spin resonance spec Flashcards
electron spin quantum number letter and value
s
1/2
how many spin states do electrons have
2
spin magnetic quantum number sign and values
ms
+1/2
-1/2
what happens to the spins of electrons when they’re paired up
the spins cancel out
neutrons and protons also have spins which means
nuclei also have spin quantum numbers
what is the nuclear spin quantum number and it’s value
I aka i
1/2
what values can I (nuclear spin quantum number) have
many values
nuclear spin magnetic quantum number value and sign
mi
+ 1/2
- 1/2
whenever the word has magnetic in it what should we had to the 1/2 values
+ and - signs
what does a spinning charge have
an associated magnetic field
it acts as a small magnet
imagine a blurry electron arrow rotating/ spinning. the blur is the associated magnetic field.
when a spin is placed in an external magnetic field, what is allowed
2 different states are allowed
one with higher energy - aligns with magnetic field
one with lower energy - is against the magnetic field
what does the external magnetic field do to the energies of the spin stated
caused them to split
what affect on splitting does a stronger magnetic field do
it splits them even more girl!!!
they’re more spaced out shelves of a book shelf, higher steps to climb.
the difference between the 2 is 🔺E
what about the energy of electrons when there is no external magnetic field
they are degenerate and at the same levels as there has been no splitting depending on if the nuclei or electron spin aligns or goes against the external magnetic field.
what is important abojt the electrons or nuclei that can split when an external magnetic field is added
they must have unpaired electrons or else the spins will cancel out.
what is a flip between the states
promotion of a nucleus of electron to a higher spin state where it then falls back to a lower state.
what happens in spin resonance spec
compound is placed in a strong external magnetic field.
if nuclei (neu + pro) or electrons are unpaired, they get separated into their spin states of diff energy.
what do the diff spin state energy’s depend on
if the electrons or nucleis magnetic spin aligns or goes against the external magnetic field
what affects the energy change aka distance between the 2 spin state energies
the strength or the magnetic field.
what EMS waves correspond to the change in energy between spin stages for nuclei
radio waves
what EMS wave corresponds to the energy change between electron spin states
MICROWAVES
what are the two types of spin resonance spec
electron spin resonance ESR
nuclear magnetic resonance NMR
what is electron spin resonance spec used for
studying species with unpaired electrons
eg: radicals
what is NMR used for
very useful in structure determination - can find functional groups
1H and 13C application (unpaired electrons)
what nuclear spin value must a nucleus have in order to show energy differences in a magnetic field
I value aka nuclear spin value must be larger than 0
aka +1/2
which can form +1/2 and -1/2 in a magnetic field
what spin do protons, neutrons and electrons all have
1/2 spin 💞💞
6 neutrons and 6 protons give the nuclear spin quantum value of …. and why
I = 0
they pair up and their spins cancel out
in 13C there is an unpaired neutron (7 total) meaning it has a nuclear spin quantum number of … meaning it is a
I = 1/2
magnetically active nucleus —> gives rise to signals in NMR spectra.
what is unfortunate about 13 C
although it is magnetically active, the proportion of 13C compared to 12C is incredibly low (1.1%)
what is special about 2H deuterium
it has an unpaired proton : 1/2
it has an unpaired neutron : 1/2
together it has I = 1
🔺E in magnetic spec issss
the energy difference between the 2 split spins
this gets bigger when u increase the strength of the magnetic field.
what is the boltzmann distribution equation
N upper/ n lower = g up / g low
x e ^( -🔺E / Kb x Temp)
E is in J, Kb is in J, temp is in K
when the ratio between n up and n low is near 1 what does this mean
the population of the two energy levels is almost the same.
this means the probability of observing a transition is low. sensitive equipment is needed to see this.
against the magnetic field
high energy
with the external magnetic field
lowest energy
what does quantum theory show about the nuclear spins
they can only align with the external magnetic field at certain, quantum angles.
eg imagine a spinning door, u cannot pass through whenever, u need to wait till it’s at a good angle. think of a group of friends being split into single ppl and walking through the door at the right angle needed
larmor frequency
the nuclear spin precesses around the direction of the magnetic field.
