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Chapter 8

Interpreting Nuclear Magnetic Resonance Spectra

Chemical Shift: Internal References and Solvent Effects
Chemical Shift: Internal References and Solvent Effects
Precise measurement of the absolute absorption frequencies of nuclei in a sample is difficult. To overcome this, a standard internal reference compound is ...
NMR Spectroscopy: Chemical Shift Overview
NMR Spectroscopy: Chemical Shift Overview
The position of the absorption signal of a sample is reported relative to the position of the signal of tetramethylsilane (TMS), which is added as an ...
Proton (¹H) NMR: Chemical Shift
Proton (¹H) NMR: Chemical Shift
Organic molecules primarily contain carbon and hydrogen atoms. While all the hydrogen isotopes are NMR-active, protium or hydrogen-1 is the most abundant. ...
Inductive Effects on Chemical Shift: Overview
Inductive Effects on Chemical Shift: Overview
The protons in unsubstituted alkanes are strongly shielded with chemical shifts below 1.8 ppm. Methine, methylene, and methyl protons appear at ...
π Electron Effects on Chemical Shift: Overview
π Electron Effects on Chemical Shift: Overview
An applied magnetic field causes loosely bound π-electrons in organic molecules to circulate, producing a local or induced diamagnetic field over a ...
π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds
π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds
In aromatic compounds, such as benzene, the circulation of (4n + 2) π-electrons sets up a diamagnetic or diatropic ring current around the perimeter ...
¹H NMR Chemical Shift Equivalence: Homotopic and Heterotopic Protons
¹H NMR Chemical Shift Equivalence: Homotopic and Heterotopic Protons
Protons in identical electronic environments within a molecule are chemically equivalent and have the same chemical shift. The replacement test is a ...
¹H NMR Chemical Shift Equivalence: Enantiotopic and Diastereotopic Protons
¹H NMR Chemical Shift Equivalence: Enantiotopic and Diastereotopic Protons
Replacing each alpha-hydrogen in chloroethane by bromine (or a different functional group) yields a pair of enantiomers. Such protons are called prochiral ...
¹H NMR Signal Integration: Overview
¹H NMR Signal Integration: Overview
The intensity of a signal, which can be represented by the area under the peak, depends on the number of protons contributing to that signal. The area ...
NMR Spectroscopy: Spin–Spin Coupling
NMR Spectroscopy: Spin–Spin Coupling
The spin state of an NMR-active nucleus can have a slight effect on its immediate electronic environment. This effect propagates through the intervening ...
¹H NMR Signal Multiplicity: Splitting Patterns
¹H NMR Signal Multiplicity: Splitting Patterns
When protons A and X are coupled, their nuclear spin energy levels are slightly modified. This is because the energy required to excite proton A to a spin ...
Interpreting &sup1;H NMR Signal Splitting: The (<em>n</em> + 1) Rule
Interpreting ¹H NMR Signal Splitting: The (n + 1) Rule
In the AX proton spin system, proton A can sense the two spin states of a coupled proton X, resulting in a doublet NMR signal with two peaks of equal ...
Spin&ndash;Spin Coupling Constant: Overview
Spin–Spin Coupling Constant: Overview
In bromoethane, the three methyl protons are coupled to the two methylene protons that are three bonds away. In accordance with the n+1 rule, the signal ...
Spin&ndash;Spin Coupling: One-Bond Coupling
Spin–Spin Coupling: One-Bond Coupling
Coupling interactions are strongest between NMR-active nuclei bonded to each other, where spin information can be transmitted directly through the pair of ...
Spin&ndash;Spin Coupling: Two-Bond Coupling (Geminal Coupling)
Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)
Two NMR-active nuclei bonded to a central atom can be involved in geminal or two-bond coupling. Geminal coupling is commonly seen between diastereotopic ...
Spin&ndash;Spin Coupling: Three-Bond Coupling (Vicinal Coupling)
Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)
Vicinal or three-bond coupling is commonly observed between protons attached to adjacent carbons. Here, nuclear spin information is primarily transferred ...
&sup1;H NMR: Long-Range Coupling
¹H NMR: Long-Range Coupling
The coupling interactions of nuclei across four or more bonds are usually weak, with J values less than 1 Hz. While these are usually not observed in ...
&sup1;H NMR: Complex Splitting
¹H NMR: Complex Splitting
A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one ...
&sup1;H NMR: Pople Notation
¹H NMR: Pople Notation
The Pople nomenclature system classifies spin systems based on the difference between their chemical shifts. Coupled spins are denoted by capital letters ...
&sup1;H NMR: Interpreting Distorted and Overlapping Signals
¹H NMR: Interpreting Distorted and Overlapping Signals
Spin systems where the difference in chemical shifts of the coupled nuclei is greater than ten times J are called first-order spin systems. These nuclei ...
Carbon-13 (&sup1;&sup3;C) NMR: Overview
Carbon-13 (¹³C) NMR: Overview
Carbon-13 is a naturally occurring NMR-active isotope of carbon with a low natural abundance of 1.1%. In contrast, carbon-12 is the most abundant isotope ...
&sup1;&sup3;C NMR: &sup1;H&ndash;&sup1;&sup3;C Decoupling
¹³C NMR: ¹H–¹³C Decoupling
The probability of having two carbon-13 atoms next to each other is negligible because of the low natural abundance of carbon-13. Consequently, peak ...
&sup1;&sup3;C NMR: Distortionless Enhancement by Polarization Transfer (DEPT)
¹³C NMR: Distortionless Enhancement by Polarization Transfer (DEPT)
When proton-coupled carbon-13 spectra are simplified by a broadband proton decoupling technique, structural information about the coupled protons is lost. ...
Other Nuclides: <sup>31</sup>P, <sup>19</sup>F, <sup>15</sup>N NMR
Other Nuclides: 31P, 19F, 15N NMR
The NMR spectroscopy of spin-half nuclei such as nitrogen-15, fluorine-19, and phosphorus-31 has wide-ranging applications in chemistry and biology. ...
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