1. Home
  2. >Organic Chemistry
  3. >Chemical analysis and structure determination
Help

Carbon NMR

Get the most by viewing this topic in your current grade. Pick your course now.

?
Intros
Lessons
  1. How does carbon NMR work?
  2. Introducing NMR: overview.
  3. NMR environments - example
  4. NMR absorptions: summary.
  5. Different ways to describe NMR spectra.
  6. Using NMR to determine structure - example
Topic Notes
?

Introduction to Carbon NMR Spectroscopy

Carbon NMR spectroscopy is a powerful analytical technique in organic chemistry, essential for elucidating molecular structures. This non-destructive method provides valuable information about carbon atoms within organic compounds. The introduction video offers a visual overview of this crucial tool. Carbon NMR, or 13C NMR, complements proton NMR, offering deeper insights into molecular frameworks. This article delves into the fundamentals of Carbon NMR, explaining its working principles, spectrum interpretation, and practical applications in structure determination. By understanding Carbon NMR, chemists can identify carbon environments, functional groups, and molecular connectivity. We'll explore how this spectroscopic method distinguishes between different types of carbon atoms, revealing crucial details about molecular composition. From simple organic molecules to complex natural products, Carbon NMR plays a pivotal role in modern chemical analysis. By mastering this technique, researchers can unravel the mysteries of molecular structures, advancing our understanding of organic compounds and their properties.

Fundamentals of Carbon NMR

Carbon Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful analytical technique used to determine the structure of organic compounds. To understand Carbon NMR, it's essential to grasp the basic principles that govern this phenomenon. At its core, Carbon NMR relies on the concept of nuclear spin, the interaction with magnetic fields, and the occurrence of resonance.

Nuclear spin is an intrinsic property of atomic nuclei, including carbon-13 isotopes. These nuclei behave like tiny magnets, aligning themselves when placed in a strong external magnetic field. The alignment can be either parallel (low energy state) or antiparallel (high energy state) to the applied field. The energy difference between these states is crucial for NMR spectroscopy.

When a sample containing carbon atoms is placed in a powerful magnetic field within an NMR machine, the nuclear spins align accordingly. The NMR spectrometer then applies a radio frequency (RF) pulse to the sample. This pulse provides energy that can cause the nuclei to transition between the low and high energy states, a phenomenon known as resonance. The frequency at which this resonance occurs is specific to the nuclear environment of each carbon atom in the molecule.

The NMR machine works by generating a strong, uniform magnetic field using a superconducting magnet. The sample is placed in this field and subjected to RF pulses. As the nuclei return to their equilibrium state after absorbing energy, they emit radio frequency signals. These signals are detected by sensitive receivers in the NMR machine and processed to produce a spectrum.

One of the most important concepts in Carbon NMR is chemical shift. This phenomenon occurs because the electrons surrounding a nucleus create their own small magnetic fields, which slightly modify the external magnetic field experienced by the nucleus. As a result, nuclei in different chemical environments resonate at slightly different frequencies. The chemical shift provides valuable information about the molecular structure and the chemical environment of each carbon atom.

To standardize chemical shift measurements, a reference compound is used. In Carbon NMR, tetramethylsilane (TMS) is the universally accepted reference point. TMS is chosen because it produces a single, sharp peak and its silicon-methyl protons are more shielded than most other protons in organic compounds. The chemical shift of TMS is defined as zero on the NMR scale.

Chemical shifts are reported on a parts per million (ppm) scale. This scale is used because it is independent of the strength of the applied magnetic field, allowing for consistent comparisons between spectra obtained on different NMR instruments. The ppm scale is calculated by dividing the frequency difference between the signal of interest and the TMS reference by the operating frequency of the spectrometer, then multiplying by one million.

Typically, Carbon NMR spectra display chemical shifts ranging from 0 to about 220 ppm. Different types of carbon atoms appear in characteristic regions of the spectrum. For example, alkyl carbons usually appear between 0-50 ppm, while carbonyl carbons are found around 160-220 ppm. These characteristic ranges help chemists identify specific functional groups and structural features in organic molecules.

The intensity of the signals in a Carbon NMR spectrum can provide information about the number of equivalent carbons in a molecule. However, unlike proton NMR, the intensities in Carbon NMR are not always directly proportional to the number of carbons due to varying relaxation times and the nuclear Overhauser effect.

