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physical 3 - spectroscopy > spin resonance spec > Flashcards

spin resonance spec Flashcards

1
Q

electron spin quantum number letter and value

A

s
1/2

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2
Q

how many spin states do electrons have

A

2

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3
Q

spin magnetic quantum number sign and values

A

ms
+1/2
-1/2

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4
Q

what happens to the spins of electrons when they’re paired up

A

the spins cancel out

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5
Q

neutrons and protons also have spins which means

A

nuclei also have spin quantum numbers

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6
Q

what is the nuclear spin quantum number and it’s value

A

I aka i
1/2

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7
Q

what values can I (nuclear spin quantum number) have

A

many values

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8
Q

nuclear spin magnetic quantum number value and sign

A

mi
+ 1/2
- 1/2

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9
Q

whenever the word has magnetic in it what should we had to the 1/2 values

A

+ and - signs

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10
Q

what does a spinning charge have

A

an associated magnetic field
it acts as a small magnet

imagine a blurry electron arrow rotating/ spinning. the blur is the associated magnetic field.

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11
Q

when a spin is placed in an external magnetic field, what is allowed

A

2 different states are allowed
one with higher energy - aligns with magnetic field
one with lower energy - is against the magnetic field

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12
Q

what does the external magnetic field do to the energies of the spin stated

A

caused them to split

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13
Q

what affect on splitting does a stronger magnetic field do

A

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

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14
Q

what about the energy of electrons when there is no external magnetic field

A

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.

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15
Q

what is important abojt the electrons or nuclei that can split when an external magnetic field is added

A

they must have unpaired electrons or else the spins will cancel out.

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16
Q

what is a flip between the states

A

promotion of a nucleus of electron to a higher spin state where it then falls back to a lower state.

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17
Q

what happens in spin resonance spec

A

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.

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18
Q

what do the diff spin state energy’s depend on

A

if the electrons or nucleis magnetic spin aligns or goes against the external magnetic field

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19
Q

what affects the energy change aka distance between the 2 spin state energies

A

the strength or the magnetic field.

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20
Q

what EMS waves correspond to the change in energy between spin stages for nuclei

A

radio waves

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21
Q

what EMS wave corresponds to the energy change between electron spin states

A

MICROWAVES

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22
Q

what are the two types of spin resonance spec

A

electron spin resonance ESR
nuclear magnetic resonance NMR

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23
Q

what is electron spin resonance spec used for

A

studying species with unpaired electrons

eg: radicals

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24
Q

what is NMR used for

A

very useful in structure determination - can find functional groups

1H and 13C application (unpaired electrons)

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25
Q

what nuclear spin value must a nucleus have in order to show energy differences in a magnetic field

A

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

26
Q

what spin do protons, neutrons and electrons all have

A

1/2 spin 💞💞

27
Q

6 neutrons and 6 protons give the nuclear spin quantum value of …. and why

A

I = 0
they pair up and their spins cancel out

28
Q

in 13C there is an unpaired neutron (7 total) meaning it has a nuclear spin quantum number of … meaning it is a

A

I = 1/2

magnetically active nucleus —> gives rise to signals in NMR spectra.

29
Q

what is unfortunate about 13 C

A

although it is magnetically active, the proportion of 13C compared to 12C is incredibly low (1.1%)

30
Q

what is special about 2H deuterium

A

it has an unpaired proton : 1/2
it has an unpaired neutron : 1/2

together it has I = 1

31
Q

🔺E in magnetic spec issss

A

the energy difference between the 2 split spins

this gets bigger when u increase the strength of the magnetic field.

32
Q

what is the boltzmann distribution equation

A

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

33
Q

when the ratio between n up and n low is near 1 what does this mean

A

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.

34
Q

against the magnetic field

A

high energy

35
Q

with the external magnetic field

A

lowest energy

36
Q

what does quantum theory show about the nuclear spins

A

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

37
Q

larmor frequency

A

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 🩰⬆️

38
Q

what happens when the EMS matches the lamor frequency

A

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.

39
Q

how is nmr spectra recorded

A

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.

40
Q

equation for the energy difference between 2 spin states of I = 1/2

A

🔺E = yBh/ 2🥧

change in energy J = ( magnetogyric ratio x strength of mag field x planks) /// 2 🥧

41
Q

relaxation time depends on

A

environment of the nucleus.
gives good info
time it takes for excited nucleus to return to ground state.

42
Q

y (magnetogyric ratio) is different in what

A

different to diff nuclei

43
Q

what does a different y (magnetogyric ratio) mean in terms of 🔺E

A

🔺E depends on what nucleus it is as the magentogyric ratio is different for each nuclei.

44
Q

the larger the mag field, the larger the 🔺E between diff spin states meaningggg

A

higher EMS energy is needed to promote a transition.

45
Q

magnet in NMR spec is usually described as an

A

operating frequency
frequency of the radiation required to bring a 1H nucleus to resonance when it experiences the full applied magnetic field.

46
Q

absorbed radiation of a nuclei is given by the energy difference between two energy spin states why is this

A

EMS can only be absorbed if it matches the energy difference between the 2 levels

47
Q

the energy absorbed = the energy gap between the two levels meaning….

A

energy gap between 2 molecules = 🔺E

48
Q

🔺E question for NMR

A

🔺E = yBh/ 2🥧

49
Q

energy difference between the 2 spin states of a nucleus depends on

A

precise local environment of the nucleus

50
Q

the mole of nuclei in the 1H atom all experience and therefore absorb energy at different frequencies, why is this

A

they are all in slightly different environments

the energy absorbed is influenced by neighbouring atoms (not equal to the external applied magnetic field)

51
Q

what does the applied magnetic field do to a molecule

A

induces the electron cloud in the molecule to start moving round and round

like a cloud tornado from above

52
Q

what does the external magnetic field inducing the electron cloud in a molecule mean

A

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

53
Q

the magnetic field a nucleus experiences is given by

A

applied magnetic field + local magnetic field

depends on the electron density around the nucleus

54
Q

value of energy absorbed by the nucleus is affected by

A

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)

55
Q

the greater the electron density near a nucleus the more …..

A

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.

56
Q

an electron withdrawing group near the nucleus decreases the amount of electrons near the nucleus andddd

A

reduces the local magnetic field + SHIELDING
nucleus experiences larger external magnetic field
energy/ frequency of radio wave radiation is INCREASED

57
Q

poor shielding means

A

more energy/ frequency needed for resonance

58
Q

better shielding means

A

less energy/ frequency needed to experience resonance.

less external magnetic field is experienced

59
Q

nuclei in diff environments absorb radiation at different energies giving rise tooo

A

NMR SPECTRA

60
Q

how does NMR spectra usually displace resonance signals

A

in terms of their difference when compared to a reference compound TMS (tetra methyl silane) or CFCl3

61
Q

why are TMS or CFCl3 used as reference compounds in NMR

A

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.

physical 3 - spectroscopy (12 decks)

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