nmr 7 Flashcards
spec for I>1/2 is different to I=1/2 in what way + why
the peaks are more broad when i >1/2.
this is due to the T2 relaxation occuring
how many different relaxations are there
theres T1 and T2
T2 is only rllu relevant to I>1/2 bc it occurs to slow in the I=1/2 nucs,, so its kinda irrelavent bc T1 finishes before T2.
describe T1
we know this one from last time.
its when the z axis component is restored aftyer being excited to the xy plane. when net magnetisation realigns with the z axis.
dscribe T2
this is when the nucs all precess at different times and start to spread out whilst in the xy plane.
as they spread out, their vectors start to cancel out. the xy axis component in net magnetistion is reduced until 0,, however the nucs are still in the xy plane. depletion of xy component
T1 general
restoration of the z axis component
T2 in general
depletion of the xy axis component
why is t2 not taken into account when u have i=1/2
occurs much slower so it doesnt rlly make a difference
what has a electric quadrupolar moment and what is it
I > 1/2 have an electric quadrupolar moment. (2 dipoles back to back in opposite directions: -++-)
this is when there is an asymmetric charge distribution in the nucleus.
what do I = 1/2 have
they have a symmetric charge distribution in the nucleus
when does fid = 0
when the z axis component is restored.
when there is no xy component in net magnetisation.
so why are peaks broad for i> 1/2
bc when we use the heisenberg uncertainty principle,,, a shorter time (fast t2) = larger change in energy // larger energy uncertainty = broad lines
a longer time = smaller change in energy = sharp peaks
an isotope with a low abundance will have wjat in its spec
it will have satellites in its spec
B nmr spectra range
-40
to
40
Li nmr spec range
-15
to
15 ppm
coupling multiplets for quadrupolar nucs
2nl + 1
but once u get to 2 bond coupling it gets tricky bc the peaks are already broad, so an increase in J = closer together peaks ,, when broad this is tricky to see.
