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Organic Chemistry
Table of Contents
Unit 13 Overview: NMR Spectroscopy for Structure Determination
13.1 Nuclear Magnetic Resonance Spectroscopy
13.2 The Nature of NMR Absorptions
13.3 Chemical Shifts
13.4 Chemical Shifts in 1H NMR Spectroscopy
13.5 Integration of 1H NMR Absorptions: Proton Counting
13.6 Spin–Spin Splitting in 1H NMR Spectra
13.7 1H NMR Spectroscopy and Proton Equivalence
13.8 More Complex Spin–Spin Splitting Patterns
13.9 Uses of 1H NMR Spectroscopy
13.10 13C NMR Spectroscopy: Signal Averaging and FT–NMR
13.11 Characteristics of 13C NMR Spectroscopy
13.12 DEPT 13C NMR Spectroscopy
13.13 Uses of 13C NMR Spectroscopy

Spin-spin splitting in 1H NMR spectra reveals how protons interact with their neighbors. This phenomenon creates multiplet patterns that provide crucial information about a molecule's structure, helping chemists decipher its layout.

The n+1 rule and coupling constants are key tools for interpreting these patterns. By analyzing peak numbers and spacings, we can determine how many nearby protons are present and their relative positions, unlocking the molecule's secrets.

Spin-Spin Splitting in 1H NMR Spectra

Spin-spin splitting in NMR spectra

  • Caused by interaction between neighboring non-equivalent protons
    • Neighboring protons exist in two spin states $\alpha$ (aligned with external magnetic field) or $\beta$ (opposed to external magnetic field)
    • Two spin states of neighboring protons affect local magnetic field experienced by proton of interest (ethanol, propane)
  • Splitting of proton signal occurs due to influence of neighboring protons
    • Number of peaks in split signal (multiplet) depends on number of neighboring non-equivalent protons (n) according to n+1 rule (doublet, triplet)
    • Intensity ratios of peaks in multiplet follow Pascal's triangle 1:1, 1:2:1, 1:3:3:1
  • Distance between peaks in multiplet called coupling constant (J)
    • Measured in Hertz (Hz) and independent of magnetic field strength
    • Magnitude depends on number of bonds separating interacting protons and dihedral angle between them (geminal, vicinal coupling)

Multiplet patterns and n+1 rule

  • n+1 rule states number of peaks in multiplet equal to number of neighboring non-equivalent protons (n) plus one
    1. Proton with no neighboring non-equivalent protons (n=0) appears as singlet
    2. Proton with one neighboring non-equivalent proton (n=1) appears as doublet
    3. Proton with two neighboring non-equivalent protons (n=2) appears as triplet
    4. Proton with three neighboring non-equivalent protons (n=3) appears as quartet
  • Equivalent protons do not cause splitting of each other's signals
    • Considered equivalent if chemically and magnetically identical
    • Chemical equivalence occurs when protons have same chemical shift and are in the same chemical environment (methyl groups)
    • Magnetic equivalence requires protons to have the same coupling relationships with other protons in molecule (para-substituted benzene)
  • Protons on same carbon (geminal) or adjacent carbons (vicinal) often non-equivalent and cause splitting (ethyl acetate, lactic acid)
    • Diastereotopic protons are non-equivalent protons on the same carbon that can have different chemical shifts and coupling patterns

Coupling constants for structural analysis

  • Coupling constants (J) provide information about number of bonds separating interacting protons and relative orientation
    • Geminal coupling (2J) between protons on same carbon typically larger (10-18 Hz) than vicinal coupling (3J) between protons on adjacent carbons (6-8 Hz)
    • Long-range coupling (4J and higher) usually smaller (<2 Hz) and less commonly observed
  • Vicinal coupling constants (3J) related to dihedral angle between interacting protons, described by Karplus equation
    • Protons in syn (0°) or anti (180°) orientation have larger coupling constants (8-14 Hz) due to greater orbital overlap
    • Protons in gauche (60°) orientation have smaller coupling constants (2-6 Hz) due to reduced orbital overlap
  • Magnitude of coupling constants helps identify relative positions and orientations of protons in molecule
    • Comparing coupling constants between different protons reveals spatial relationships and helps determine molecular structure (cis and trans isomers)

Key Terms to Review (33)

