, residual solvents, by-products, grease, or insufficiently cleaned equipment may also appear.

If there are peaks that cannot be explained by the structure of the target compound, the possibility of impurities or solvent should be considered.
However, it is not necessary to force an assignment for every small peak.
The discussion should focus primarily on whether the major peaks correspond to the target structure.

Discussion When Peaks Overlap

In NMR, peaks may overlap when multiple hydrogens appear at similar chemical shifts.
Signals are particularly likely to overlap in the aromatic region and complex alkyl regions.
When peaks overlap, it becomes difficult to accurately read integration values and splitting patterns.

When overlap is present, integration values should not be interpreted too rigidly.
If possible, an enlarged spectrum, two-dimensional NMR, measurement in another solvent, comparison with literature spectra, or similar methods should be used.

Discussion When Peaks Are Broad

Causes of broad NMR peaks include exchange of OH or NH, sample concentration, viscosity, slow molecular motion, impurities, magnetic-field inhomogeneity, and insufficient dissolution of the sample.
In polymers and highly viscous samples, molecular motion becomes slower and peaks may broaden.

OH and NH peaks readily become broad because of hydrogen bonding and proton exchange.
When a peak is broad, not only poor measurement but also the effects of exchangeable protons and molecular motion can be discussed.

Comparison of the NMR Spectra of Starting Materials and Products

In organic synthesis experiments, it is important to compare the NMR spectra of the starting material and product.
Check whether signals that should disappear as a result of the reaction have disappeared and whether signals characteristic of the product have appeared.
In esterification, acetylation, reduction, oxidation, substitution reactions, and similar reactions, the chemical shifts of hydrogens near functional groups change.

If peaks originating from the starting material remain, the reaction may be incomplete, purification may be insufficient, or starting material may be mixed with the product.
If peaks originating from the product appear as expected and the integration ratios also agree, this supports the conclusion that the target substance was obtained.

NMR Discussion of Ester Synthesis

In ester synthesis, an alcohol and carboxylic acid react to form an ester.
In NMR, signals from methylene or methyl groups adjacent to oxygen, signals from ester substituents, and changes in OH protons from the starting material are examined.
In ethyl esters, a 3H triplet and a 2H quartet are often observed.

If partial structures characteristic of an ester are confirmed by NMR and C=O and C-O absorptions are also confirmed by IR, ester formation is more strongly supported.
If unreacted alcohol or carboxylic acid remains, peaks originating from the starting materials may remain.

NMR Discussion of Acetanilide Synthesis

In amide compounds such as acetanilide, aromatic hydrogens, methyl hydrogens of the acetyl group, and the NH proton are important.
Aromatic hydrogens appear near 6–8 ppm, while the CH3 of the acetyl group appears relatively upfield.
The NH proton may appear broad because of hydrogen bonding and solvent effects.

Compared with the starting material aniline, amidation changes the electronic environment around nitrogen and may cause changes in the aromatic region and NH signal.
Confirmation together with the amide C=O absorption in IR strengthens the evidence for the product.

How to Proceed With Structural Estimation

When estimating a structure from NMR, first check the number of signals.
Next, estimate the types of hydrogens from the chemical shift of each signal.
Then confirm the numbers of hydrogens from the integration ratios and estimate the numbers of neighboring hydrogens from the splitting patterns.
Finally, check whether this information is consistent with the structural formula.

NMR is a very powerful structural-analysis method, but complete structural determination may be difficult from ¹H NMR alone for complex molecules.
When necessary, results from ¹³C NMR, IR, MS, two-dimensional NMR, melting-point measurements, TLC, and other methods are combined.

How to Write a Peak-Assignment Table

In an NMR spectrum report, preparing a peak-assignment table makes the discussion easier to understand.
The table includes chemical shifts, integration values, splitting patterns, assigned hydrogens, and the basis for assignment.
It is not necessary to force assignments for every small impurity peak.
The table should focus on the major peaks needed to explain the structure of the target compound.