In conclusion, Carbon NMR spectroscopy is based on the principles of nuclear spin, magnetic field interactions, and resonance. The NMR machine exploits these principles to generate spectra that reveal the chemical environment of carbon atoms in a molecule. The concept of chemical shift, measured relative to TMS on a ppm scale, is crucial for interpreting these spectra and elucidating molecular structures. Understanding these fundamentals is essential for anyone working with or studying organic chemistry and structural analysis.

Interpreting Carbon NMR Spectra

Carbon Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful analytical technique used to determine the structure of organic compounds. Understanding how to read and interpret Carbon NMR spectra is crucial for chemists and researchers in various fields. This article will explore the key aspects of Carbon NMR interpretation, focusing on peak position, intensity, multiplicity, and the concept of molecular symmetry.

One of the most important features of a Carbon NMR spectrum is the peak position, also known as chemical shift. The chemical shift is measured in parts per million (ppm) and provides valuable information about the chemical environment of each carbon atom in a molecule. Generally, carbon atoms bonded to electronegative elements or involved in multiple bonds appear at higher chemical shifts (downfield), while those in more shielded environments appear at lower chemical shifts (upfield). For example, carbonyl carbons typically appear around 170-220 ppm, while aliphatic carbons are usually found between 0-50 ppm.

The intensity of peaks in a Carbon NMR spectrum is another crucial factor to consider. Unlike proton NMR, where peak integration directly correlates to the number of protons, Carbon NMR peak intensities are not as straightforward. However, they can still provide valuable information. In general, the intensity of a carbon signal is influenced by factors such as the number of attached protons and relaxation times. Quaternary carbons (those without attached protons) often appear as smaller peaks compared to carbons with attached protons. This is due to the absence of nuclear Overhauser enhancement (NOE) and longer relaxation times for quaternary carbons.

Multiplicity in Carbon NMR spectra is primarily observed when using techniques like DEPT (Distortionless Enhancement by Polarization Transfer) or when analyzing coupled spectra. In a standard broadband decoupled Carbon NMR spectrum, all signals appear as singlets. However, DEPT experiments can provide information about the number of attached protons. DEPT-135, for instance, shows CH and CH3 groups as positive peaks, CH2 groups as negative peaks, and quaternary carbons are absent. This multiplicity information is invaluable for structure elucidation.

The concept of symmetry in molecules plays a significant role in Carbon NMR spectroscopy. Symmetrical molecules often display fewer signals than expected based on the total number of carbon atoms. This is because chemically equivalent carbons produce the same signal in the spectrum. For example, benzene, a highly symmetrical molecule, shows only one signal in its Carbon NMR spectrum despite having six carbon atoms. Understanding symmetry can greatly simplify spectrum interpretation and provide insights into molecular structure.

To illustrate these concepts, let's consider the Carbon NMR spectrum of ethanol (CH3CH2OH). We would expect to see three distinct signals: 1. A peak around 18 ppm for the methyl carbon (CH3) 2. A peak around 58 ppm for the methylene carbon (CH2) 3. A peak around 62 ppm for the carbon attached to the hydroxyl group (C-OH) The methyl and methylene peaks would likely have higher intensities compared to the C-OH peak due to the presence of attached protons. In a DEPT-135 experiment, the CH3 and CH peaks would appear positive, while the CH2 peak would be negative.

Another example is acetone ((CH3)2CO). Its Carbon NMR spectrum would show two signals: 1. A peak around 30 ppm for the two equivalent methyl groups 2. A peak around 205 ppm for the carbonyl carbon The methyl peak would have a higher intensity than the carbonyl peak. The presence of only two signals instead of three demonstrates the effect of symmetry on the spectrum.

In conclusion, interpreting Carbon NMR spectra requires a thorough understanding of peak position, intensity, multiplicity, and molecular symmetry. Peak positions provide information about the chemical environment of carbon atoms, while intensities can offer insights into the number of attached protons. Multiplicity, especially in DEPT experiments, helps determine the types of carbon atoms present. Lastly, recognizing symmetry in molecules can simplify spectrum interpretation and provide valuable structural information. By mastering these concepts and practicing with various

Chemical Shifts and Functional Groups

Chemical shifts play a crucial role in understanding the structure and environment of carbon atoms in organic molecules. These shifts are influenced by various factors, with functional groups being one of the most significant. In this detailed explanation, we'll explore how different carbon environments and functional groups affect chemical shifts, providing valuable insights for spectroscopic analysis.

