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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

Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful tool for determining molecular structures. It reveals the number and types of hydrogen atoms in a compound, their chemical environments, and how they're connected to each other.

1H NMR spectroscopy focuses on hydrogen atoms, providing key information through chemical shifts, splitting patterns, and integration values. This data helps identify functional groups, distinguish isomers, and confirm reaction products, making it essential for organic chemistry analysis.

1H NMR Spectroscopy

Interpretation of 1H NMR spectra

  • Provides structural information about organic compounds
    • Number of signals corresponds to number of unique hydrogen environments
    • Integration values indicate relative number of hydrogens in each environment (1:2:3 ratio)
    • Splitting patterns reveal number of neighboring hydrogens
      • Singlet (s) no neighboring hydrogens
      • Doublet (d) one neighboring hydrogen (ethanol -OH)
      • Triplet (t) two neighboring hydrogens (ethyl acetate -CH2-)
      • Quartet (q) three neighboring hydrogens (ethyl acetate -CH3)
      • Multiplet (m) complex splitting pattern or overlapping signals (benzene ring)
  • Chemical shift ($\delta$) values in ppm reflect electronic environment of hydrogens
    • Electronegative atoms or groups cause downfield shifts to higher $\delta$ (aldehydes, carboxylic acids)
    • Electron-donating groups cause upfield shifts to lower $\delta$ (alkyl groups)
    • Aromatic and vinylic hydrogens typically appear at $\delta$ 6-8 ppm (benzene, styrene)
    • Aliphatic hydrogens usually resonate at $\delta$ 0-5 ppm (cyclohexane, fatty acids)
  • Comparing spectra of reactants and products determines reaction products
    • Disappearance or appearance of signals indicates changes in hydrogen environments (alcohol to ketone)
    • Shifts in peak positions reflect changes in electronic environment (ester hydrolysis)
    • Integration values provide information about stoichiometry and yield (1:1 adduct formation)

Analysis of chemical shifts and patterns

  • Chemical shift trends help identify functional groups
    • Alcohols and carboxylic acids have exchangeable protons with variable shift (phenol, acetic acid)
    • Amines have exchangeable protons that may appear as broad singlets (aniline, piperidine)
    • Aldehydes show a characteristic downfield singlet around $\delta$ 9-10 ppm (benzaldehyde)
    • Aromatic protons appear as multiplets in the $\delta$ 6-8 ppm range (toluene, naphthalene)
  • Coupling constants ($J$) provide information about dihedral angles and stereochemistry
    • Vicinal coupling ($^3J$) depends on dihedral angle between protons
      • Large $^3J$ ~ 12-14 Hz indicates anti orientation (trans alkenes)
      • Small $^3J$ ~ 0-2 Hz suggests gauche orientation (cis alkenes)
    • Geminal coupling ($^2J$) between diastereotopic protons can reveal chiral centers (lactic acid)
  • Peak multiplicities and integration values determine substituents and symmetry
    • Highly symmetric molecules have fewer unique hydrogen environments (p-xylene vs o-xylene)
    • Substituents identified by characteristic splitting patterns and integrations (t-butyl group)

Applications of NMR spectroscopy

  • Distinguishes between constitutional isomers with different connectivity and unique spectra
    • Peak positions, multiplicities, and integrations differ between isomers (butanol isomers)
    • Symmetry differences lead to variations in number of signals (ortho vs para-disubstituted benzenes)
  • Stereoisomers may have similar spectra but exhibit differences in peak splitting
    • Diastereomers have distinct chemical and magnetic environments
      1. Chemical shift differences distinguish diastereomers (meso vs dl-tartaric acid)
      2. Unique coupling constants differentiate diastereomers (cis vs trans-decalin)
    • Enantiomers have identical spectra in achiral solvents (D vs L-alanine)
      • Chiral shift reagents or solvents can induce differences in enantiomer spectra (Mosher's acid)
  • Analyzing product NMR spectra confirms regiochemistry of reactions
    • Selective disappearance or shift of signals indicates site of reaction (Markovnikov addition)
    • Integration values confirm relative amounts of regioisomers formed (electrophilic aromatic substitution)
    • Coupling patterns provide insight into neighboring substituents and stereochemistry (diastereoselective reduction)

