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Proton NMR

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Intros
Lessons
  1. Using 1H NMR.
  2. Splitting patterns
  3. Integration in 1H NMR
  4. Using 1H NMR to find structure - example.
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Introduction to Proton NMR

Proton NMR spectroscopy, also known as hydrogen NMR, is a powerful analytical technique used to determine the structure of organic compounds. Our introduction video provides an essential foundation for understanding this crucial tool in chemistry. Proton NMR focuses on the magnetic properties of hydrogen atoms within molecules, offering valuable insights into their chemical environment. Unlike carbon-13 NMR, which examines carbon atoms, proton NMR is more sensitive and requires smaller sample sizes. The key differences include the natural abundance of the isotopes (1H vs 13C) and the resulting signal strength. Proton NMR spectra typically show more complex splitting patterns due to spin-spin coupling between nearby hydrogen atoms. This technique is widely used in organic chemistry, biochemistry, and pharmaceutical research for structure elucidation, purity analysis, and reaction monitoring. By mastering proton NMR, chemists can unlock a wealth of molecular information, making it an indispensable skill in modern chemical analysis.

Basics of Proton NMR Spectroscopy

Proton Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful analytical technique used to determine the structure of organic compounds. This method relies on the fundamental principles of nuclear magnetic resonance, which occurs when the nuclei of certain atoms are immersed in a static magnetic field and exposed to a second oscillating magnetic field. In proton NMR, we focus specifically on the behavior of hydrogen atoms (protons) within molecules.

One of the key concepts in proton NMR is the chemical shift. The chemical shift refers to the resonance frequency of a nucleus relative to a standard reference compound. This phenomenon arises because the electrons surrounding a nucleus shield it from the applied magnetic field, and the degree of shielding depends on the chemical environment of the proton. The chemical shift is measured in parts per million (ppm) and provides valuable information about the molecular structure.

The reference compound used in proton NMR is typically tetramethylsilane (TMS). TMS is chosen as the reference because it produces a single, sharp peak and its protons are highly shielded, resulting in a chemical shift defined as 0 ppm. All other chemical shifts in a spectrum are measured relative to TMS, allowing for standardization across different instruments and experiments.

The typical range of proton NMR spectra spans from 0 to 12 ppm. This range encompasses various chemical environments commonly found in organic molecules. Understanding the typical chemical shifts for different types of protons is crucial for spectral interpretation. Here are some examples of common chemical environments and their corresponding chemical shifts:

1. Alkyl protons: Protons attached to sp3 hybridized carbon atoms typically appear in the range of 0-4 ppm. For instance, methyl (CH3) protons often resonate around 0.7-1.2 ppm, while methylene (CH2) protons are usually found between 1.2-1.4 ppm.

2. Alkene protons: Protons attached to sp2 hybridized carbon atoms in alkenes generally appear in the range of 4.5-6.5 ppm. The exact chemical shift depends on the substitution pattern of the alkene.

3. Aromatic protons: Protons on aromatic rings resonate in the range of 6.5-8.5 ppm due to the strong deshielding effect of the aromatic ring current.

4. Aldehyde protons: The proton in an aldehyde group (CHO) typically appears as a singlet around 9-10 ppm due to the strong deshielding effect of the adjacent carbonyl group.

5. Alcohol and amine protons: OH and NH protons can appear over a wide range, typically between 1-5 ppm, but their exact position can vary depending on concentration and solvent effects.

6. Carboxylic acid protons: The proton in a carboxylic acid group (COOH) usually appears as a broad singlet around 10-13 ppm, although this can vary significantly depending on hydrogen bonding and exchange effects.

It's important to note that these ranges are general guidelines, and the actual chemical shifts can be influenced by various factors such as neighboring functional groups, ring strain, and hydrogen bonding. Additionally, the multiplicity of signals (singlets, doublets, triplets, etc.) provides information about the number of neighboring protons, further aiding in structure elucidation.

In practice, interpreting proton NMR spectra involves analyzing the chemical shifts, integration (which relates to the number of protons responsible for each signal), multiplicity, and coupling constants. This information, combined with other spectroscopic techniques, allows chemists to determine the structure of unknown compounds and verify the purity and identity of synthesized molecules.

As analytical techniques continue to advance, proton NMR spectroscopy remains an indispensable tool in organic chemistry, biochemistry, and materials science. Its non-destructive nature, high sensitivity, and ability to provide detailed structural information make it an essential technique for researchers and industry professionals alike. By mastering the fundamental principles of chemical shifts,

Splitting Patterns in Proton NMR

Splitting patterns in proton Nuclear Magnetic Resonance (NMR) spectroscopy are crucial for understanding the structural information of organic molecules. These patterns arise from the interaction between neighboring protons and provide valuable insights into the molecular environment of the observed nuclei. One of the fundamental concepts in interpreting these patterns is the N+1 rule, which helps determine the multiplicity of signals.