frequency of this precession is the lamour frequency.
spinner spinning shoulder to shoulder to magnetic field 🩰⬆️
what happens when the EMS matches the lamor frequency
the nucleus comes into resonance with the EMS And absorbs energy
this promotes the nucleus to a higher energy spin state.
when energy is removed, it relaxes back to its original state and releases energy into the solvent.
how is nmr spectra recorded
sample is placed in a tube in a strong magnetic field.
solvent is used to dissolve sample but this doesn’t give an NMR signal (no unpaired nuc or electrons?)
EMS radiation irridates it and excites some nuclei to a higher energy state. energy given out when the nuclei returns to the ground state is detected on a sensitive radio frequency receiver.
equation for the energy difference between 2 spin states of I = 1/2
🔺E = yBh/ 2🥧
change in energy J = ( magnetogyric ratio x strength of mag field x planks) /// 2 🥧
relaxation time depends on
environment of the nucleus.
gives good info
time it takes for excited nucleus to return to ground state.
y (magnetogyric ratio) is different in what
different to diff nuclei
what does a different y (magnetogyric ratio) mean in terms of 🔺E
🔺E depends on what nucleus it is as the magentogyric ratio is different for each nuclei.
the larger the mag field, the larger the 🔺E between diff spin states meaningggg
higher EMS energy is needed to promote a transition.
magnet in NMR spec is usually described as an
operating frequency
frequency of the radiation required to bring a 1H nucleus to resonance when it experiences the full applied magnetic field.
absorbed radiation of a nuclei is given by the energy difference between two energy spin states why is this
EMS can only be absorbed if it matches the energy difference between the 2 levels
the energy absorbed = the energy gap between the two levels meaning….
energy gap between 2 molecules = 🔺E
🔺E question for NMR
🔺E = yBh/ 2🥧
energy difference between the 2 spin states of a nucleus depends on
precise local environment of the nucleus
the mole of nuclei in the 1H atom all experience and therefore absorb energy at different frequencies, why is this
they are all in slightly different environments
the energy absorbed is influenced by neighbouring atoms (not equal to the external applied magnetic field)
what does the applied magnetic field do to a molecule
induces the electron cloud in the molecule to start moving round and round
like a cloud tornado from above
what does the external magnetic field inducing the electron cloud in a molecule mean
it means that it forms a smaller local magnetic field that acts against the external magnetic field + reduced the overall field a nucleus experiences.
aka the local magnetic field due to the cloud protects/ SHIELDS the nucleus from the external magnetic field
the magnetic field a nucleus experiences is given by
applied magnetic field + local magnetic field
depends on the electron density around the nucleus
value of energy absorbed by the nucleus is affected by
the electron density around the nucleus that forms a local magnetic field.
this depends on the position of the h in the molecule (next to electronegative or not)
the greater the electron density near a nucleus the more …..
shielded it is
can occur due to an electron donating group near the nucleus
reduces the external magnetic field experienced + energy of radio wave radiation needed for resonance.
an electron withdrawing group near the nucleus decreases the amount of electrons near the nucleus andddd
reduces the local magnetic field + SHIELDING
nucleus experiences larger external magnetic field
energy/ frequency of radio wave radiation is INCREASED
poor shielding means
more energy/ frequency needed for resonance
better shielding means
less energy/ frequency needed to experience resonance.
less external magnetic field is experienced
nuclei in diff environments absorb radiation at different energies giving rise tooo
NMR SPECTRA
how does NMR spectra usually displace resonance signals
in terms of their difference when compared to a reference compound TMS (tetra methyl silane) or CFCl3
why are TMS or CFCl3 used as reference compounds in NMR
symmetric ch3 arrangement around the Si means Hs are shielded well.
their signal is out the range normally seen in other compounds.
12 nuclei are in the same environment and so only have 1 peak.
4 carbons have the same environment and therefore also only have 1 peak.
different peaks = diff environments = diff shielding experienced due to the different environments.