Spin–spin splitting: Spin-spin splitting is a phenomenon observed in Nuclear Magnetic Resonance (NMR) spectroscopy where the magnetic environment of a nucleus affects the resonance frequency of nearby nuclei, leading to the splitting of their NMR signals into multiple peaks. This effect allows for the determination of molecular structures by providing information about the number and arrangement of neighboring hydrogen atoms.
Dihedral angle: A dihedral angle is the angle between two intersecting planes, which in organic chemistry, often refers to the angle between planes through two sets of three atoms, usually involving a bond between two carbon atoms. In the context of ethane's conformations, it describes the rotational angle around the carbon-carbon bond.
Coupling: In the context of Nuclear Magnetic Resonance (NMR) Spectroscopy, coupling refers to the interaction between different nuclear spins that leads to the splitting of NMR signals into multiplets. This phenomenon is essential for determining the structure of organic compounds by providing information about the number and arrangement of neighboring hydrogen atoms.
Coupling constant: The coupling constant (denoted as J) in Nuclear Magnetic Resonance (NMR) spectroscopy is a measure of the interaction strength between neighboring nuclear spins, influencing the splitting pattern in an NMR spectrum. It is expressed in hertz (Hz) and provides information about the spatial relationship and number of bonds separating adjacent atoms.
N + 1 rule: The n + 1 rule is a guideline in Nuclear Magnetic Resonance (NMR) spectroscopy that predicts the number of peaks (splitting pattern) a signal will split into due to spin-spin coupling with neighboring hydrogen atoms. It states that if a hydrogen atom is coupled with n neighboring hydrogens, the NMR signal will split into n + 1 peaks.
Coupling constant, J: The coupling constant, J, is a measure in Hertz (Hz) that quantifies the strength of interaction between neighboring nuclear spins in Nuclear Magnetic Resonance (NMR) spectroscopy. It indicates the separation between split peaks in an NMR spectrum and is independent of the magnetic field strength.
Chemical shift: In nuclear magnetic resonance (NMR) spectroscopy, a chemical shift is a measure of the change in the resonant frequency of a nucleus relative to a standard reference. It provides insights into the electronic environment surrounding a nucleus, helping to identify molecular structures.
Ethanol: Ethanol, also known as ethyl alcohol, is a colorless, volatile, and flammable liquid that is the principal type of alcohol found in alcoholic beverages. It is an important organic compound with diverse applications in various fields, including as a fuel, solvent, and chemical feedstock.
Ethyl Acetate: Ethyl acetate is a versatile organic compound that serves as a common solvent, a flavoring agent, and an important intermediate in various chemical reactions. It is the ester formed by the reaction between acetic acid and ethanol, and its diverse applications make it a crucial compound in both laboratory and industrial settings.
Propane: Propane is a simple alkane hydrocarbon with the chemical formula C3H8. It is a colorless, odorless, and flammable gas that is widely used as a fuel source for heating, cooking, and transportation. Propane's properties and behavior are important in understanding the topics of alkanes, alkane isomers, and the interpretation of 1H NMR spectra.
Dihedral Angle: The dihedral angle is the angle between two intersecting planes, specifically the angle between the planes formed by the bonds in a molecule. It is a crucial concept in understanding the three-dimensional structure and conformations of organic compounds.
Lactic Acid: Lactic acid is a chemical compound produced in the body during anaerobic respiration, where glucose is broken down without the use of oxygen. It is a key player in various biochemical processes, including the reason for handedness in molecules, spin-spin splitting in 1H NMR spectra, cyanohydrin formation, and the regulation of biological acids through the Henderson-Hasselbalch equation.
Chemical Equivalence: Chemical equivalence refers to the concept in nuclear magnetic resonance (NMR) spectroscopy where chemically identical protons or atoms exhibit the same chemical shift and coupling patterns, indicating that they are in the same chemical environment within a molecule.
J-Coupling: J-coupling, also known as spin-spin coupling, is a phenomenon observed in nuclear magnetic resonance (NMR) spectroscopy where the magnetic moments of neighboring nuclei interact, resulting in the splitting of NMR signals. This interaction provides valuable information about the chemical structure and connectivity of molecules.
Coupling Constant: The coupling constant is a measure of the strength of the spin-spin interaction between two nuclei in a molecule, which is observed in nuclear magnetic resonance (NMR) spectroscopy. It describes the magnitude of the splitting patterns seen in NMR spectra, providing valuable information about the structure and connectivity of molecules.
Diastereotopic Protons: Diastereotopic protons are a pair of chemically nonequivalent hydrogen atoms (protons) that are attached to the same carbon atom in a molecule. These protons exhibit different chemical shifts and coupling patterns in the $^1$H NMR spectrum, allowing for their identification and characterization.