NMR Spectroscopy Fundamentals

  • Shielding and deshielding effects influence chemical shifts
    • Electron-withdrawing groups cause deshielding, resulting in downfield shifts
    • Electron-donating groups increase shielding, leading to upfield shifts
  • Tetramethylsilane (TMS) is used as a reference compound for chemical shift calibration
  • Deuterated solvents are employed to avoid interference from solvent proton signals
  • The Larmor frequency determines the resonance condition for nuclei in the magnetic field

Key Terms to Review (36)

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.
Constitutional isomers: Constitutional isomers are compounds that have the same molecular formula but differ in the sequence in which their atoms are connected. These variations lead to molecules with distinct physical and chemical properties, despite having the same numbers of each type of atom.
Electrophilic aromatic substitution: Electrophilic aromatic substitution is a chemical reaction in which an atom, typically hydrogen, attached to an aromatic system, such as benzene, is replaced by an electrophile. This process preserves the aromaticity of the compound while introducing a functional group.
Electrophilic Aromatic Substitution: Electrophilic aromatic substitution is a fundamental organic reaction in which an electrophile (a species that is attracted to electrons) replaces a hydrogen atom on an aromatic ring, resulting in the formation of a new carbon-electrophile bond. This reaction is crucial in understanding the behavior and reactivity of aromatic compounds, which are prevalent in many organic molecules and have widespread applications.
Constitutional Isomers: Constitutional isomers are a type of structural isomerism where molecules have the same molecular formula but differ in the connectivity or arrangement of their atoms. This concept is essential in understanding the properties and behavior of organic compounds across various topics in chemistry.
Enantiomers: Enantiomers are a pair of stereoisomers that are non-superimposable mirror images of each other. They have the same molecular formula and connectivity, but differ in the spatial arrangement of their atoms, resulting in a unique handedness or chirality.
Stereoisomers: Stereoisomers are molecules that have the same molecular formula and connectivity, but differ in the three-dimensional arrangement of their atoms in space. This spatial arrangement of atoms leads to different physical and chemical properties, even though the atoms are connected in the same way.
Diastereomers: Diastereomers are a type of stereoisomer that have the same molecular formula and connectivity, but differ in their three-dimensional arrangement of atoms in space. They are not mirror images of each other and do not exhibit the property of chirality.
Markovnikov Addition: Markovnikov addition is a fundamental organic chemistry concept that describes the regiochemical outcome of the addition of a polar molecule, such as hydrogen halides or water, to an unsymmetrical alkene or alkyne. It predicts the formation of the more stable carbocation intermediate, leading to the addition of the electrophilic component to the carbon atom that can best stabilize the resulting positive charge.
Larmor Frequency: Larmor frequency, also known as the nuclear precession frequency, is a fundamental concept in nuclear magnetic resonance (NMR) spectroscopy. It describes the rate at which the magnetic moments of nuclei, such as hydrogen (1H) or carbon (13C), precess or rotate around an applied magnetic field.
Shielding: Shielding is a phenomenon that occurs in nuclear magnetic resonance (NMR) spectroscopy, where the applied magnetic field interacts with the electrons surrounding a nucleus, altering the effective magnetic field experienced by that nucleus. This shielding effect influences the chemical shift, a crucial parameter in NMR analysis.
Deshielding: Deshielding is a phenomenon in nuclear magnetic resonance (NMR) spectroscopy where the magnetic environment of a nucleus is altered, causing it to experience a weaker shielding effect and resulting in a change in the observed chemical shift. This concept is central to understanding the nature of NMR absorptions, chemical shifts, and the interpretation of 1H and 13C NMR spectra.
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.
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.
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.
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.
Tetramethylsilane (TMS): Tetramethylsilane (TMS) is a chemical compound commonly used as a reference standard in proton nuclear magnetic resonance (1H NMR) spectroscopy. It is a volatile, colorless liquid with a characteristic silicone-like odor. TMS plays a crucial role in the interpretation and analysis of 1H NMR spectra, particularly in the context of integration of absorptions and the various uses of 1H NMR spectroscopy.