Chemical shifts are measured in parts per million (ppm) and represent the difference in resonance frequency between a specific nucleus and a reference compound. For carbon-13 NMR spectroscopy, tetramethylsilane (TMS) is commonly used as the reference, with its chemical shift set at 0 ppm. The chemical shift of a carbon atom is determined by its electronic environment, which is influenced by neighboring atoms and functional groups.

Shielding and deshielding effects play a significant role in determining chemical shifts. Shielding occurs when electrons around a nucleus create a magnetic field that opposes the applied magnetic field, resulting in a lower chemical shift. Conversely, deshielding happens when electrons are drawn away from a nucleus, leading to a higher chemical shift. The presence of electronegative atoms or pi bonds can cause deshielding effects on nearby carbons.

Different carbon environments exhibit distinct chemical shift ranges:

  • Alkyl carbons (sp3 hybridized) typically appear between 0-50 ppm
  • Alkene carbons (sp2 hybridized) usually resonate between 100-150 ppm
  • Alkyne carbons (sp hybridized) are found around 65-90 ppm
  • Aromatic carbons generally appear between 110-160 ppm
  • Carbonyl carbons have higher chemical shifts, ranging from 160-220 ppm

Functional groups significantly impact the chemical shifts of nearby carbons. Here's a list of common functional groups and their effects on chemical shifts:

  1. Alkyl groups: Slightly shield nearby carbons, resulting in lower chemical shifts
  2. Halogens: Deshield adjacent carbons, causing higher chemical shifts
  3. Alcohols: The electronegative oxygen deshields nearby carbons
  4. Ethers: Have a similar effect to alcohols but to a lesser extent
  5. Aldehydes and Ketones: The carbonyl group strongly deshields the carbon directly attached to it
  6. Carboxylic acids: Cause significant deshielding due to the carbonyl and hydroxyl groups
  7. Esters: Similar to carboxylic acids but with slightly less deshielding
  8. Amines: The nitrogen atom causes moderate deshielding of adjacent carbons
  9. Nitriles: The triple bond results in significant deshielding of the attached carbon
  10. Aromatic rings: Cause deshielding due to ring current effects

To illustrate the impact of functional groups on chemical shifts, here's a table of common chemical shift ranges for different carbon environments:

Carbon Environment Chemical Shift Range (ppm)
Alkyl (CH3, CH2, CH) 0-50
Alkene (C=C) 100-150
Alkyne (CC) 65-90
Aromatic 110-160
Alcohol (C-OH)

Applying Carbon NMR to Structure Determination

Carbon Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful tool for structure determination in organic chemistry. This step-by-step guide will walk you through the process of using Carbon NMR data to elucidate molecular structures, incorporating key concepts such as molecular formula analysis, symmetry considerations, and functional group identification.

Step 1: Obtain the Molecular Formula
The first crucial step in structure determination in organic chemistry is to obtain the molecular formula of the compound. This information is typically provided alongside the NMR data or can be derived from other analytical techniques such as mass spectrometry. The molecular formula gives you the exact number of carbon atoms present in the molecule, which should match the number of signals in your Carbon NMR spectrum (unless symmetry is present).

Step 2: Analyze the Carbon NMR Spectrum
Carefully examine the Carbon NMR spectrum, noting the number of signals, their chemical shifts, and relative intensities. Each signal typically represents a unique carbon environment in the molecule. Pay attention to the following key features:

  • Number of signals: This indicates the number of distinct carbon environments.
  • Chemical shifts: These provide information about the electronic environment of each carbon.
  • Signal intensities: These can give clues about the number of equivalent carbons.

Step 3: Perform Symmetry Analysis
Consider the possibility of molecular symmetry, which can reduce the number of signals observed in the spectrum. If the number of signals is less than the total number of carbons in the molecular formula, symmetry is likely present. Look for patterns in the spectrum that might indicate equivalent carbons due to symmetry elements such as planes of symmetry or rotation axes.

Step 4: Identify Functional Groups
Use the chemical shift values to identify potential functional groups present in the molecule. Certain ranges of chemical shifts are characteristic of specific functional groups:

  • 0-50 ppm: Typically aliphatic carbons
  • 50-90 ppm: Often indicates carbons attached to electronegative atoms (e.g., alcohols, ethers)
  • 110-160 ppm: Aromatic or alkene carbons
  • 160-220 ppm: Carbonyl carbons (e.g., aldehydes, ketones, carboxylic acids)

Step 5: Construct Partial Structures
Based on the identified functional groups and carbon environments, begin to construct partial structures that are consistent with the spectral data. Consider how these partial structures might fit together to form the complete molecule.