Chemical Shift: Chemical shift is a fundamental concept in nuclear magnetic resonance (NMR) spectroscopy that describes the position of a signal in the NMR spectrum relative to a reference signal. It provides information about the chemical environment of a nucleus, allowing for the identification and characterization of different functional groups and molecular structures.
Triplet: A triplet refers to a group of three equivalent hydrogen atoms or protons that appear as a distinct signal in a proton nuclear magnetic resonance (1H NMR) spectrum. This term is particularly relevant in the context of understanding proton counting, spin-spin splitting, and the uses of 1H NMR spectroscopy.
Spin-Spin Splitting: Spin-spin splitting is a phenomenon observed in nuclear magnetic resonance (NMR) spectroscopy where the signal for a particular nucleus is split into multiple peaks due to the magnetic interactions between that nucleus and the neighboring nuclei. This splitting pattern provides valuable information about the molecular structure and connectivity within a compound.
Quartet: A quartet is a group of four related elements or entities that function together as a unit. In the context of nuclear magnetic resonance (NMR) spectroscopy, a quartet refers to a specific pattern observed in the 1H NMR spectrum when a proton is coupled to three neighboring protons.
NMR Spectra: NMR (Nuclear Magnetic Resonance) spectra is a powerful analytical technique used to determine the structure of organic compounds by analyzing the unique electromagnetic signals emitted by the nuclei of atoms within a molecule when exposed to a strong magnetic field.
N+1 Rule: The n+1 rule is a fundamental principle in nuclear magnetic resonance (NMR) spectroscopy that describes the pattern of spin-spin splitting observed in the 1H NMR spectra of organic compounds. It is a crucial concept for understanding the relationship between the number of neighboring equivalent protons and the resulting signal multiplicity.
Geminal Coupling: Geminal coupling, also known as geminal splitting, is a type of spin-spin coupling that occurs between hydrogen atoms directly attached to the same carbon atom in a molecule. This coupling pattern is observed in the proton nuclear magnetic resonance (1H NMR) spectra and provides valuable information about the molecular structure.
Vicinal Coupling: Vicinal coupling refers to the spin-spin splitting pattern observed in proton nuclear magnetic resonance (1H NMR) spectroscopy when two adjacent, non-equivalent hydrogen atoms interact with each other. This coupling phenomenon provides valuable information about the structure and connectivity of organic molecules.
Spin States: Spin states refer to the quantized angular momentum of an electron or a nucleus, which can exist in discrete energy levels. This concept is particularly important in the context of 1H NMR spectroscopy, as the spin states of hydrogen nuclei determine the splitting patterns observed in the NMR spectra.
Multiplet: A multiplet is a group of closely spaced signals observed in the 1H NMR spectrum of a molecule, resulting from the spin-spin coupling between the proton of interest and the neighboring protons. The pattern of the multiplet provides information about the number and relative positions of the coupled protons.
Pascal's Triangle: Pascal's triangle is a triangular array of numbers where each number is the sum of the two numbers directly above it. It is a fundamental concept in mathematics with applications in various fields, including organic chemistry, particularly in the context of spin-spin splitting patterns in 1H NMR spectra and more complex spin-spin splitting patterns.
Magnetic Equivalence: Magnetic equivalence refers to the phenomenon where chemically distinct protons in a molecule exhibit the same chemical shift in the 1H NMR spectrum due to their identical magnetic environments. This concept is crucial in understanding the spin-spin splitting patterns observed in 1H NMR spectra.
Karplus Equation: The Karplus equation is a mathematical relationship that describes the coupling constant (J) between two vicinal (adjacent) protons in a 1H NMR spectrum. It allows for the prediction of the magnitude of spin-spin splitting patterns observed in proton nuclear magnetic resonance spectroscopy.
Singlet: In the context of nuclear magnetic resonance (NMR) spectroscopy, a singlet is a type of signal observed in the 1H NMR spectrum when a proton is not coupled to any other protons. This means the proton experiences a single, unspilt absorption peak in the spectrum.
Doublet: A doublet is a splitting pattern observed in proton nuclear magnetic resonance (1H NMR) spectroscopy, where a single signal is split into two distinct signals of equal intensity. This splitting occurs due to the spin-spin coupling interaction between a proton and an adjacent proton with a different spin state.
Para-Substituted Benzene: para-Substituted benzene refers to a benzene ring with a substituent group attached in the para position, which is the position directly opposite the hydrogen atom on the ring. This structural arrangement has important implications for the analysis of 1H NMR spectra.
Hertz: Hertz (Hz) is the unit used to measure the frequency of a periodic phenomenon, such as the oscillation of an electromagnetic wave or the vibration of a physical object. It represents the number of cycles or repetitions that occur within one second.