nmr 2 Flashcards
what is resonance in nmr (short version)
when a spin flip occurs
from low energy and aligned with Bo (+1/2)
to high energy and opposing Bo (-1/2)
radiation is absorbed when this occurs
what does resonance depend on
the type of nuclei being studied
the strength of the applied magnetic field Bo.
why does resonance depend on Bo
larger the Bo = the larger the energy gap between the high and low energy spin states
meaning more energy is needed to be absorbed for resonance to occur.
are nuclei isolated in a vacuum
nope,, theyre surrounded by electrons
what do the electrons do to the nucleus
they shield it from the applied magnetic field Bo,,
reduces the total applied magnetic field that the nucleus experiences.
what magnetic field reaches the nucleus then
effective magnetic field reaches the nucleus.
effective magnetic field < applied magnetic field
what equation links magnetic field and frequency
v = y. Bo /2n
if the effective magnetic field is lower than the applied magnetic field,, what about frequency
the observed frequency of precession ‘v’,, will be smaller than the theoretical frequency
why is effective magnetic field smaller than applied magnetic field.
why is observed frequency of precession//larmor frequency smaller than the theoretical frequency
bc electrons are charged and when they spin that create their own magnetic field.
this magnetic field protects the nucleus from the applied magnetic field.
a smaller effective magnetic field means a smaller frequency // speed of procession // larmor frequency.
electron cloud affects the precession / larmor frequency and theeee
the chemical shift
curly partial positive looking thingy <3
chemical shifts differ between
differ between different nuclei
chemical shifts are reported relative to aaaa
a standard
usually at 0ppm
theyre not acc 0ppm but we just say they are lols.
and other things are compared to the 0ppm compounds
0ppm is usually on theee … side of the spectrum
on the right hand side of the spectrum
ppm increases going to the left
the larger the ppm value,,
the less shielded the nuclei
the larger chemical shift
more to the left hand side of the spectrum
the smaller the ppm value
the closer to 0,, the rhs it is
the smaller the chemical shift
the more shielded it is.
what affects ppm
the e- density.
larger ppm meansss
LHS
larger chemical shift
less shielded,, bc it undergoes resonance at more parts per million
a smaller ppm means
to the rhs
larger chemical shift
more shielded,, bc it undergoes resonance at less parts per minute
bc less effective Bo reaches the nuc bc of shielding
what else can affect a nucleus’ chemical shift // signal position
the spin of neighbouring nuclei!!
when 2 neighbouring nuclei have the same magnetism // same alignment what happens
local magnetic field is strengthened
faster precession
higher chemical shift
when 2 neighbouring nuclei have opposite magnetism // opposite alignment what happens
local magnetic field is weakened
slower precession
lower chemical shift
1 signal splits into how many peaks
2I + 1
I as in 👁️
what does each peak represent
a unique combo of nuclear spins
what is I 👁️
spin quantum number of the main character nucleus.
AX2,, whats special about the x’s
they must be the same isotope,, the same nucleus // equivalent nuclei
must have the same environment
when main character nuc is coupled to 2 equivalent nuclei,, how many nucs are there in total
3
when the main nuc is coupled to 2 equivalent nuclei,, how many diff combos are there + explain the spectrum
draw x-A-x
bc its AX2 bc its coupled to 2 equivalent nucs - so theyre both called X.
A must always have an up arrow,, so draw the up arrow for each A.
the X’s can either:
both align (strengthen Bo,, higher shift)
both oppose (weaken Bo, lower shift)
a mix of both : up + down or down + up.
one variation = short
2 variations = tall
one variation = short
so itll be 1:2:1 triplet 💗
coupling to 3 equivalent nuclei AX3,, how many diff combos are there
jot down what we have
x x x A
A will always align 💗
X’s dont always align
4 together
1 vs 3 up
2 vs 2
3 vs 1 up
4 diff ones,, some have more combos tho
this gives a quartet: 1:3:3:1
if all the couples nuclei are aligned in the same way,, will this have a higher or lower chemical shift
higher chemical shift as they will strengthen the magnetic field,, giving a faster precession // larmor frequency which will in turn increase ppm (parts per minute)
coupled nuclei which have the same net magnetism will whatttt
have a larger ppm,,, bc they strengthen eachothers magnetic field,,, they feel a greater effective magnetic field due to the additive nature of their magnetic moments,, they therefore experience a greater magnetic field strength leading to a larger ppm and a larger larmor//precession frequency.
multiplicity refers to…..
the number of peaks there is in the spectrum
how do we find the multiplicity
2nI+1
n is the number of X’s
I is the spin of the X’s
AXn
how do we figure out the intensity of these peaks
we need to use the lovely tree diagram yayaya 🌴
5 peaks
quintet
6 peaks
sextet
7 peaks
septet
when can nuclei couple
when they are connected via bondsss
dont have to be directly bonded together tho!!
what does 3JHH mean
3 bond coupling between 2 H nuclei
theres 3 bonds between the 2 H’s.
coupling constant issss
the gap between peak signals in nmr
measured in hz as this distance differs with magnet strength.
the closer the bond coupling (bonds between the nuclei) theee …. the coupling constant
further apart the coupling constant
so if the nuclei are close together,, the coupling constant increases in length in hz
if the coupling nuclei are further apart,, what happens to the coupling constant
the coupling constant gets smaller
the value of nJXY gets whatt as the number of n increases
as n increases
the nuclei are further apart
meaning the coupling constant,, the nJXY value,, will decrease.
when is the coupling constant too small to see
when the nuclei are too far apart – 3 to 4 bonds are normally the max <3
coupling to non identical nuclei will give
different coupling constants (different gaps between the peaks)
why is hz used for the coupling constant?
bc hz does not change with the strength of the magnetic field.
but ppm will change with the strength of the magnetic field.
coupling constants are constant
equation for ppm
ppm = hz/Bo
when are signals well defined
when using a higher Bo
bc of the ppm = hz/Bo equation
theyre more crispy and skinny
on a low Bo theyre more round and not as crispy.
when I 👁️ is 1 what can ml be
-1, 0, +1
how do we draw the stick diagram for I = 1
draw one line to start
then do 2I + 1
to give 3,, so u should end with 3 lines
so 3 peaks
each peak corresponds to one of the ml values
one for -1, one for 0 and one for +1.
AX and AX2 defines what
the spin system
tells us which nuclei and how many are coupling
what defines the signals appearance,, aka how the spectrums peaks will look like
u need the quantum numbers I and mI to see how many peaks yu have
u also need the spin system
what is a spin active nuclei
a nuclei where I does not equal 0,, but more than 0
bc 2nI + 1 would give 1 if I =0
and 1 means no coupling
it gives a singlet
only one ml value aswell : 0
conditions needed for coupling to occur
- I > 0 or must be 1/2 (2nI+1)
- nucleus X must have a population difference between Na and Nb. nb/na cannot be 1.
if nuclei can couple to other nuclei if theyre I is 1/2 or more,, what does this mean about the spectrum
it means that there can be a bucnh of peaks and it can be overwhelming
we sometimes supress coupling to prevent this.
whats it called when u supress coupling from occuring
decoupling.
how can we decouple something
saturating nucleus with radio frequencies of nucleus X (to prevent nucleus x from having a difference in population)
measure nucleus A.
no coupling can occur bc nucleus x doesnt have a popukation difference and this is one of the things needed for coupling to occur.
decoupling nucleus equation type thing and its explanation
A{X}
A is the observed nucleus
X is the saturated//irradated one that can no longer couple
u can only decouple one nucleus at a time
