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year 2 spec > nmr 1 > Flashcards

nmr 1 Flashcards

1
Q

when do nuclear spins arise

A

from any unpaired neutrons or protons in the nucleus

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2
Q

if nuclear spins occur due to unpairs protons and neutrons in the nucleus,, when will an atom have a nuclear spin

A

when they have an odd atomic weight // atomic mass

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3
Q

what is spectroscopy

A

measuring alterations in the molecular structure when it interacts with light

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4
Q

what alterations in the molecular structure are seen in spectroscopy

A

movement to different vibrational // electronic energy levels

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5
Q

alterations seen in spectroscopy are

A

quantised - not random - specific

cannot be in the middle of energy levels

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6
Q

what happens when smt moves to a higher energy level

A

it has absorbed energy

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7
Q

what happens when smt moves to a lower energy level

A

it emits energy
it gives out energy

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8
Q

radiation (uv, bis, infrared, microwave, radio wave) is used to name spec whyyyy

A

bc it corresponds to the energy used for transitions to occur at that energy level.

easier to say the radiation type rather than the amount of KJmol-1

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9
Q

what radiation is used for nuclear spins

A

radio waves

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10
Q

describe radio waves

A

low energy (so temp can be used for it)
high wavelength

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11
Q

nuclei spin quantum number issss

A

I (eye) 👁️

= n/2

where n is an integer

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12
Q

isotopes have specific values of what

A

they have specific values of I 👁️
specific nuclei spin numbers

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13
Q

individual nuclei have a spin offff

A

ml
where ml is from +I to -I 👁️

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14
Q

individual nuclei have a spin offff

A

ml
where ml is from +I to -I 👁️

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15
Q

nuclei are said to behave like spin magnets bc they’re charged meaning when they spin they create aaaaa

A

they create a magnetic field 🫶
their own magnetic field

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16
Q

Is I 👁️ always an integer

A

NOPE 🫶🫶

remember it’s n/2 where n is an integer

so the values of I can be 1/2 etccc

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17
Q

if a nucleus has a spin of I = 1/2,, what would ml be

A

ml = + I to - I

so ml = 1/2 to -1/2

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18
Q

if there’s no magnetic field itssss

A

degenerate - meaning equal in energy ‼️‼️‼️‼️

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19
Q

if there is a magnetic field,, the spins are

A

no longer degenerate aka no longer equal

they split into high and low energy levels

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20
Q

Bo is theeee

A

magnetic field strength (hz)

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21
Q

when they split into higher and lower energy levels,, what is the ml for the higher energy level

A
  • I
    higher in energy
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22
Q

what is the ml for the lower energy when they split

A

+ I

more stable

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23
Q

the magnetic strength,, Bo alters what

A

alters the severity of splitting,, the higher the Bo,, the more they split

the larger the energy gap between the high(-) and low energy gap (+)

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24
Q

what is the selection rule for NMR spec

A

change in ml = +- 1

difference between splits of high and low energy can only be +-1

frequency applied must equal the energy gap between the high and low split

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25
Q

high energy split is alpha or beta

A

betaaaaaa b

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26
Q

low energy split is alpha or beta

A

alphaaaaaa ( more stable so better)

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27
Q

Nb/Na is population at equilibrium,, what else does it equal

A

= e ^ -🔺E/kT

this cannot equal 1 or else the populations are the same and NMR will not work.

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28
Q

normally there is a higher population on the

A

Na (lower energy level)

29
Q

is nmr sensitive,, and why

A

not sensitive

  • sensitivity depends on the population difference (Na-Nb) which depends on Bo(affects splitting l) and Bo is very small in NMR spec.

which is why heat can be used - bc the energy gap is so small. bc Bo is small

30
Q

in nmr, can heat be used to transition nuclei into higher energy levels

A

yessss.

bc the energy between them is low bc NMR uses radio waves which are low in energy

31
Q

what is y

A

proportionality constant,, gyromagnetic ratio

32
Q

v (frequency) equals

A

y Bo / 2n

n = pi 🥧

33
Q

when are the ml values degenerate

A

when there is no external magnetic field

34
Q

why are the ml values degenerate when there is no external magnetic field

A

bc there is nothing for them to interact with

think of magnets when they attract//repel

this means the energy of the system is not affected - bc theyre degenarate bc they have nothing to interact with.

35
Q

when a nucleus is placed in a magnetic field,, are all ml values the same

A

nope girl <3

they will be different based on their different orientations.

different orientations = different energies as they will react with the external field differently.

36
Q

are all orientations of the nuclear magnetic moment allowed

A

nopeee,, nuclear spins are quantised so only some orientations will work.

these also depend on the external/ applied magnetic field.