Step 6: Apply Additional Spectroscopic Data
If available, incorporate data from other spectroscopic techniques such as proton NMR or IR spectroscopy to further refine your structural hypothesis. These additional data points can help confirm the presence of specific functional groups or structural features.

Step 7: Propose a Complete Structure
Combine all the information gathered in the previous steps to propose a complete molecular structure that is consistent with the Carbon NMR data, molecular formula, and any additional spectroscopic evidence.

Step 8: Verify the Proposed Structure
Double-check that your proposed structure accounts for all the observed NMR signals, matches the molecular formula, and is consistent with any symmetry considerations. Ensure that the structure makes chemical sense and doesn't violate any fundamental principles of bonding or stability.

Worked Example:
Let's apply this process to a simple example from the video:

  • Given: Molecular formula C4H8O2
  • Carbon NMR spectrum shows 3 signals: 14 ppm, 60 ppm, and 171 ppm
Analysis:
  1. The molecular formula indicates

    Advanced Carbon NMR Techniques

    Carbon Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful analytical tool for elucidating molecular structures. While basic 13C NMR provides valuable information, advanced techniques offer even deeper insights into molecular composition and connectivity. In this section, we'll explore some of these advanced Carbon NMR techniques, including DEPT experiments, 2D NMR spectroscopy, and the significance of coupling constants.

    DEPT (Distortionless Enhancement by Polarization Transfer) is a crucial advanced technique in Carbon NMR. This experiment allows for the differentiation of carbon atoms based on the number of attached protons. DEPT-45, DEPT-90, and DEPT-135 are common variants, each providing specific information. DEPT-45 shows all protonated carbons as positive peaks, DEPT-90 displays only CH groups, and DEPT-135 shows CH and CH3 as positive peaks while CH2 appears as negative peaks. By comparing these spectra, analysts can determine the number of hydrogens attached to each carbon, significantly aiding in structure elucidation.

    Two-dimensional (2D) NMR spectroscopy represents another leap forward in Carbon NMR capabilities. These experiments correlate different nuclei, providing information about connectivity and spatial relationships within molecules. HSQC (Heteronuclear Single Quantum Coherence) and HMBC (Heteronuclear Multiple Bond Correlation) are two essential 2D NMR techniques. HSQC correlates directly bonded 1H and 13C nuclei, allowing for unambiguous assignment of carbon signals to their corresponding protons. HMBC, on the other hand, shows long-range correlations between carbons and protons separated by two or three bonds, crucial for determining molecular connectivity and identifying quaternary carbons.

    Coupling constants play a vital role in advanced Carbon NMR analysis. While often associated with proton NMR, coupling constants in 13C NMR provide valuable structural information. Carbon-carbon coupling (JCC) and carbon-proton coupling (JCH) can be observed in high-resolution spectra. These coupling constants are sensitive to factors such as bond angles, hybridization, and substituent effects. For instance, the magnitude of 1JCH coupling constants can indicate the hybridization state of the carbon atom, with sp3 carbons typically showing smaller coupling constants than sp2 or sp carbons.

    Another advanced technique worth mentioning is INADEQUATE (Incredible Natural Abundance Double Quantum Transfer Experiment). Despite its low sensitivity due to the low natural abundance of 13C, this powerful experiment directly reveals carbon-carbon connectivity, making it invaluable for structure determination of complex organic molecules. INADEQUATE can unambiguously establish the carbon skeleton of a molecule, a feat not easily accomplished by other NMR methods.

    The application of these advanced techniques significantly enhances the information obtainable from Carbon NMR experiments. For example, in the analysis of natural products or complex synthetic molecules, the combination of DEPT, 2D NMR, and coupling constant analysis can lead to complete structure elucidation. DEPT experiments quickly reveal the types of carbons present, 2D NMR techniques like HSQC and HMBC establish connectivity, and careful analysis of coupling constants provides insights into spatial arrangements and bond characteristics.

    Moreover, these advanced techniques are not limited to static structures. They can also be applied to study dynamic processes in molecules. For instance, variable temperature NMR experiments combined with these advanced techniques can reveal information about molecular motion, conformational changes, and chemical exchange processes. This makes Carbon NMR an indispensable tool not just for structural analysis but also for studying molecular dynamics and reaction mechanisms.