The N+1 rule states that the number of peaks in a signal is equal to the number of equivalent neighboring protons plus one. This rule is essential for predicting and interpreting splitting patterns in proton NMR spectra. For example, if a proton has two equivalent neighboring protons, it will appear as a triplet (2+1 = 3 peaks) in the spectrum.

Multiplicity refers to the number of peaks in a signal and is directly related to the number of neighboring protons. Understanding multiplicity is crucial for identifying the molecular environment of specific protons within a compound. The most common splitting patterns observed in proton NMR spectra include singlets, doublets, triplets, quartets, and multiplets.

A singlet appears as a single peak in the spectrum and indicates that the observed proton has no neighboring protons or is in a unique chemical environment. This pattern is often seen in isolated protons or those surrounded by equivalent nuclei. Examples include the protons in chloroform (CHCl3) or the methyl group in acetone ((CH3)2CO).

A doublet consists of two peaks of equal intensity and suggests that the observed proton has one neighboring proton. This pattern is commonly observed in compounds with adjacent CH2 groups or in aromatic systems. For instance, the protons in ethyl bromide (CH3CH2Br) would appear as a doublet due to coupling with the adjacent CH2 group.

Triplets are characterized by three peaks with an intensity ratio of 1:2:1. This pattern indicates that the observed proton has two equivalent neighboring protons. A classic example is the CH2 group in ethanol (CH3CH2OH), which appears as a triplet due to coupling with the adjacent CH3 group.

Quartets consist of four peaks with an intensity ratio of 1:3:3:1 and suggest that the observed proton has three equivalent neighboring protons. This pattern is often seen in compounds with a CH3 group adjacent to a CH group. For example, the CH proton in ethyl acetate (CH3COOCH2CH3) would appear as a quartet due to coupling with the neighboring CH3 group.

Multiplets are more complex splitting patterns that arise when a proton has multiple non-equivalent neighboring protons or when higher-order coupling effects are present. These patterns can be challenging to interpret and may require advanced NMR techniques for full analysis. Multiplets are commonly observed in complex organic molecules with multiple functional groups or in aromatic systems.

Understanding splitting patterns and the N+1 rule is essential for accurately interpreting proton NMR spectra and elucidating molecular structures. By analyzing the multiplicity of signals and considering the chemical shifts, chemists can gain valuable information about the connectivity and arrangement of atoms within a molecule. This knowledge is crucial for structure determination, reaction monitoring, and quality control in various fields of chemistry and related disciplines.

In practice, the application of splitting patterns and the N+1 rule requires careful consideration of factors such as chemical equivalence, coupling constants, and potential overlapping signals. Advanced NMR techniques, such as decoupling experiments and two-dimensional spectroscopy, can provide additional information to resolve complex splitting patterns and confirm structural assignments.

As chemists become more proficient in interpreting splitting patterns, they can quickly identify common structural features and functional groups based on characteristic signal multiplicities. This skill is invaluable for rapid structure elucidation and quality control in organic synthesis, pharmaceutical research, and materials science. By mastering the concepts of splitting patterns and the N+1 rule, researchers can unlock the full potential of proton NMR spectroscopy as a powerful analytical tool in modern chemistry.

Integration in Proton NMR

Integration is a crucial concept in proton Nuclear Magnetic Resonance (NMR) spectroscopy that provides valuable information about the relative number of protons contributing to each signal in a spectrum. This feature of NMR spectroscopy is essential for determining molecular structures and confirming the identity of organic compounds. Understanding integration is key to interpreting proton NMR spectra accurately and extracting meaningful structural information.

In proton NMR, integration refers to the measurement of the relative areas under the peaks or signals in the spectrum. These areas are directly proportional to the number of protons responsible for each signal. This relationship between signal intensity and proton count is fundamental to the power of NMR as an analytical tool. By analyzing the integration values, chemists can determine the relative number of protons in different chemical environments within a molecule.

The process of integration involves measuring the area under each peak or group of peaks in the NMR spectrum. Modern NMR spectrometers perform this integration automatically, providing numerical values that represent the relative areas. These values are typically displayed as a stepped line above the spectrum, with the height of each step corresponding to the relative number of protons.

