Scalar coupling L5 Flashcards
What is resonant frequency of a nucleus affected by
Spins of nearby nuclei
What is the correlation of electrons and nuclear spins mediated by
Fermi contact interactions
Describe what the effect of coupling is diminished by
Diminishes rapidly with distance, ie through the number of bonds. Coupling across more than 3 bonds is unusual except in conjugated/aromatic compounds
What is scalar coupling in NMR spectroscopy?
Scalar coupling refers to the interaction between nuclear spins that occurs through chemical bonds.
How is scalar coupling transmitted?
Scalar coupling is transmitted through the bonding electrons between nuclei.
What is the importance of scalar coupling constants?
Scalar coupling constants provide information about the bonding environment, connectivity, and dihedral angles between coupled nuclei.
What are the units for scalar coupling constants?
Scalar coupling constants are measured in Hertz (Hz).
What determines the magnitude of scalar coupling constants?
The magnitude depends on the number of bonds between coupled nuclei, bond angles, and the hybridization of the atoms involved.
What factors influence the magnitude of the J-coupling constant?
Bonding distance between the coupled nuclei
Hybridization of the atoms involved
Electron density around the nuclei.
Dihedral angle for multi-bond couplings
Describe how coupling to non equivalent nuclei occurs
occurs when nuclei are in different chemical environments, leading to different J-coupling constants. This results in distinct splitting patterns in the NMR spectrum, doublets of doublets
What is 1J
1J represents the one-bond scalar coupling constant between two nuclei directly bonded by a single bond. Usually positive
What is 2J
2J (Two-Bond Coupling)refers to the scalar coupling constant between nuclei that are separated by two chemical bonds. In NMR spectra, 2J coupling leads to signal splitting, often resulting in patterns like doublets of doublets (dd). j is usually negative and often quite small
What is 3J
3J (Three-Bond Coupling) refers to the scalar coupling between nuclei separated by three chemical bonds. 3J coupling results in multiplet splitting in NMR spectra. J is usually positive and smaller that 1J and 2J
What is J independent of
B0
It is the effect of on nucleus on another
Define spectral dispersion
Spectral dispersion in NMR refers to the spread of nuclear resonance signals across a range of chemical shifts, allowing for the differentiation of nuclei in various chemical environments.
How do you increase spectral dispersion
Using a higher field spectrometer
How can thou separate overlapping multiplets
Higher B0 can separate overlapping multiplets
How does the number of bonds separating nuclei affect J-coupling?
As the number of bonds separating nuclei increases, the J-coupling constant decreases. One-bond coupling is the strongest followed by two-bond coupling, and three-bond coupling. Coupling becomes weaker with more bonds separating the nuclei.
How does the nature of bonds separating nuclei affect the magnitude of J-coupling?
J value is typically larger in conjugated systems
How does the dihedral angle affect the magnitude of J-coupling?
J-coupling is strongest at 0° or 180° (anti-periplanar or syn-periplanar) and coupling = 0 at 90° (gauche), due to changes in orbital overlap between the coupled nuclei.
What is the Karplus relationship?
The Karplus relationship shows that 3J-coupling depends on the dihedral angle: it is strongest at 0° or 180° and weakest at 90°.
Describe nomenclature of spin systems
Letters used to indicate relative chemical shifts of nuclei. The closed the letter the more similar the nuclei
Describe the AX spin system
Two nuclei with very different chemical shifts
Describe the AM spin system
Nuclei are well separated in chemical shifts of but are more similar than AX
Describe the AB spin systems
Two nuclei with very similar chemical shifts
Describe AX2 spin system
One nucleus A couples to two X nuclei
What is tenting in NMR spectroscopy?
Tenting refers to a distortion of NMR signals caused by the interaction of scalar couplings and magnetic fields. This typically occurs in systems with strong couplings or non-equivalent nuclei.
What is first-order spectra in NMR?
First-order spectra in NMR occur if two coupled nuclei have well separated chemical shifts. Spectra are simple.
What is second order spectra
If two coupled nuclei have similar chemical shifts then the spectra are not simple. This will result in tenting in the spectra where the signals tent towards coupled nuclei
What are fermi contact interactions
magnetic interactions between electrons and the nucleus. It is only exhibited by electrons in the s orbital. since only s orbitals have probability at the nucleus
What is the equation for the number of lines in a multiplet
= 2nI+1
Where n=number of equivalent nuclei with spin I coupled to observed nucleus
For coupling to an I=1/2 nuclei what fo line intensities depend on
depend on Pascal’s triangle
How does the spatial arrangement of nuclei effect the value of J
trans isomers usually have the higher value of J followed by the cis isomers then the gem isomer