    In conclusion, advanced Carbon NMR techniques such as DEPT, 2D NMR spectroscopy, and the analysis of coupling constants significantly expand the capabilities of NMR spectroscopy in molecular structure determination. These methods provide a wealth of information about carbon environments, molecular connectivity, and spatial relationships within molecules. As technology continues to advance, we can expect even more sophisticated Carbon NMR techniques to emerge, further enhancing our ability to unravel complex molecular structures and dynamics.

    Conclusion

    Carbon NMR spectroscopy stands as a cornerstone technique in organic chemistry, offering unparalleled insights into molecular structures. This article has explored the fundamental principles, key features, and practical applications of Carbon NMR in structure determination. We've delved into chemical shift ranges, coupling patterns, and the interpretation of spectra, highlighting how these elements contribute to elucidating molecular frameworks. The importance of Carbon NMR in organic chemistry cannot be overstated; it provides crucial information about carbon environments, helping chemists identify compounds and verify synthetic products. To truly master this technique, regular practice in interpreting spectra is essential. We encourage readers to apply the knowledge gained here to real-world examples and to revisit the introductory video for visual demonstrations of concepts. By honing your Carbon NMR interpretation skills, you'll enhance your capabilities in organic chemistry and structure determination, opening doors to more advanced research and applications in the field.

    How does carbon NMR work?

    Introducing NMR: overview.

    Step 1: Introduction to NMR Spectroscopy

    NMR spectroscopy, or Nuclear Magnetic Resonance spectroscopy, is a powerful analytical technique used to determine the structure of organic molecules. In this section, we will focus specifically on carbon NMR, which is used to study carbon atoms within a molecule. The primary objective of NMR spectroscopy is to understand the molecular structure by analyzing the interaction of nuclei with magnetic fields.

    Step 2: Understanding Nucleus Spin

    The fundamental principle behind NMR spectroscopy is the property of nucleus spin. Certain nuclei, such as carbon-13, possess a non-zero spin value, making them spin-active. This means that these nuclei can interact with magnetic fields. In carbon NMR, we focus on the carbon-13 isotope, which has a natural abundance of about 1%. These carbon-13 nuclei spin in a specific direction and can be influenced by external magnetic fields.

    Step 3: Interaction with Magnetic Fields

    When a sample is placed in an NMR machine, it is exposed to a strong magnetic field. This magnetic field affects the spin-active carbon-13 nuclei, causing them to flip their spin direction. Additionally, low-energy radiation, typically in the form of radio waves, is introduced. The carbon-13 nuclei absorb this radiation and resonate at a specific frequency. This resonance can be measured and used to detect the presence and behavior of the nuclei.

    Step 4: The Concept of Chemical Shift

    The concept of chemical shift is crucial in NMR spectroscopy. Electrons surrounding the nucleus create their own magnetic fields, which can shield the nucleus from the external magnetic field. This shielding effect causes the nucleus to experience a different magnetic field than the one produced by the NMR machine. The difference in the magnetic field experienced by the nucleus is known as the chemical shift. The chemical shift depends on the bonding and electronic environment of the nucleus.

    Step 5: Measuring the NMR Spectrum

    The NMR spectrum is measured in units called parts per million (ppm). This unit represents the fraction of the resonance frequency of the nuclei compared to a reference compound, typically tetramethylsilane (TMS). TMS is assigned a chemical shift value of zero ppm, and all other chemical shifts are measured relative to this reference. The NMR spectrum provides valuable information about the molecular structure based on the chemical shifts observed.

    Step 6: Analyzing the NMR Spectrum

    Analyzing the NMR spectrum involves interpreting the chemical shifts and understanding the bonding environment of the carbon atoms. Different types of carbon atoms, such as those in different functional groups or bonding environments, will have distinct chemical shifts. By comparing the observed chemical shifts to known reference values, we can deduce the structure of the molecule.

    Step 7: Application to Molecular Structure Determination

    The ultimate goal of carbon NMR spectroscopy is to determine the molecular structure of organic compounds. By analyzing the chemical shifts and resonance frequencies of carbon-13 nuclei, we can identify the types of carbon atoms present and their bonding environments. This information is crucial for understanding the overall structure and properties of the molecule.

    FAQs

    1. What is Carbon NMR spectroscopy?

      Carbon NMR spectroscopy is an analytical technique used to determine the structure of organic compounds by examining the chemical environment of carbon atoms. It provides information about the number, type, and arrangement of carbon atoms in a molecule.