What Is Difficult to Determine From NMR Alone?

NMR is extremely useful for structural estimation, but it cannot necessarily determine everything by itself.
When peaks overlap, it becomes difficult to accurately read integration ratios and splitting patterns.
In addition, when trace impurities or isomers are mixed in, it may be difficult to judge them from the major peaks alone.

Combining NMR with other analytical results increases reliability, such as IR for confirmation of functional groups, MS for molecular weight, melting point or HPLC for purity, and TLC for reaction progress.
In a report, a good discussion explains how far the NMR results support the structure and where their limitations lie.

Comparison With Literature Spectra

When discussing an NMR spectrum, comparison with literature spectra or spectra of standard substances improves reliability.
If chemical shifts, integration ratios, and splitting patterns generally agree with literature values, the sample is more likely to be the target compound.
However, chemical shifts may change slightly depending on the measurement solvent, concentration, temperature, and instrument conditions.

When comparing with literature values, it is not necessary to demand perfect agreement; instead, confirm that the major signals do not contradict the structure.
OH and NH protons in particular may change greatly depending on conditions.

When NMR Results Can Be Considered Good

NMR results can be considered good when signals corresponding to the target compound are clearly observed and the chemical shifts, integration ratios, and splitting patterns agree with the structural formula.
It is also important that there be few extra peaks originating from starting materials or impurities and that the major peaks can be explained after excluding solvent peaks and water peaks.

In synthesis experiments, if signals originating from the starting material decrease or disappear and signals characteristic of the product are observed, this provides evidence that the reaction has proceeded.
However, because it may be difficult to evaluate purity completely using NMR alone, the result should be judged together with other analytical results.

Example Discussion When the Experiment Did Not Go Well

When an NMR measurement does not go well, possible causes are considered from results such as weak peaks, broad peaks, overlapping peaks, incorrect integration ratios, large solvent or water peaks, remaining starting-material peaks, or inability to confirm the target peaks.
Organizing the causes into sample preparation, insufficient dissolution, concentration, insufficient purification, measurement solvent, impurities, and peak overlap makes the discussion easier.

Discussion When Integration Ratios Do Not Agree

If the integration ratios do not agree with the numbers of hydrogens expected from the structural formula, possible causes include peak overlap, insufficient baseline correction, exchange of OH or NH, inclusion of solvent peaks, impurities, concentration, and relaxation-time effects.
Particularly in complex aromatic regions, multiple signals may overlap and make the integration value appear larger.

Small deviations in integration ratios are not unusual.
What is important is whether the ratios of the major signals are substantially consistent with the structural formula and whether the cause of any deviation can be explained.

Discussion When Splitting Is Difficult to Observe

If splitting is difficult to observe, possible causes include peak overlap, increased line width, instrument resolution, concentration, exchange reactions, and complex coupling caused by nonequivalent hydrogens.
In the aromatic region, multiple hydrogens couple with one another and may form multiplets that are difficult to explain using a simple n+1 rule.

If splitting cannot be clearly read, it may be treated as a multiplet rather than being forced into a classification such as singlet or doublet.
In structural estimation, chemical shift and integration ratio may be given priority, with splitting used as supplementary information.

Discussion When the Number of Peaks Differs From the Expected Number

If the number of peaks differs from that predicted from the structural formula, possible causes include overlooking equivalent hydrogens, insufficient consideration of symmetry, peak overlap, impurities, mixtures of isomers, remaining starting material, and inclusion of solvent peaks.
In symmetrical molecules, the number of signals may be smaller than expected.

Conversely, if more peaks than expected are present, the sample may contain impurities or unreacted starting material.
The judgment should take into account not only the structural formula but also the state of purification and reaction conditions.

How to Write Points for Improvement

In a discussion of NMR spectra, including not only sources of error but also points for improvement makes the report easier to organize.
Improvements can be considered separately for sample preparation, purification, measurement, and analysis.