It's important to note that integration values are relative, not absolute. They indicate the ratio of protons between different signals rather than the exact number of protons. For example, if one signal has an integration value of 2 and another has a value of 3, it means the second signal represents 1.5 times as many protons as the first signal.

To use integration effectively in structure determination, chemists often set the smallest integration value to 1 and scale the other values accordingly. This approach simplifies the interpretation and allows for easier comparison between different parts of the molecule. For instance, if the smallest integration value is set to 1 and corresponds to one proton, a signal with an integration of 3 would represent three protons.

Integration is particularly useful when combined with other information from the NMR spectrum, such as chemical shifts and splitting patterns. By correlating the number of protons indicated by integration with the chemical environment suggested by the chemical shift, chemists can piece together the structural components of a molecule.

Let's consider some examples to illustrate how integration is used in molecular structure determination:

1. Ethanol (CH3CH2OH): In the proton NMR spectrum of ethanol, you would typically observe three distinct signals. The integration values might show a ratio of 3:2:1. This corresponds to the three protons in the methyl group (CH3), the two protons in the methylene group (CH2), and the one proton in the hydroxyl group (OH), respectively.

2. Acetone (CH3COCH3): The spectrum of acetone would show a single peak with an integration value of 6, representing the six equivalent protons in the two methyl groups.

3. 2-Bromopropane (CH3CHBrCH3): This molecule would display two signals in its proton NMR spectrum. One signal, corresponding to the six protons in the two methyl groups, would have an integration value three times that of the other signal, which represents the single proton on the carbon bonded to bromine.

Integration becomes even more powerful when analyzing more complex molecules. For instance, in a substituted benzene ring, the integration values can help determine the substitution pattern by indicating the number of protons in different positions on the ring.

While integration is a powerful tool, it's important to be aware of its limitations. Factors such as impurities, overlapping signals, or exchange processes can affect the accuracy of integration. Additionally, protons attached to heteroatoms like oxygen or nitrogen may not always integrate reliably due to exchange phenomena.

In conclusion, integration in proton NMR spectra is a fundamental technique for elucidating molecular structures. By providing information about the relative number of protons contributing to each signal, integration allows chemists to piece together the structural puzzle of organic molecules. When combined with other spectral data, such as chemical shifts and coupling patterns, integration becomes an indispensable tool in the arsenal of analytical chemists and organic structure analysts. Mastering the interpretation of integration values is essential for anyone working with NMR spect

Interpreting Proton NMR Spectra

Proton Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful analytical technique used for structure determination in NMR. Interpreting proton NMR spectra requires a systematic approach, combining information from chemical shifts, splitting patterns, and integration to deduce molecular structures. This guide will walk you through the process of spectrum analysis, providing a step-by-step example to illustrate the key concepts.

Step 1: Analyze Chemical Shifts

The first step in interpreting a proton NMR spectrum is to examine the chemical shifts of the peaks. Chemical shifts provide information about the electronic environment of each hydrogen atom in the molecule. Different functional groups and nearby atoms influence these shifts, allowing you to identify specific structural features. For example, aromatic protons typically appear between 6-8 ppm, while alkyl protons are found between 0-4 ppm.

Step 2: Examine Splitting Patterns

Next, focus on the splitting patterns of each peak. These patterns, also known as multiplicities, arise from spin-spin coupling between neighboring protons. Common patterns include singlets (s), doublets (d), triplets (t), and quartets (q). The number of peaks in a splitting pattern is determined by the n+1 rule, where n is the number of equivalent neighboring protons. Analyzing these patterns helps identify the relative positions of hydrogen atoms within the molecule.

Step 3: Interpret Integration Values

Integration values represent the relative number of protons responsible for each peak or group of peaks. By comparing these values, you can determine the ratio of different types of hydrogen atoms in the molecule. This information is crucial for piecing together the overall structure and confirming the presence of specific functional groups.

Step 4: Combine Information for Structure Determination

To deduce the molecular structure, combine the information gathered from chemical shifts, splitting patterns, and integration. Look for consistent patterns and relationships between different peaks. Consider the molecular formula (if known) and any additional spectroscopic data available, such as IR or mass spectrometry results.