    2. How does Carbon NMR differ from Proton NMR?

      While both techniques are forms of Nuclear Magnetic Resonance spectroscopy, Carbon NMR focuses on 13C nuclei, providing information about carbon skeletons in molecules. Proton NMR, on the other hand, examines 1H nuclei, offering insights into hydrogen environments. Carbon NMR typically shows fewer, simpler peaks but requires more sample and longer acquisition times.

    3. What information can be obtained from chemical shifts in Carbon NMR?

      Chemical shifts in Carbon NMR provide crucial information about the electronic environment of carbon atoms. They help identify functional groups, distinguish between different types of carbons (e.g., alkyl, aromatic, carbonyl), and reveal details about molecular structure and bonding.

    4. How are Carbon NMR spectra interpreted?

      Interpretation involves analyzing the number of signals (indicating distinct carbon environments), their chemical shifts (revealing the type of carbon), and sometimes their intensities. Advanced techniques like DEPT experiments can provide additional information about the number of hydrogens attached to each carbon.

    5. What are some advanced Carbon NMR techniques?

      Advanced techniques include DEPT (Distortionless Enhancement by Polarization Transfer) for determining the number of attached hydrogens, 2D NMR methods like HSQC and HMBC for establishing connectivity, and INADEQUATE for directly observing carbon-carbon bonds. These techniques provide more detailed structural information and are crucial for complex molecule analysis.

    Prerequisite Topics

    Understanding Carbon NMR (Nuclear Magnetic Resonance) is a crucial skill for chemists and spectroscopists, but it requires a solid foundation in several fundamental concepts. While there are no specific prerequisite topics provided for this article, it's important to recognize that Carbon NMR builds upon various basic principles of chemistry and physics.

    To fully grasp Carbon NMR, students should have a strong understanding of organic chemistry, including the structure and bonding of carbon-containing compounds. Familiarity with different functional groups and their electronic properties is essential, as these factors significantly influence the chemical shifts observed in NMR spectra.

    Additionally, a basic knowledge of quantum mechanics and atomic structure is beneficial. These concepts help explain the underlying principles of NMR spectroscopy, including the behavior of atomic nuclei in magnetic fields and the energy transitions that give rise to NMR signals.

    Students should also be comfortable with spectroscopic techniques in general. While not a direct prerequisite, experience with other forms of spectroscopy, such as IR or UV-Vis, can provide a helpful context for understanding the principles of NMR.

    Mathematical skills, particularly in interpreting graphs and analyzing complex data, are also valuable. Carbon NMR spectra can be intricate, and the ability to read and interpret these spectra is a key skill that builds on basic mathematical and analytical abilities.

    Furthermore, a general understanding of laboratory techniques and safety procedures is important for those who will be conducting NMR experiments. This includes knowledge of sample preparation and handling of sensitive equipment.

    While these topics are not explicitly listed as prerequisites, they form the foundation upon which Carbon NMR knowledge is built. Students who have a solid grasp of these underlying concepts will find it easier to understand the principles of Carbon NMR, interpret spectra, and apply this powerful analytical technique in their studies and research.

    It's worth noting that learning is often an iterative process. As students delve deeper into Carbon NMR, they may find themselves revisiting and strengthening their understanding of these fundamental concepts. This reinforcement of basic principles through the lens of a more advanced topic like Carbon NMR can lead to a more comprehensive and nuanced understanding of chemistry as a whole.

    In conclusion, while there may not be a strict list of prerequisites for studying Carbon NMR, a strong foundation in organic chemistry, quantum mechanics, spectroscopy, and analytical skills will greatly enhance a student's ability to master this important analytical technique. Recognizing the interconnectedness of these topics with Carbon NMR can motivate students to approach their studies holistically, seeing each new concept as a building block towards a more comprehensive understanding of chemical analysis and structure determination.

    In this lesson, we will learn:

    • To understand how NMR works as an analytical process.
    • To understand the descriptive terms used when interpreting NMR spectra.
    • To apply principles of NMR to determine molecular structure.