Improvements to Sample Preparation

• Purify the sample sufficiently
• Remove residual solvents
• Dry the sample completely
• Dissolve it completely in the measurement solvent
• Adjust the concentration appropriately
• Clean the NMR tube

Improvements to Measurement

• Select an appropriate deuterated solvent
• Check solvent peaks and water peaks
• Increase the number of scans when necessary
• Check whether the sample is turbid
• Match the measurement conditions to those used for literature values

Improvements to Analysis

• Organize major peaks in an assignment table
• Interpret chemical shifts, integration ratios, and splitting together
• Exclude solvent peaks and water peaks from consideration
• Take peak overlap into account
• Compare with literature spectra
• Do not make a definitive judgment from NMR alone; combine it with IR, MS, and other methods

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of NMR spectra, simply writing that “there was a peak” or “the integration ratio matched” results in a superficial discussion.
A good discussion relates chemical shifts, integration ratios, and splitting patterns to the structural formula and explains, with evidence, which hydrogens they originate from.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of NMR spectra.
Adjust the necessary parts according to your own experimental results.

• Chemical shifts reflect the electronic environment surrounding each hydrogen atom.
• Signals appearing downfield are considered to originate from hydrogens affected by electronegative atoms, aromatic rings, or carbonyl groups.
• Integration ratios indicate the ratios of the numbers of hydrogens corresponding to the individual signals.
• Because the integration ratio agreed with the ratio of the numbers of hydrogens in the structural formula, the result supports the structure of the target compound.
• Splitting patterns provide clues for estimating the number of neighboring hydrogens.
• According to the n+1 rule, a triplet indicates the presence of two equivalent neighboring hydrogens.
• OH and NH protons are readily affected by exchange and may produce broad peaks.
• Solvent peaks and water peaks must be distinguished from signals of the target compound.
• When peaks overlap, it becomes difficult to accurately read integration ratios and splitting patterns.
• The structure should not be determined from the NMR spectrum alone but judged in combination with other analytical results such as IR and MS.

Points to Check When Discussing NMR Spectra

Checking the following points before writing the report makes the discussion easier to write.

• Have the nucleus measured and measurement solvent been stated?
• Have the chemical shifts of the major signals been recorded?
• Have the integration ratios been compared with the numbers of hydrogens in the structural formula?
• Has the number of neighboring hydrogens been explained from the splitting patterns?
• Have coupling constants been checked when necessary?
• Have solvent peaks and water peaks been distinguished?
• Have the possibilities of impurity peaks and starting-material peaks been considered?
• Has the effect of peak overlap been taken into account?
• Have the spectra of the starting material and product been compared?
• Has the spectrum been compared with literature spectra?
• Are the points that are difficult to determine from NMR alone understood?
• Do the points for improvement correspond to the sources of error?

Summary

NMR spectroscopy is an analytical method for estimating the structures of compounds by examining the chemical environments of hydrogens and carbons in molecules.
In ¹H NMR, the types of hydrogens can be read from chemical shifts, the numbers of hydrogens from integration ratios, and the numbers of neighboring hydrogens from splitting patterns.
Combining this information makes it possible to judge whether the spectrum agrees with the structure of the target compound.

Chemical shifts are affected by functional groups and the surrounding electronic environment, integration ratios indicate ratios of the numbers of hydrogens, and splitting reflects coupling with neighboring hydrogens.
However, OH and NH protons are readily affected by exchange and may produce broad peaks or inaccurate integration values.
Attention must also be paid to solvent peaks, water peaks, impurity peaks, and peak overlap.

In a report, rather than simply writing that “a peak appeared,” relate the chemical shifts, integration ratios, and splitting patterns to the structural formula and explain, with evidence, which hydrogens they originate from.
NMR is extremely useful for structural estimation, but when necessary, it is important to combine it with other analytical results such as IR, MS, melting point, and TLC to produce a more reliable discussion.