nmr 1 Flashcards
when do nuclear spins arise
from any unpaired neutrons or protons in the nucleus
if nuclear spins occur due to unpairs protons and neutrons in the nucleus,, when will an atom have a nuclear spin
when they have an odd atomic weight // atomic mass
what is spectroscopy
measuring alterations in the molecular structure when it interacts with light
what alterations in the molecular structure are seen in spectroscopy
movement to different vibrational // electronic energy levels
alterations seen in spectroscopy are
quantised - not random - specific
cannot be in the middle of energy levels
what happens when smt moves to a higher energy level
it has absorbed energy
what happens when smt moves to a lower energy level
it emits energy
it gives out energy
radiation (uv, bis, infrared, microwave, radio wave) is used to name spec whyyyy
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
what radiation is used for nuclear spins
radio waves
describe radio waves
low energy (so temp can be used for it)
high wavelength
nuclei spin quantum number issss
I (eye) 👁️
= n/2
where n is an integer
isotopes have specific values of what
they have specific values of I 👁️
specific nuclei spin numbers
individual nuclei have a spin offff
ml
where ml is from +I to -I 👁️
individual nuclei have a spin offff
ml
where ml is from +I to -I 👁️
nuclei are said to behave like spin magnets bc they’re charged meaning when they spin they create aaaaa
they create a magnetic field 🫶
their own magnetic field
Is I 👁️ always an integer
NOPE 🫶🫶
remember it’s n/2 where n is an integer
so the values of I can be 1/2 etccc
if a nucleus has a spin of I = 1/2,, what would ml be
ml = + I to - I
so ml = 1/2 to -1/2
if there’s no magnetic field itssss
degenerate - meaning equal in energy ‼️‼️‼️‼️
if there is a magnetic field,, the spins are
no longer degenerate aka no longer equal
they split into high and low energy levels
Bo is theeee
magnetic field strength (hz)
when they split into higher and lower energy levels,, what is the ml for the higher energy level
- I
higher in energy
what is the ml for the lower energy when they split
+ I
more stable
the magnetic strength,, Bo alters what
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 (+)
what is the selection rule for NMR spec
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
high energy split is alpha or beta
betaaaaaa b
low energy split is alpha or beta
alphaaaaaa ( more stable so better)
Nb/Na is population at equilibrium,, what else does it equal
= e ^ -🔺E/kT
this cannot equal 1 or else the populations are the same and NMR will not work.
normally there is a higher population on the
Na (lower energy level)
is nmr sensitive,, and why
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
in nmr, can heat be used to transition nuclei into higher energy levels
yessss.
bc the energy between them is low bc NMR uses radio waves which are low in energy
what is y
proportionality constant,, gyromagnetic ratio
v (frequency) equals
y Bo / 2n
n = pi 🥧
when are the ml values degenerate
when there is no external magnetic field
why are the ml values degenerate when there is no external magnetic field
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.
when a nucleus is placed in a magnetic field,, are all ml values the same
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.
are all orientations of the nuclear magnetic moment allowed
nopeee,, nuclear spins are quantised so only some orientations will work.
these also depend on the external/ applied magnetic field.
ml = 1/2 will have how many orientations
2 orientations : +-1/2
2I 👁️ + 1 energy levels.
in nmr,, what does the signal intensity show
the number of nuclei giving rise to a transition
weaker signals are linked to
low gyromagnetic values
small y values
larger signals are linked to
higher gyromagnetic values
larger y values
why is nmr not sensitive
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.
magnetisation
- 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.
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*
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
scroot for net magnetisation
when most nuclei are aligned with the z axis
when they look up
scroot for what happens when EMR of frequency, v, is applied to the z axis at 90*
the nuclei are distubed from their z axis alignement,, they are moved towards the xy plane and rotate around the z axis
what can be measured to give a spectrum
the oscillating magnetisation (the regular movement) induces a current.
this can be measured to give a spectrum
what to think of when we say precession
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.
precession frequency = lamour frequency equation
v = y Bo / 2n
dependent on magnetogyric ratio and the magnetic field strength
do we need to know the gyromagnetic number
nope,, its a constant that differs with each isotope.
applying a radiofrequency pulse at right angles to Bo (the line following the z axis,, that the nuclei are spinning along) what happens
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
unpaired protons// neutrons have different FIDs at different frequencies due to what
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)
high electron density around nucleus
low larmor frequency
slow spin
shielded from Bo
low electron density
less shielding from Bo
larger larmor frequency
faster rate of spinning around the z axis // Bo.
at equilibrium the nuclei is aligned with what (rotating frame)
the z axs
when pulsing / excitation occurs the nuclei are what (rotating frame)
pushed into the xy plane
when relaxation happens what happens (rotating frame)
the nuclei align themslf back with the z axis.
laboratory frame considers theeee
precession of magnetisation
the spinning
this is the photo with the cone above and below the circle/ nuc along the axis
what does the rotating frame do
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.
is the resonance frequency equal to the excitation frequency
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.
what do we like the measure the difference of
the difference between resonance and excitation frequencies
what is induced current
current that is generated
what happens to the induced current over time
it decays,, causing the nuclei to go back into equilibrium ,, they align back with the z axis.
free induction decay - what is measured
RELATIVE FREQUENCIES
diff between excitation and the frequency of the NMR signal
different signals give
different free induction decay
what can we meaure simultaneously
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
raw nmr data // many overlapping signals // fid is converted from a time domain to a
frequency domain
as it separates the different overlapping signals
how does the fourier transform occur
used to separate different signals
signal frequency / operating spec frequency
to give a universal scale
different magnet strength will give a different nmr spectra.