Step-by-Step Example: Spectrum Analysis

Let's walk through an example of interpreting a proton NMR spectrum for a simple organic compound:

Given spectrum data: - Peak A: δ 1.2 ppm, triplet, integration 3 - Peak B: δ 2.5 ppm, quartet, integration 2 - Peak C: δ 3.7 ppm, singlet, integration 3

Step 1: Analyze Chemical Shifts - Peak A (1.2 ppm): Likely an alkyl group, possibly a methyl (-CH3) - Peak B (2.5 ppm): Slightly deshielded, possibly adjacent to an electronegative group - Peak C (3.7 ppm): More deshielded, could be a methoxy group (-OCH3)

Step 2: Examine Splitting Patterns - Peak A: Triplet suggests two neighboring protons - Peak B: Quartet indicates three neighboring protons - Peak C: Singlet implies no neighboring protons

Step 3: Interpret Integration Values - Peak A: 3 protons - Peak B: 2 protons - Peak C: 3 protons

Step 4: Combine Information for Structure Determination - The triplet (A) and quartet (B) are likely coupled, suggesting an ethyl group (-CH2CH3) - The singlet (C) is consistent with a methoxy group (-OCH3) - Proposed structure: CH3CH2-X-OCH3, where X is likely an electronegative atom or group

Based on this analysis, a possible molecular formula could be C4H10O2, representing ethyl methyl ether or a similar compound. However, additional spectroscopic data would be needed to confirm the exact structure.

Advanced Concepts in Proton NMR

Proton Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful analytical technique used to determine the structure of organic compounds. While basic concepts like chemical shift and spin-spin coupling provide valuable information, more advanced topics can offer deeper insights into molecular structure. In this section, we'll explore second-order effects, coupling constants, and exchangeable protons, and how these concepts contribute to structural analysis.

Second-order effects in proton NMR occur when the chemical shift difference between coupled nuclei is comparable to the coupling constant. This situation leads to more complex splitting patterns that deviate from the simple first-order multiplets. In second-order spectra, peak intensities become distorted, and additional lines may appear, making interpretation challenging. However, these effects can provide valuable information about the relative positions of protons in a molecule. Chemists use advanced software and simulation techniques to analyze second-order spectra and extract precise structural details.

Coupling constants (J values) play a crucial role in proton NMR analysis. These values, measured in Hertz (Hz), represent the magnitude of interaction between coupled nuclei. Coupling constants provide information about the relative spatial arrangement of protons in a molecule. For instance, vicinal coupling constants (3J) in alkenes can indicate whether the protons are in a cis or trans configuration. The Karplus equation relates the dihedral angle between vicinal protons to their coupling constant, allowing researchers to deduce conformational information. Additionally, geminal coupling constants (2J) between protons on the same carbon atom can offer insights into hybridization and substituent effects.

Exchangeable protons are another important aspect of advanced proton NMR analysis. These are protons that can readily exchange with the solvent or other molecules in the sample. Common examples include -OH, -NH, and -SH groups. The behavior of exchangeable protons in NMR spectra can provide valuable information about hydrogen bonding, acidity, and molecular dynamics. In protic solvents like water or methanol, exchangeable protons may appear as broad peaks or may not be visible at all due to rapid exchange. However, in aprotic solvents or at low temperatures, these protons can be observed and analyzed. Deuterium exchange experiments, where D2O is added to the sample, can help identify exchangeable protons and confirm their assignment.

The analysis of second-order effects, coupling constants, and exchangeable protons significantly enhances structural elucidation capabilities. For example, in complex natural products or pharmaceutical compounds, these advanced concepts can help determine stereochemistry, conformation, and the presence of specific functional groups. Coupling constants can differentiate between isomers or confirm the relative positions of substituents on a ring system. Second-order effects, while challenging to interpret, can provide precise information about closely spaced protons in complex spin systems. Exchangeable protons can offer insights into intramolecular hydrogen bonding, which is crucial for understanding the three-dimensional structure and properties of biomolecules.

To fully utilize these advanced concepts, researchers often employ two-dimensional NMR techniques such as COSY (Correlation Spectroscopy) and NOESY (Nuclear Overhauser Effect Spectroscopy). These methods provide additional correlations between protons, helping to unravel complex coupling patterns and confirm structural assignments. Moreover, variable temperature NMR experiments can reveal dynamic processes within molecules, such as conformational changes or the behavior of exchangeable protons.

In conclusion, mastering advanced topics in proton NMR, including second-order effects, coupling constants, and exchangeable protons, greatly enhances a chemist's ability to perform detailed structural analysis. These concepts, when combined with other spectroscopic techniques and chemical knowledge, enable researchers to solve complex structural puzzles and gain deeper insights into molecular behavior. As NMR technology continues to advance, with higher field strengths and more sensitive probes, the application of these advanced concepts will become even more powerful in fields ranging from organic synthesis to drug discovery and materials science.

