nmr 4 Flashcards
whats the problem with the most abundant carbon - C12
such high abundance but it has a spin quantum umber of i = 0
which means its invisible in nmr
what carbon can we use then
we can use the less abundant carbon - 13C
bc it has an i = 1/2
whats the problem with the 13C carbon that is nmr active bc it has i=1/2
its abundance = 1%
what is receptivity
sensitivity of the nucleus regardless of its abundance
is 13C more or less receptive than 1H
receptivity of 13C is much less than that of 1H
less detectedddd
less sensitive than 1H
overall sensitivity equation
receptivity x abundance
is the precession frequency of 13C less than 1H
yess the precession frequency is lesssss
is 13C nmr run alongside 1H nmr
yessss
similarites between 13C and 1H nmr
the same solvents are used
deuterated solvents are used (ones with Deuterium instead of H - although impurities are still seen).
deuterated solvent examples
C6D6
CDCl3
the C in these solvents is detected
does 13C couple to deuterium
yesss
they couple and coupling pattern can be seen using 2ni+1
if most of the deuterated solvent is deuterium,, the 13C signal will be in the
deutero solvent and not the protio solvent
when 13C couples to deuterium in CDCl3 where will the coupling signal be and what will it be
triplet at 77ppm.
when 13C couples to deuterium in CD2Cl2,, what type of peak will it be
13C couples to the 2 D
so we use 2 for n
so 2 x 2 x 1 + 1
which gives 5,, so theres a quintet
diff between 13C and 1H nmr spectra
13C has a larger range of chemical shift values
between 0 - 220 ppm
can be outside of this range tho.
what reference / standard is used for 13C
tetramethylsilane
TMS
a chem shift of 0
baseline of a spectra that is less sensitive
fuzzy,, kinda zigzag like
what do we expect 13C to be coupled to generally
1H
theyre normally bonded together in molecules
1H is very abundant
1H = i = 1/2
what spin system is C in a methyl group + explain
AX3
a = C
bonded to 3 H’s
gives a quartet (2ni+1)
this range is rlly big so we want to supress coupling for it to look nicer
what do we affect when we want to decouple smt
either change the population difference
make i less than 0 or make it 0 .
what coupling do we want to decouple
the non 13C nucleus by saturating it with ITS resonance frequency
normally saturating the 1H nucleus to supress 13C-1H coupling
if we want to saturate H,, what frequency must be supress it using
H,, we must use its resonance frequency.
if the nuclei is in an excited state,, it cannot couple to other things. this prevents coupling
what is proton decoupled spectra // nmr
where we irradiate the 1H to prevent stuff from coupling to it.
this removes X-H peaks
also called X[1H] nmrrrr
typical chem shift range 0-50
aliphatic
C-C=O
C-N
C- halogen
still heavily shielded
typical chem shift 5-100
CtripleC
C-O
typical chem shift 100-150
benzene
C=C
typical chem shift 150-200
C=O
little shielding
shielding is independent offfff
independent of the nucleusss
the nucleus doesnt affect the shielding
the e- affect the shielding
proton nmr shifts are XXXX than 13C chem shifta
they have a lower ppm range
what can symmetry do the the total amount of signals seen
symmetry can reduce the amount of signals seen in a spectraaaa
a spectrum can have low signal / noise after collection of data aka irridation has occurred,, how can we improve the spectrum quality
making multiple identical measurements and averaging the signal
scanning it a bunch of times
for a less sensitive nuc we needddd
we need more scans for a less sensitive nuc
how many scans does 1H need
8-16 scans to make an average and to improve the quality of the nmr spec
how many scans des 13C need
13C is a less sensitive nuclei
it needs more scans
256-2000 scans
between each scan what must we wait for
we must wait for the nuc to relax and for net magnetisation to be aligned with the z axis once again
what nuc relaxes faster
the 1H nuc relaxes faster,, allowing less time between scans and therefor a faster time to get a an imoproved spectra
does 13C nuc relax fast
nopeee
it relaxes slower meaning we need to do more scans bc its less sensitive and wait longer between the scan in order to give a strong signal in the spectra when it gets scanned.
how can we make a nucleus relax faster + increase signal strength
irridate // saturate the nuclei around the wanted nuclei
aka if we want to increase relaxation speed for C,, we would saturate the Hs its bonded to. - to decouple it from them
the more H’s bonded to the C, the larger the integration,, higher the peak.
the more protons a Carbon is bonded to…
the more protons
= faster relaxation
= stronger signals
= larger intergration
signal intensity trend
C< CH<CH2<CH3
larger integration means
a taller peak // a stronger signal
what does DEPT stand for
distortionless enhancement by polarisation transfer
what should we think of when we think of DEPT
Cash is low = 13C
low = some peaks point down
which peaks in DEPT point down
the ones with an even amount of protons.
even 1Hs.
which peaks in DEPT point up
those with a odd number of protons
odd 1H’s.
whats a quaternary carbon
one that isnt bonded to any H’s
what happens to the quaternary C’s in the 13C nmr DEPT experiment
they arent included in ittt
theyre peaks are not visible.
describe cool things about 19F
100% abundance
I = 1/2
its also very sensitive : less scans needed for a large intergration signal.
does 19F behave similar to 1H in a coupling perspective
yes bc they both have i 1/2 and 100% abundance.
what does 19F normally couple to
normally couples to 1H bc compounds with F normally contain H
nmr range of 19F
+ 500 —> -500ppm
can we use the 2ni + 1 rule for F coupling
yesss bc its 100% abundant and has an i=1/2 which is similar to 1H!!!!!
29Si spin and abundance percentage
i = 1/2
abundance = 4.7%
in a 1H nmr spectrum of a silane what bonds would be seen
Si-H
bonds
they form satellites bc the abundance is very low and the rest of Si is inactive due to being i=0.
so nmr would detect all nucs but only some will bc nmr active ::: if the nucs that are coupled to 1H are of low abundance,, satellites will be seen
if Si is 4.7% abundant,, what % of the molecules will contain a 29Si nucleus
4.7% of the molecules will contain a 29Si nuccccc.
this will show a doublet bc
i =1/2
and its coupling to Si
2ni+1
2 x 1 x 1/2 + 1 = 2 (doublet peak)
if in 1H theres smt very abundant and smt not abundant
but the not abundant is nmr active
what will be seen
satellites for the low abundant isotope thats nmr active
also a singlet for the 1H due to the high abundance isotope being nmr inactive.
2 x 1 x 0 + 1 = 1 (singlet)
in the 1H and 29Si situation of silane,, what peaks will be seen
29Si : abundance = 4.7% and 93.3%
singlet for the 1H thats bonded to the 29Si thats inactive!!! 2 x 1 x 0 + 1 = 1
high intensity due to abundance of the inactive isotope!!
satellites with low intensity due to the nmr active low abundance isotope.
these peaks are superimposed
intensity corresponds to abundance of each isotope.
the satellites have what intensity
4.7%
divided by 2
2.35 intensity for each satellite.
describe the peaks seen in the Silane 1H nmr
satellites: 1 on the left of the singlet, 1 on the right of the singlet
satellite coupling constants,, how do we do them
cc will be from one satellite to the other as the 3 peaks are not a triplet but a singlet and satellite.
what does superimposed mean
that the satellites are laid on top of the singlet peakkk
the singlet peak is in the middle of the singletttt.
