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.
What is the purpose of the spectrometer frequency?
To encourage nuclei to become excited
What happens during decoupling in NMR
Coupling nucleus irradiated at its resonance frequency -> rapid transition between all spin states -> other nuclei sees it as 0 spin (average) -> decoupled nucleus does not cause peaks to split!
Notation for a decoupled 11B NMR spectra with BH4-
11B{1H} NMR
What happens to spectrum where only 10% nuclei are spin active?
One singlet of 90% in middle and 2 smaller SATELLITE PEAKS of 5% on either side.
If nucleus has 2 isotopes that are active, which one gives the largest coupling constant of the satellite peaks?
Heavier nuclei TEND to give bigger coupling constants but depends on magnetogyric ratio.
Which coupling constant is bigger in B NMR if A-B-C and A-B is covalent and B-C is dative
A-B will be the main split and bigger coupling constant
Does BX3 or BX4 have a higher ppm
BX3 is more deshielded to higher ppm
Why does BF3 not follow the halide sequency of the most shielded
F has similar sized full pz orbital as B’s empty pz orbital -> returns some e- density in sigma bonding -> greater expected shielding
What happens if 2 nuclei of I=3/2 get coupled
1:2:3:4:3:2:1 as each 1:1:1:1 gets split into 4 other ones
What is quadrupole moment + its symbol
Measure of distortion from spherical charge distribution, eQ
What is Heisenberg Uncertainty Principle
ΔE*Δt >= h/4π where ΔE is uncertainty in energy of state and Δt is uncert in time specified
Why does I=1/2 give well defined spectra but others may not
I=1/2 has a long lived ground and excited state. Therefore Δt is large so ΔE is small so well defined energies -> sharp line in spectrum. For quadrupolar, it transitions through spin states rapidly so Δt is very small so it gives a larger ΔE and broader and shorted peaks in spectrum since the area stays the same
When can quadrupolar nuclei still give sharp peaks
A symmetrical molecule so there is no field gradient (dipole) at the nucleus
When will quadrupolar nuclei not be seen in spectra
Large eQ + unsymmetrical environ. Also if measuring I=1/2 spectra and has a quadrupolar neighbour
What does it mean if a molecule is fluxional
Equatorial and axial atoms are seen as the same as they exchange rapidly giving a different spectra because NMR only sees an average.
What happens to the spectra as you raise the temperature for a fluxional molecule
- Loss of fine structure. 2. Coalescence: it collapses to a single broad peak. 3. Sharpening to a single narrow peak
Why does fluxionality affect NMR
It is a slow technique so it only sees averaged out set of atoms