Conclusion

In this comprehensive guide to proton NMR, we've explored the fundamental principles and techniques essential for spectral interpretation. The introduction video serves as a crucial foundation, offering a visual and auditory explanation of proton NMR basics. We've covered key concepts such as chemical shifts, integration, multiplicity, and coupling constants, all vital for deciphering molecular structures. The importance of practice cannot be overstated; interpreting spectra is a skill honed through experience. We encourage readers to seek out additional resources, such as online databases of NMR spectra, practice problems in NMR, and advanced textbooks, to further their understanding. Remember, proton NMR is a powerful tool in organic chemistry, and mastering its interpretation opens doors to deeper insights into molecular structures. As you continue your journey in spectroscopy, keep revisiting these concepts and challenging yourself with increasingly complex spectra. The world of proton NMR awaits your exploration!

Using 1H NMR: Splitting Patterns

Proton NMR, also known as 1H NMR, is a powerful tool in organic chemistry for determining the structure of molecules. One of the key features of 1H NMR is the splitting patterns observed in the spectra. These patterns provide valuable information about the number of neighboring hydrogen atoms and the environment of the protons in a molecule. In this guide, we will walk through the steps to understand and predict splitting patterns in 1H NMR.

Step 1: Introduction to Proton NMR

Proton NMR is similar to carbon-13 NMR but with a few additional features that make it more useful for analyzing organic molecules. The basic principles of NMR involve the interaction of nuclear spins with an external magnetic field, resulting in signals that can be measured and interpreted. In proton NMR, these signals are produced by the protons (hydrogen nuclei) in the molecule.

Key concepts to remember from carbon-13 NMR that also apply to proton NMR include:

  • Signals in the NMR spectrum are produced due to the spin of the nucleus.
  • These signals are related to the chemical environments of the protons.
  • The chemical shift, measured in parts per million (ppm), indicates the position of the signal in the spectrum.

Step 2: Understanding Chemical Shifts

Chemical shifts in proton NMR are influenced by the electronic environment around the protons. Different types of protons (e.g., alkyl, alkenyl, aromatic, carbonyl) appear in distinct regions of the NMR spectrum. For example:

  • Alkyl protons typically appear between 0-2 ppm.
  • Alkenyl protons are found around 5-6 ppm.
  • Aromatic protons appear in the region of 6-8 ppm.
  • Carbonyl protons (e.g., aldehydes) are found between 9-10 ppm.
  • Acidic protons (e.g., carboxylic acids) can appear between 9-12 ppm.

These regions help in identifying the types of protons present in the molecule and their respective environments.

Step 3: Introduction to Splitting Patterns

Splitting patterns, also known as multiplicity, arise due to the interaction between non-equivalent protons on adjacent carbon atoms. This phenomenon is described by the n+1 rule, where 'n' is the number of neighboring protons. The resulting splitting pattern will have (n+1) peaks.

For example:

  • A proton with no neighboring protons (n=0) will appear as a singlet (1 peak).
  • A proton with one neighboring proton (n=1) will appear as a doublet (2 peaks).
  • A proton with two neighboring protons (n=2) will appear as a triplet (3 peaks).
  • A proton with three neighboring protons (n=3) will appear as a quartet (4 peaks).

Step 4: Predicting Splitting Patterns

To predict the splitting pattern of a proton, follow these steps:

  1. Identify the proton of interest in the molecule.
  2. Determine the number of non-equivalent neighboring protons (n).
  3. Apply the n+1 rule to predict the number of peaks in the splitting pattern.

For example, in ethanol (CH3CH2OH):

  • The CH3 group has two neighboring protons (CH2), so it will appear as a triplet (3 peaks).
  • The CH2 group has three neighboring protons (CH3) and one proton (OH), so it will appear as a quartet (4 peaks).

Step 5: Integration Values

Integration values in proton NMR provide information about the relative number of protons contributing to each signal. The area under each peak is proportional to the number of protons. By comparing the integration values, you can determine the ratio of different types of protons in the molecule.

For example, in ethanol:

  • The CH3 group will have an integration value of 3 (three protons).
  • The CH2 group will have an integration value of 2 (two protons).
  • The OH group will have an integration value of 1 (one proton).

Step 6: Practical Application

By combining the information from chemical shifts, splitting patterns, and integration values, you can deduce the structure of an unknown molecule. Here is a step-by-step approach:

  1. Analyze the chemical shifts to identify the types of protons present.
  2. Examine the splitting patterns to determine the number of neighboring protons.
  3. Use the integration values to find the relative number of each type of proton.
  4. Piece together the information to propose a structure that fits all the observed data.

With practice, interpreting 1H NMR spectra becomes a valuable skill in organic chemistry, allowing you to identify and confirm the structures of complex molecules.