    Notes:

    • Nuclear magnetic resonance (NMR) spectroscopy is an extremely versatile tool to determine chemical structure. NMR works because of a property called nuclear spin:
      • If a nucleus has non-zero spin, it will interact with a magnetic field.
      • NMR applies a magnetic field that flips the ‘spin state’ of the nucleus. In this state the nucleus resonates due to applied radiation (radio waves) being absorbed. The frequency of this resonance can be measured.
      • The electrons around a nucleus are also affected by the NMR field. They set up their own magnetic field that interferes with the original NMR field; we say the electrons shield \, the nucleus with their magnetic field.
      • As a result, the field the nucleus actually experiences is different – shifted – from the NMR field. This is seen in the change in frequency of energy the nuclei emit (see the second point in this list).
      • Depending on how the electrons are arranged around a nucleus – how it is bonding - the chemical shift, symbol δ\delta, will be a different value.
      • Because NMR machines come in and can run at a variety of frequencies (their field strength), the units on an NMR spectrum is parts per million (ppm). Ppm is a fraction of how far away the resonance frequency is from the resonance of a reference sample called tetramethylsilane (TMS), which is a signal chosen to be 0 ppm.
      Using NMR, over time chemists have built up a large catalogue of chemical shifts that are evidence for a nucleus’s environment – what it is bonded to – so we learn the structure of the molecule around these nuclei. This is how NMR is used to find molecular structure. NMR can be run for any nucleus that has non-zero spin, so conveniently for organic chemists, NMR works for carbon AND hydrogen because they both have a ‘spin active’ isotope. NMR for carbon detects the minor isotope 13C ^{13}C, or carbon-13, which is spin active (I = 1/2\scriptsize1/2 )

    • A  \, 13C ^{13}C spectra shows an absorption peak for every carbon environment – not necessarily every atom!
      To identify a carbon environment, “tell a story” of how the specific atom is connected to the rest of the molecule. If it is connected in a unique way, it is a unique environment.
      • NMR environments, when assigning peaks to atoms or vice versa will normally be given a unique label for each environment, e.g. CaC_a, CbC_b, CcC_c, CdC_d.
      • If a molecule has symmetry, look for atoms in symmetrical environments. Nuclei in symmetrical environments will produce the same NMR spectrum. This is how NMR spectra sometimes show ‘less peaks’ than the number of atoms in the molecule being analyzed.
      See below for an example:

    • A  \, 13C ^{13}C NMR spectra has absorption ‘regions’ where certain carbon environments are found. NMR absorption tables with specific ppm values are widely available online and in chemistry textbooks. Below is a general description:
      • Between 0-100 ppm, saturated carbon signals will be found. This region can be split into two further parts:
        • 0-50 ppm: saturated alkanes. From lower to higher ppm, this includes terminal carbon atoms (-CH3), secondary carbon (-CH2-) and tertiary carbon. Most of the halogens (R3C-X) are also found here.
        • 50-100 ppm: saturated carbon bonded to oxygen, such as ethers (R3C-O-R’) and alcohols (R2C-OH). Alkynes (C≡\equiv C) are also found here.
      • Between 100-200 ppm, unsaturated carbon signals will be found. This region can also be split into two parts:
        • 100-150 ppm: unsaturated hydrocarbon environments. This includes alkenes (R2C=CR2) and aromatic carbon.
        • 150-200 ppm: unsaturated carbon bonded to oxygen. This includes ketones (R2C=O) aldehydes (RCHO), esters (RCOOR'), amides (RCONR2) and carboxylic acids (RCOOH).

    • Below is a step by step outline to working out an NMR spectrum using a worked example, if you are given / know the molecular formula:
      Determine the structure of a molecule with the formula C5H10O2

      There are five signals in the  \,13C ^{13}C NMR spectrum: a signal at 180 ppm; 34 ppm; 27 ppm; 22 ppm and 13 ppm.

      STEP ONE: With your known molecular formula, look for a fit to a general formula. You are trying to find out the hydrocarbon backbone! For example:
      • Alkane (CnH2n+2).
      • Alkene (CnH2n).
      • Aromatic compounds (C:H ratio generally nearer to 1:1 due to the C6H6 benzene ring).

      In this C5H10O2 example, we have C5H10 which resembles an alkene, however there are also two oxygen atoms. We need to consider how they are connected in the molecule. So far, we could say we have either an alkene or alkane chain with one or more functional groups containing oxygen.

      STEP TWO: Compare your formula with the number of peaks in the spectrum. You are looking for signs of symmetry. If there is not one signal for every carbon atom in your molecule, you have some form of symmetry! This could be alkyl chain branches or clues to where the substituents on the aromatic ring are.
      When a molecule is symmetrical, carbon environments become equivalent. The same carbon environments will produce the same NMR signal. This is how you may have less signals than you do carbon atoms in the molecule.
      We have five signals for five carbon atoms, so each carbon environment is unique. This likely means any functional group(s) are not attached in the middle of the chain. Alone though, this is not concrete evidence.

