Scalar coupling L5

Question 1

What is resonant frequency of a nucleus affected by

Answer 1

Spins of nearby nuclei

Question 2

What is the correlation of electrons and nuclear spins mediated by

Answer 2

Fermi contact interactions

Question 3

Describe what the effect of coupling is diminished by

Answer 3

Diminishes rapidly with distance, ie through the number of bonds. Coupling across more than e bonds is unusual except in conjugated/aromatic compounds

Question 4

What is scalar coupling in NMR spectroscopy?

Answer 4

Scalar coupling refers to the interaction between nuclear spins that occurs through chemical bonds.

Question 5

How is scalar coupling transmitted?

Answer 5

Scalar coupling is transmitted through the bonding electrons between nuclei.

Question 6

What is the importance of scalar coupling constants?

Answer 6

Scalar coupling constants provide information about the bonding environment, connectivity, and dihedral angles between coupled nuclei.

Question 7

What are the units for scalar coupling constants?

Answer 7

Scalar coupling constants are measured in Hertz (Hz).

Question 8

What determines the magnitude of scalar coupling constants?

Answer 8

The magnitude depends on the number of bonds between coupled nuclei, bond angles, and the hybridization of the atoms involved.

Question 9

What factors influence the magnitude of the J-coupling constant?

Answer 9

Bonding distance between the coupled nuclei
Hybridization of the atoms involved
Electron density around the nuclei.
Dihedral angle for multi-bond couplings

Question 10

Describe how coupling to non equivalent nuclei occurs

Answer 10

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

Question 11

What is 1J

Answer 11

1J represents the one-bond scalar coupling constant between two nuclei directly bonded by a single bond. Usually positive

Question 12

What is 2J

Answer 12

2J (Two-Bond Coupling)refers to the scalar coupling constant between nuclei that are separated by two chemical bonds. The coupling occurs between non-adjacent nuclei 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

Question 13

What is 3J

Answer 13

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

Question 14

What is J independent of

Answer 14

B0
It is the effect of on nucleus on another

Question 15

Define spectral dispersion

Answer 15

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.

Question 16

How do you increase spectral dispersion

Answer 16

Using a higher field spectrometer

Question 17

How can thou separate overlapping multiplets

Answer 17

Higher B0 can separate overlapping multiplets

Question 18

How does the number of bonds separating nuclei affect J-coupling?

Answer 18

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.

Question 19

How does the nature of bonds separating nuclei affect the magnitude of J-coupling?

Answer 19

Single bonds typically result in stronger coupling while double bonds and triple bonds lead to weaker coupling. The coupling strength also depends on factors like bond length. Double and triple bonds reduce the overlap of electron clouds, weakening the transmission of the coupling.

Question 20

How does the dihedral angle affect the magnitude of J-coupling?

Answer 20

J-coupling is strongest at 0° or 180° (anti-periplanar or syn-periplanar) and weakest at 90° (gauche), due to changes in orbital overlap between the coupled nuclei.

Question 21

What is the Karplus relationship?

Answer 21

The Karplus relationship shows that 3J-coupling depends on the dihedral angle: it is strongest at 0° or 180° and weakest at 90°.

Question 22

Describe nomenclature of spin systems

Answer 22

Letters used to indicate relative chemical shifts of nuclei. The closed the letter the more similar the nuclei

Question 23

Describe the AX spin system

Answer 23

Two nuclei with very different chemical shifts

Question 24

Describe the AM spin system

Answer 24

Nuclei are well separated in chemical shifts of but are more similar than AX

Question 25

Describe the AB spin systems

Answer 25

Two nuclei with very similar chemical shifts

Question 26

Describe AX2 spin system

Answer 26

One nucleus A couples to two X nuclei

Question 27

What is tenting in NMR spectroscopy?

Answer 27

Tenting refers to a distortion of NMR signals caused by the interaction of scalar couplings and magnetic fields, which results in an asymmetrical pattern resembling a tent. This typically occurs in systems with strong couplings or non-equivalent nuclei.

Question 28

What is first-order spectra in NMR?

Answer 28

First-order spectra in NMR occur when the coupling constants are significantly different in magnitude, leading to simple, well-resolved peaks.

Question 29

What is second order spectra

Answer 29

Second-order spectra in NMR occur when coupling constants are similar in magnitude causing complex interactions between the peaks. This results in splitting patterns that are not simple and require correction for overlap and distortion of signals