FAQs

  1. What does 1H NMR tell you?

    1H NMR (Proton Nuclear Magnetic Resonance) provides information about the chemical environment of hydrogen atoms in a molecule. It reveals the number of distinct types of hydrogen atoms, their relative abundance, and their connectivity to neighboring atoms. This information helps determine the structure of organic compounds, including the presence of functional groups and the arrangement of atoms within the molecule.

  2. What do the peaks on the H NMR mean?

    Peaks in an H NMR spectrum represent different types of hydrogen atoms in the molecule. The position of a peak (chemical shift) indicates the chemical environment of the hydrogen, while the shape and splitting pattern (multiplicity) provide information about neighboring hydrogen atoms. The area under the peak (integration) corresponds to the number of equivalent hydrogen atoms contributing to that signal.

  3. What is the H NMR chemical shift?

    The H NMR chemical shift is a measure of the resonance frequency of a hydrogen nucleus relative to a reference compound, typically tetramethylsilane (TMS). It is expressed in parts per million (ppm) and reflects the degree of magnetic shielding experienced by the hydrogen atom. Chemical shifts help identify the type of chemical environment in which a hydrogen atom resides, such as alkyl, alkene, or aromatic regions.

  4. Is H2 NMR-active?

    H2 (molecular hydrogen) is not typically NMR-active in standard proton NMR experiments. This is because the two hydrogen nuclei in H2 are chemically and magnetically equivalent, resulting in no net magnetic moment. However, under special conditions or using advanced NMR techniques, it is possible to observe signals from H2 in certain experiments, such as para-hydrogen-induced polarization (PHIP) studies.

  5. How can I practice interpreting H NMR spectra?

    To practice interpreting H NMR spectra, you can use online resources, textbooks, and problem sets. Many universities offer practice problems and spectral databases. Start with simple molecules and gradually move to more complex structures. Analyze real spectra from scientific literature or databases like the SDBS. Practice identifying chemical shifts, integration values, and splitting patterns. Solve structure determination problems and compare your answers with provided solutions to improve your skills.

Prerequisite Topics

Understanding Proton Nuclear Magnetic Resonance (NMR) spectroscopy requires a solid foundation in several fundamental concepts of chemistry and physics. While there are no specific prerequisite topics provided for this article, it's important to recognize that a comprehensive grasp of basic organic chemistry, atomic structure, and quantum mechanics can significantly enhance your understanding of Proton NMR.

Proton NMR is a powerful analytical technique used to determine the structure of organic compounds. It relies on the magnetic properties of hydrogen atoms (protons) within molecules. To fully appreciate the intricacies of this technique, students should have a strong background in organic chemistry, including knowledge of molecular structures, functional groups, and chemical bonding.

Additionally, a basic understanding of atomic structure is crucial. This includes familiarity with electron configurations, orbitals, and the behavior of electrons in atoms. These concepts form the basis for understanding how protons interact with magnetic fields in NMR spectroscopy.

Quantum mechanics also plays a significant role in Proton NMR. While a deep dive into quantum theory isn't necessary, having a general understanding of energy levels, spin states, and the principles of spectroscopy can greatly aid in comprehending the underlying physics of NMR.

Furthermore, basic principles of magnetism and electromagnetic radiation are essential for grasping how NMR spectrometers work and how they generate the spectra used for analysis. This includes concepts such as magnetic fields, resonance, and the interaction between matter and electromagnetic waves.

Lastly, a foundation in analytical chemistry techniques and spectroscopic methods can provide valuable context for understanding Proton NMR. Familiarity with other spectroscopic techniques like infrared (IR) spectroscopy or mass spectrometry can help students appreciate the unique advantages and applications of NMR in chemical analysis.

While these topics are not explicitly listed as prerequisites, they form the conceptual framework upon which Proton NMR knowledge is built. Students who have a solid grasp of these fundamental areas will find it easier to navigate the complexities of NMR spectroscopy, interpret spectra, and apply this powerful technique in their studies and research.

As you delve into the world of Proton NMR, remember that revisiting and strengthening your understanding of these foundational concepts can significantly enhance your ability to master this advanced analytical technique. The interconnectedness of these topics with Proton NMR highlights the importance of a well-rounded chemistry education and the value of building upon prior knowledge in the sciences.

In this lesson, we will learn:

  • How to apply the principles learned in 13C NMR for use in 1H NMR.
  • How to predict and understand the splitting patterns and integration of 1H NMR signals.
  • How 1H NMR is used to determine the structure of organic molecules.