37
Q

ml = 1/2 will have how many orientations

A

2 orientations : +-1/2

2I 👁️ + 1 energy levels.

38
Q

in nmr,, what does the signal intensity show

A

the number of nuclei giving rise to a transition

39
Q

weaker signals are linked to

A

low gyromagnetic values
small y values

40
Q

larger signals are linked to

A

higher gyromagnetic values
larger y values

41
Q

why is nmr not sensitive

A

the difference in population between high energy and low energy are very similar.

they have nealry identical populations in their energy levels - this gives a weak NMR signal as the gyromagnetic ratio would be small.

42
Q

magnetisation

A
  • net alignment of nuclei in the direction of the field.

aka most of the nuclei are aligned in the z axis direction aka UPPP.

nuclei are aligned up,, following the z axis.

43
Q

once the nuclei are aligned with the direction of the field (z axis) what happens if electromagnetic radiation is applied to the z axis at 90*

A

the net magnetisation,M, is perturbated ( it moves from its usual course of motion) towards the xy plane where it precesses (moves around) around the z axis at v=yBo/2n

44
Q

scroot for net magnetisation

A

when most nuclei are aligned with the z axis

when they look up

45
Q

scroot for what happens when EMR of frequency, v, is applied to the z axis at 90*

A

the nuclei are distubed from their z axis alignement,, they are moved towards the xy plane and rotate around the z axis

46
Q

what can be measured to give a spectrum

A

the oscillating magnetisation (the regular movement) induces a current.

this can be measured to give a spectrum

47
Q

what to think of when we say precession

A

think of like a spinner spinning

Bo arrow going up with the Z axis and the spinners spinning aligned to it.

this happens when a magnetic field is applied to the nuclei.

48
Q

precession frequency = lamour frequency equation

A

v = y Bo / 2n

dependent on magnetogyric ratio and the magnetic field strength

49
Q

do we need to know the gyromagnetic number

A

nope,, its a constant that differs with each isotope.

50
Q

applying a radiofrequency pulse at right angles to Bo (the line following the z axis,, that the nuclei are spinning along) what happens

A

nuclei flip down to the xy plane together whilst continuing to spin // precess.

this produces (the magnetisation vector) a current, a signal is detected and amplified.

the oscillating signal has sin wave shape with decreasing magnitude,,, the magnitude decreases as the nucleus realigns with the Bo, magnetic field. FREE INDUCTION DECAY

51
Q

unpaired protons// neutrons have different FIDs at different frequencies due to what

A

due to them being different as theyre in different chemical environments - due to different shielding from electron densities around them.

the electron densities shield the nucleus from the applied magnetic field, Bo, giving it a lower larmor frequency (speed at which it spins around Bo // z axis)

52
Q

high electron density around nucleus

A

low larmor frequency

slow spin

shielded from Bo

53
Q

low electron density

A

less shielding from Bo

larger larmor frequency

faster rate of spinning around the z axis // Bo.

54
Q

at equilibrium the nuclei is aligned with what (rotating frame)

A

the z axs

55
Q

when pulsing / excitation occurs the nuclei are what (rotating frame)

A

pushed into the xy plane

56
Q

when relaxation happens what happens (rotating frame)

A

the nuclei align themslf back with the z axis.

57
Q

laboratory frame considers theeee

A

precession of magnetisation

the spinning

this is the photo with the cone above and below the circle/ nuc along the axis

58
Q

what does the rotating frame do

A

removes the time dependence

removes the rotation

this is the graph type photo where z is the y axis and x is the x axis.

and it shows the arrow being aligned with the z at equilibrium, then excited towards the x, then relaxed back towards the z.

59
Q

is the resonance frequency equal to the excitation frequency

A

nope

bc of shielding due to electrons.

diff nuclei in the same nucleus will have different frequencies bc they may have more e- density around them etc.

60
Q

what do we like the measure the difference of

A

the difference between resonance and excitation frequencies

61
Q

what is induced current

A

current that is generated

62
Q

what happens to the induced current over time

A

it decays,, causing the nuclei to go back into equilibrium ,, they align back with the z axis.

63
Q

free induction decay - what is measured

A

RELATIVE FREQUENCIES

diff between excitation and the frequency of the NMR signal

64
Q

different signals give

A

different free induction decay

65
Q

what can we meaure simultaneously

A

we can measure many resonance frequencies

results in a free induction decay made up of many overlapping signals

they decrease in intensity

gives raw nmr data

66
Q

raw nmr data // many overlapping signals // fid is converted from a time domain to a

A

frequency domain
as it separates the different overlapping signals

67
Q

how does the fourier transform occur

A

used to separate different signals

signal frequency / operating spec frequency

to give a universal scale

68
Q

different magnet strength will give a different nmr spectra.

A

year 2 spec (8 decks)

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