NMR Spectroscopy: Fine Structure and Spin-Spin Coupling

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Questions and Answers

What is the purpose of applying a gradient of magnetic field in Magnetic Resonance Imaging?

To detect only in three dimensions
To detect only in a certain plane (correct)
To detect protons in all directions
To detect protons in the entire body

What is the purpose of changing the plane in Magnetic Resonance Imaging?

To detect protons in the entire body
To detect protons in a certain direction
To detect only in one direction
To reconstruct a three-dimensional image (correct)

What is the result of the reaction in a tubular reactor BrO3 -+ MA+ H+ -> MA-Br + CO2 + H2O?

Production of BrO3
Production of CO2 and H2O
Production of radicals
Production of MA-Br (correct)

What is the significance of light shading in MRI?

Indicates high proton concentration (D) Signup and view all the answers

What is the main difference between NMR and ESR?

ESR uses electron spin, NMR uses nuclear spin (B) Signup and view all the answers

What is the limitation of ESR?

It can only be used for molecules with unpaired electrons (B) Signup and view all the answers

Why are shielding constants not significant in ESR?

Because they do not change much (D) Signup and view all the answers

Why is the derivative of absorption usually shown in ESR?

Because it is a peculiarity of ESR (D) Signup and view all the answers

What is the main reason for the fine structure in the NMR spectrum of a molecule?

The magnetic nucleus contributes to the local field experienced by the other nuclei (B) Signup and view all the answers

What is the effect of spin-spin coupling on the energy levels of a nucleus in an NMR experiment?

It splits the energy levels of the nucleus into multiple levels (C) Signup and view all the answers

What is the expression for the energy difference between the energy levels of a nucleus A in an AX spin system with spin-spin coupling?

$∆E = γ ħ B0(1-σA) + 0.5 hJ$ (D) Signup and view all the answers

What is the intensity ratio of the lines in the NMR spectrum of an AX2 spin system?

1:2:1 (A) Signup and view all the answers

What is the coupling constant for an AX spin system?

JAX (D) Signup and view all the answers

What is the effect of the Karplus equation on the coupling constant J?

It varies the coupling constant J with the angle between the bonds (B) Signup and view all the answers

What is the significance of the 3JHH coupling constant in NMR spectroscopy?

It is used to determine bond angles (A) Signup and view all the answers

What is the effect of hydrogen bonding on the NMR spectrum of a molecule?

It averages out the coupling effects (D) Signup and view all the answers

What is the energy of interaction between an electron and a magnetic field?

E = -gμB mI B0 (D) Signup and view all the answers

What is the spin quantum number of a proton?

I = 1/2 (C) Signup and view all the answers

What is the Larmor frequency?

ν = γ ħ B0/h (A) Signup and view all the answers

What is the intensity of NMR lines proportional to?

B0^2 (D) Signup and view all the answers

What is the chemical shift?

δ = (ν-ν0) / ν0 x 10^6 (D) Signup and view all the answers

What is the shielding constant?

σ = σ(local) + σ(neighbor) + σ(solvent) (C) Signup and view all the answers

What is the energy difference between the two energy levels in a magnetic field?

ΔE = γ ħ B0 (D) Signup and view all the answers

What is the magnetic moment of a nucleus in a magnetic field?

μz = γ ħ mI (D) Signup and view all the answers

What is the nuclear magneton?

μN = eħ/(2mp) (D) Signup and view all the answers

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Study Notes

Magnetic Resonance Imaging (MRI)

Uses a gradient of magnetic field to detect protons in a certain plane
By changing the plane, a 3D reconstruction of proton concentrations is possible
Applications in medical imaging, where large proton concentrations correspond to soft tissue

Electron Spin Resonance (ESR or EPR)

Similar to Nuclear Magnetic Resonance (NMR), but uses the spin of electrons
Only applicable for molecules with unpaired electrons (e.g., O2, radicals, d-metal complexes)
Frequencies fall in the microwave range, with multiple peaks due to interaction with nuclei
Derivative of absorption spectrum is usually shown

Fine Structure of NMR Spectra

Spin-spin coupling: interaction between spins of nucleus A and X contributes to local magnetic field
Energy of interaction depends on magnetic spin quantum numbers and coupling constant J
In an A-X spin system, resonance frequencies of A and X nuclei are affected by spin-spin coupling
Two lines are observed instead of one, with frequency difference J

Fine Structure of AX2, AX3, and AXN Spin Systems

AX2: doublet, with intensity ratio 1:1
AX3: triplet, with intensity ratio 1:2:1
AXN: Pascal's triangle determines intensities

Coupling Constant (J)

Notation: NJAX, representing coupling between nuclei A and X over N bonds
Depends on bond angle and can be used to determine bond angles
Karplus Equation: J = A + B cos ϕ + C cos 2ϕ

Magnetic Resonance Transition

Energy levels in presence of magnetic field
Related to interaction of magnetic field with spin angular momentum
One of the most commonly used spectroscopic techniques

Nuclear Spin Properties

Spin quantum number, I, inherent property of subatomic particles
I = 1/2 for electron, neutron, and proton
I = 0 for 16O, 32S, and 12C

Nuclei in Magnetic Fields

Energy of interaction depends on extent of magnetic field, magnetic quantum number, and type of nucleus
Magnetic moment around z-axis: μz = γ ħ mI

Electron Spin Resonance (ESR)

Interaction between electron spin and magnetic field
Energy of interaction depends on g-factor, Bohr magneton, and magnetic field
Energy difference: ΔE = ge μB B0

Nuclear Magnetic Resonance (NMR)

Interaction between nuclear spin and magnetic field
Energy of interaction depends on γ, ħ, and magnetic field
Energy separation: ΔE = γ ħ B0

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