Notes:

  • Nuclear magnetic resonance (NMR) spectroscopy is an extremely versatile tool to determine chemical structure. The details of how NMR spectroscopy works was covered in Carbon NMR

  • NMR for hydrogen nuclei is called proton (1H) NMR. This is the major isotope of hydrogen which is spin active.

  • All of the essentials you learned in 13C NMR apply to 1H NMR:
    • Signals are produced in an NMR spectrum which are related to ‘environments’, or how the H atoms in a molecule are bonded to the rest of the structure.
    • These environments are measured in ppm with TMS as a reference.
    • There are regions of the 1H NMR spectrum which reflect ‘proton environments’. Some include:
      • 0-2 ppm is the alkyl proton (R3CH, R2CH2, RCH3) region, with -CH3 closest to the 0 ppm reference.
      • 5-6 ppm is the alkenyl proton region (R2C=C(H)R).
      • 6.5 – 7.5 ppm is the aromatic proton region.
      • 9-11 ppm is generally the acid/carbonyl proton region. Carboxylic acid protons appear at 10-11 ppm, while aldehyde ones closer to 9-10 ppm. Ketones do not have any protons directly attached to them; a 9-10 ppm signal in the 1H NMR spectrum will settle if you have a ketone or an aldehyde!
      This and more can all be found in a 1H NMR absorption table.

  • Proton NMR spectra are a bit more complicated than 13C NMR. Proton NMR spectra show splitting patterns in the peaks.
    This is when proton environments ‘couple’ with other environments on carbons adjacent to them. This causes the signals to ‘split’ in a way that is sometimes called the n + 1 rule because the type of split depends on the number of adjacent protons (n), plus one. For example:
    • In a molecule with R–CH2CH3, the –CH3 protons will couple to the –CH2CH3 protons: there’s two of them. Using n + 1, 2 + 1 = 3, so the –CH3 signal becomes a triplet signal. This ‘triplet’ is known as a splitting pattern. .
    • In a molecule with the fragment R–C(CH3)3, a signal due to the –C(CH3)3 protons will not split with any other signal, because the adjacent carbon is tertiary – it has no protons attached to it. Using n + 1, 0 + 1 = 1, so the C(-CH3)3 signal remains a singlet.

    See the table for the names of the split signals:

    N + 1 value

    Splitting pattern

    Example

    1

    Singlet

    -C(CH3)3

    2

    Doublet

    -CH(CH3)2

    3

    Triplet

    -CH2CH3

    4

    Quartet

    -CH(Cl)CH3

    5+

    Multiplet

    -CH2CH2CH3


  • Another difference from carbon NMR to proton NMR is the assigning of integration values to peaks. Integration values are found from the area under the peaks, and roughly relate to the number of protons that produce this signal. This is different from the splitting pattern!
    • For example, the protons -CH(OH) CH2CH3 would have an integration of 2.
    • The protons -CH2CH2CH3 would have an integration of 3.

    These ‘differences’ once you understand them are really just more information, or stronger evidence for the structure of a molecule – 1H NMR is more useful than 13C NMR for a lot of organic molecules!

  • Worked example: Find the structure of a molecule using 1H NMR spectra and the molecular formula.
    Determine the structure of a molecule with the formula C5H10O2.
    There are five signals in the 1H NMR spectrum:

    • Chemical shift (ppm)

    Splitting

    Integration

    0.8

    Triplet

    ~3

    1.2

    Multiplet

    ~2

    1.5

    Multiplet

    ~2

    2.2

    Triplet

    ~2

    11.8

    Singlet

    ~1



    This format of question can come with a lot of other information (such as mass spectrum, the 13C NMR or some test-tube reaction results) to make it easier, or it might not. The less information you’re given, the more you can give the examiner to get marks!

    STEP ONE: Just like we did with 13C NMR, start by using the molecular formula to spot functional groups!
    Recall some general formulae:
    • CnH2n+2 for an alkane. Think of this as your starting molecule or a blank slate with no functional groups to create obvious NMR signals.
    • CnH2n+2O for an alcohol. Alcohol groups won’t remove any H from the molecule because the oxygen has effectively ‘inserted’ between a C-H bond.
    • CnH2n for an alkene.
    • CnH2nO for a carbonyl e.g. ketone or aldehyde. Each carbonyl group will replace two hydrogens from the molecule, compared to if the carbon was just a -CH2- unit instead.
    • CnH2nO2 for an ester or acid. Both these groups have a carbonyl bond that has replaced two hydrogens.

    We have the formula C5H10O2, so two H atoms or C-H bonds have been replaced by the two oxygen atoms now present. This only reasonably matches a carbonyl (C=O) group as part of an ester or carboxylic acid functional group. We have some evidence for an ester or acid.