      STEP THREE: Try to identify the functional groups present from the rest of the formula. For example:
      • A carboxylic acid or ester will have two oxygen atoms, and an aldehyde or ketone will have one. For all three, due to the C=O bond, any molecule with these groups will have two less hydrogens than an alkane analogue.
      • An alkene will have two less hydrogens than the alkane analogue because of the double bond to carbon that was otherwise making C-H bonds.
      From here, we have options for the two oxygen atoms in the molecule. Functional groups containing oxygen atoms include:
      • Aldehydes (RCHO) and ketones (R2CO), but these are only one oxygen atom each.
        We’d need two of them to get two oxygens as per our molecular formula and that would give us less hydrogen atoms than we know we have.
        The information that we have means it is unlikely that an aldehyde or ketone is present in the molecule.
      • Alcohols (R3C-OH), which we could have two of. We could then have an alkene double bond which explains the two less hydrogens than an ordinary diol would have. This is a possibility going forward.
      • Carboxylic acids (RCOOH) and esters (RCOOR’), one of which would explain the two oxygen atoms and the two less hydrogens that we have in our formula, compared to the alkane.
        This is another possibility for the structure.

      STEP FOUR: Use the information you’ve gathered from the molecular formula to pick out the appropriate signals. This is where you rule out structure ‘candidates’ and find evidence for the remaining structure. You should be able to use an NMR absorption table and your molecular formula to know where to expect signals.
      We have carried two possibilities for the structure into this step of our working:
      • An alkene with two alcohol groups attached.
      • A carboxylic acid.
      • An ester
      There are no signals from 34 to 180 ppm. This leaves no evidence for a C=C bond (100-150ppm) or carbon atoms bonded to an alcohol group (around 50-75 ppm). The typical range that an ester carbon resonates at (160-170 ppm) also shows no peaks. The ester and alkene with alcohol groups can be ruled out.
      • An alkene with two alcohol groups attached.
      • Ester.
      • A carboxylic acid.
      We are left with the possibility of a carboxylic acid. The NMR spectrum contains a single signal at 180ppm, which is the range where carbon atoms with carboxylic acid groups resonate. This is substantial supporting evidence.
      We can now assign part of our predicted structure to this NMR signal. We can also ‘strike off’ part of the formula.
      From C5H10O2 \, removing an acid COOH leaves the remaining molecule fragment C4H9.

      We have four signals left: 34 ppm; 27 ppm; 22 ppm and 13 ppm, to assign to four carbon atoms. 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 probably just the four remaining carbon atoms in a straight alkane chain; the higher the ppm, the closer it is to the carboxylic acid group. This would be assigning the rest of the molecule.

      Finally, draw out your predicted structure with the NMR peaks assigned to the specific carbon atoms. You should be able to account for all of the peaks – one for each carbon environment! See the picture below:

    • Aside from just ‘ppm’, chemists use a few different descriptive terms when analyzing an NMR spectrum. Here is a look at the different NMR descriptions, compared to the ppm scale and the TMS reference you’ll see on every spectrum:
      • Chemical shift \, just means the different frequency a nucleus resonates at compared to TMS, the reference at 0 ppm. We would say the reference TMS has zero chemical shift.
        • Higher ppm \, is further from 0 ppm, so this is a larger chemical shift.
        • Lower ppm \, is a smaller chemical shift.
      • Shielding \, is talking about how a nucleus is ‘shielded’ from the NMR magnetic field by the electrons surrounding it. Compared to the vast majority of carbon environments, the carbon nuclei in TMS (the reference at 0 ppm) is shielded.
        • A signal at higher ppm reflects a deshielded \, nucleus.
        • A signal at lower ppm reflects a shielded \, nucleus.
      • The field \, means the magnetic field strength that makes the nucleus resonate. The slightly electropositive silicon atom in the reference TMS makes the four carbons surrounding it more shielded by electrons; they need a stronger (higher) field than most carbon atoms to resonate.
        • A signal at higher ppm is further from 0 ppm, so a lower field strength made that nucleus resonate. This is a downfield \, signal.
        • A signal at lower ppm is closer to 0 ppm, so a relatively higher field strength made it resonate. This is an upfield \, signal.