NMR Flashcards
What happens in an applied magnetic field for nucleus
It spins on its axis. In mag field, the axis of rotation will precess around the mag field.
What is the frequency of precession called
Resonance or Larmor frequency
Nuclear magnetic moment equation
μ = Magnetogyric ratio (γ) * Spin quantum number (I)
What is magnetogyric ratio. Meaning if negative or positive
A number given for every nucleus. Signs determines anti/clockwise precession. Sort of how fast its going
What does nuclear spin quantum number I result in
The orientation of nuclear spin
Equation which quantises orientation of magnetic moment in z azis
μz prop to m*(h/2π) where m = +I .. -I and h is Plancks
How many allowed nuclear spin states of μz are there
2I + 1 where I is the nuclear spin
What happens to μz orientations when magnetic field is absent
All have same energy ie degenerate
Energy of nuclei in applied field equation
E = γBm*h/2π where m is +I .. -I
How to work out Larmor frequency using difference in energy of spin states
ΔE = hν = γBh/2π so frequency v = γB/2π
What is the importance of the Larmor frequency of a nucleus in NMR
Irradiation causes transitions between nuclear spin energy levels
What is the Zeemen effect
The splitting of energy levels/spin states in a magnetic field
Effective magnetic field equation
Beff = B0(1 - σ) where σ is the shielding constant
What causes the shielding constant
Electrons opposing the induced B due to their motion
Where can e- reinforce applied B (deshielding)
Benzene ring
Nomenclature for higher frequency NMR
Desheilded and downfield!
Why does F cause more desheilding than I in 1H NMR?
Fs electronegativity pulls e- density away from carbon and H bond
What is the coupling constant
Written like 1JAX where 1 is how many bonds and AX are the 2 nuclei. Measured in Hz. Same if looking at X or A spectrum.
Amount of NMR splitting peaks
2nI + 1 where n is how many neighbours the nucleus sees. I = spin
What can affect coupling constant magnitude?
Intervening bonds, delocalised orbitals, heavy nuclei
Find coupling constant
Difference in ppm betw 2 peaks x resonance frequency in MHz gives constant in Hz
AMX system for X. Explain its spectrum if I=1/2
X sees M first -> doublet. Then X sees A so causes original lines to split into doublets again. In total 4 lines where coupling constant is smaller for second couplet. Called doublet of doubles.
In H-P-F, would HP or PF have the bigger coupling constant
PF because coupling occurs through intervening electrons. F has more than H.
Splitting for AMX2 systems
There will be either a triplet of doublets or doublet of triplets depending on what the chosen molecule sees first. If A sees M first then it will be a doublet split into triplets aka a doublet of triplets.