    STEP TWO: Compare the number of peaks to the molecular formula. You are looking for clues about symmetry!
    For 1H NMR, integration values help a lot because alkyl chain branching and aromatic rings can produce many protons in the same environment.

    We have five proton environments for ten protons over five carbon atoms. Apart from the CH3 chain ends, carbon atoms without any functional groups attached are just -CH2- so we have five signals for five “-CH2-” units. This just means each CH2 or CH3 section of the chain is a unique environment.
    It could be a straight carbon chain with the oxygen functional group(s) on one carbon atom. This makes every proton environment different (one is attached to the functional group, one is one carbon away, another two away, another three...).

    Five different signals works for a carboxylic acid, but not an ester. An ester would need the ten protons to be spread over only four carbons because the ester carbon won’t be bonded to any hydrogens at all. The number of signals is strong evidence that we don’t have an ester.

    STEP THREE: Now we can look for the expected peaks in the NMR spectrum, starting with the functional groups:

    A carboxylic acid is the major functional group that still fits both the formula and the number of expected proton signals. Carboxylic acid 1H NMR peaks are obvious singlet signals at around 10-12 ppm. We have a signal in exactly that range! It is a singlet, too. This is the -COOH proton environment.

    From here, we have assigned COOH of our molecule. What else do we have left? Try and ‘subtract’ COOH from the molecular formula: C5H10O2 – COOH = C4H9. So we have C4H9 of an organic molecule left – that’s just a four-carbon butyl chain.

    We have four signals left too: 0.8 ppm; 1.2 ppm; 1.5 ppm and 2.2 ppm, to assign to four proton environments. These are all in the region where saturated carbon is found – a typical alkane chain. The deshielding effect of the carboxylic acid group (producing higher ppm) gets weaker the further away from the group you go. These four signals are the four remaining carbon atoms in a straight chain; the higher the ppm, the closer it is to the carboxylic acid group.
    • The triplet at 2.2 ppm is due to the –CH2 protons of the carbon adjacent to the COOH group. The closest protons to the deshielding effect would predictably make them resonate further downfield (higher ppm) than the rest of the protons in the chain.
    • The two multiplets – coupling both sides of themselves in the straight chain – are predictably the middle peaks of the spectrum. They are more shielded than the adjacent carbon, the triplet at 2.2 ppm, but less shielded than the terminal carbon.
    • The signal at 0.8 ppm, the terminal carbon attached to a CH2 group, would couple for a triplet. This would be assigning the rest of the molecule.

    STEP FOUR:Draw out your predicted structure with the NMR peaks assigned to the specific carbon atoms. See the picture below:

    Notice that by seeing the formula and the number of NMR peaks we have a good idea about what our molecule is before we even look at the actual ppm values.
    You have seen a lot more molecular formulae and general formulae/functional groups than you have seen NMR spectra, so start with what you know best! There are millions of organic molecules but by starting with ruling out possible functional groups you will quickly narrow down your search for what the structure is, and that is important in a timed exam paper.

  • Worked example: Compare the 1H NMR spectra of two molecules:
    Below is the structure of two molecules. Describe and explain the differences expected in the 1H NMR of the two compounds.

    • In this question format, you know the structure already. What are the obvious differences? Start with those and try to add as much detail as possible:
    • The molecule on the left contains a ketone (RCOR’).
    • The molecule on the right contains an aldehyde (-CHO).
    • The molecule on the left will have an extra -CH3 group that the aldehyde does not.

    Think about these differences – how do they change the NMR spectrum?

    • The aldehyde will have a sharp singlet around 10-11 ppm with an integration value of 1. This corresponds to the proton attached to the carbonyl carbon. This signal will simply not be there in the ketone.
    • The ketone will have an extra -CH3 carbon environment around 1.5-2 ppm. This will also be a singlet with an integration of 3, whereas the other alkyl proton signals (from the ethyl chain attached to the benzene ring) will be a triplet or quartet. This singlet alkyl proton signal won’t be present in the aldehyde.

    Because you were given the structure of the molecules already, you need to give as much detail as possible on the actual signals. Don’t waste time on the signals they both have – those aren’t going to be used to identify the compounds are they? Always give:

    • The expected ppm values of the peaks that the molecules don’t have in common.
    • The splitting pattern (if it is 1H NMR).
    • The integration (if it is 1H NMR).


1 Source for 1H NMR data: https://www2.chemistry.msu.edu/courses/cem251/SS13_HOVIG/Spectroscopy%20tables.pdf