Ppm: ppm, or parts per million, is a unit of measurement used to express the concentration or amount of a specific substance within a larger substance or mixture. It is commonly used in various contexts, including in the analysis of chemical shifts in nuclear magnetic resonance (NMR) 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.
1H NMR Spectroscopy: 1H NMR (Proton Nuclear Magnetic Resonance) Spectroscopy is an analytical technique used to identify and characterize organic compounds by detecting the magnetic properties of hydrogen (proton) nuclei within a molecule. It provides valuable information about the structure, connectivity, and environment of hydrogen atoms in a sample.
Aliphatic Hydrogens: Aliphatic hydrogens refer to the hydrogen atoms bonded to carbon atoms in straight-chain or branched, non-aromatic organic compounds. These hydrogens are important in the context of 1H NMR spectroscopy, as their chemical shifts and coupling patterns provide valuable information about the structure and environment of organic molecules.
Integration Values: Integration values refer to the quantitative information obtained from the integrated signals in a proton nuclear magnetic resonance (1H NMR) spectrum. These values provide insights into the relative amounts of different hydrogen environments within a molecule, which is crucial for understanding the structure and composition of organic compounds.
Coupling Constants: Coupling constants, in the context of nuclear magnetic resonance (NMR) spectroscopy, refer to the quantitative measure of the interaction between nuclear spins within a molecule. They provide valuable information about the connectivity and spatial arrangement of atoms within a compound, which is crucial for understanding its structure and properties.
Aromatic Hydrogens: Aromatic hydrogens refer to the hydrogen atoms attached to the carbon atoms in an aromatic ring structure. These hydrogens exhibit unique chemical and spectroscopic properties due to the delocalized pi-electron system in the aromatic compound.
Exchangeable Protons: Exchangeable protons refer to hydrogen atoms that are attached to highly electronegative atoms, such as oxygen, nitrogen, or sulfur, and can readily exchange with protons from the solvent, typically water. These protons are easily displaced and can be detected in 1H NMR spectroscopy.
Downfield Shifts: Downfield shifts refer to the phenomenon in proton nuclear magnetic resonance (1H NMR) spectroscopy where the resonance signals of protons are observed at higher chemical shift values, indicating a deshielding effect on the protons. This is an important concept in the context of the various uses of 1H NMR spectroscopy.
Vinylic Hydrogens: Vinylic hydrogens refer to the hydrogen atoms attached to the carbon-carbon double bond in alkene molecules. These hydrogens exhibit distinct chemical and spectroscopic properties that are crucial in understanding the uses of 1H NMR spectroscopy.
Upfield Shifts: Upfield shifts refer to the phenomenon in proton nuclear magnetic resonance (1H NMR) spectroscopy where the signals for certain protons appear at a higher frequency (lower ppm) on the spectrum compared to their expected positions. This shift is caused by the shielding effect of the surrounding electrons, which reduces the effective magnetic field experienced by the protons.
Diastereoselective Reduction: Diastereoselective reduction is a type of organic reaction where a specific stereoisomer (diastereomer) is selectively formed from a starting material containing multiple stereogenic centers. This process is crucial in the synthesis of complex organic compounds with desired stereochemical configurations.
Deuterated Solvents: Deuterated solvents are organic compounds where one or more of the hydrogen atoms have been replaced with the heavier isotope of hydrogen, known as deuterium. These modified solvents are commonly used in nuclear magnetic resonance (NMR) spectroscopy, particularly in the context of 1H NMR spectroscopy.
Splitting Patterns: Splitting patterns refer to the characteristic ways in which the signals from hydrogen atoms in a molecule are split or divided in the 1H NMR spectrum. This splitting is a result of the magnetic interactions between neighboring hydrogen atoms, providing valuable information about the molecular structure.
Chiral Shift Reagents: Chiral shift reagents are compounds used in nuclear magnetic resonance (NMR) spectroscopy to help determine the stereochemistry of organic molecules. They interact with the analyte in a way that separates the signals for diastereomeric protons, allowing for the identification of the relative configuration of stereogenic centers.