nmr 6 Flashcards
is nmr a slow process
yupppp
how do we normally see the difference between signals in nmr
we usually see them in ppm units which may look further apart than when we convert it to hz
when we use hz as the units to measure the difference between signals,, what are we measuring
we’re measuring the difference in signal position as a frequency,, in hz
when we measure the difference in frequency between signals in nmr,, what units do we use
we use hz to measure frequency
as nmr is slow,, what sometimes happens
sometimes the inequivalent parts of the molecule rotate faster than their frequency difference
what should we think of when we see dynamic nmr
think of molecules that can rotate faster than the frequency difference between their signals
what does fluxional mean
it can moveeee
explain fluxionality in nmr
it’s when,, let’s say the 3 H’s on a methyl are in 2 diff environments bc in the newman projection one of the Hs is between 2 Br,, although they’re in diff environments,, nmr counts them as one peak and as equal and when the bond rotates,, they move environments. and bc this rotation is faster than the frequency difference of their signals,, it gets counted as one peak
frequency difference between signals numberrrr
triangle v
aka change in frequency
in hz
frequency difference between signals numberrrr
triangle v
aka change in frequency
in hz
when will distinct signals be seen
distinct signals will be seen when the bond rotation is slower than the frequency difference between signals
aka when the rate of exchange (hz) is lower than 2pi delta v
aka change in frequency x 2n
fluxionality and exchange difference
fluxionality is movement
exchange is a change in the chemical environment
what type of change is a bond rotation
a chemical change
what’s special about bond rotations
they have an activation energy
rotations are governed by kinetics,, what does this mean
higher temp = faster rotation
lower temp = slower rotation
why is methyl group always seen as one signal,, even tho they could be in diff environments
bc the rotation of a methyl group has a really low activation energy meaning it rotates fast,, even in lower temperatures
how can we ‘split’ nmr signals and why does this happen
if the bond Ea isn’t that large,, lower temps can cause fast bond rotations,, which could occur faster then the difference in frequency between signals and will lead to 1 signal instead of multiple.
when this happens,, we can lower the temp in nmr,, which reduces the amount of energy it has,, making it less likely to provide it with sufficient energy to overcome the Ea barrier.
which slows // stops rotation
and allows peaks to be separated as the rotation frequency is no longer faster than the frequency diff between signals
when we decrease temp in nmr what happens ( short edition)
the averaged signals
change into separate signals
due to less energy to overcome the Ea of bond rotations
bc the activation energy or the methyl rotation is so low,, how does this affect the nmr spectra
bc the activation energy is so low,, even at low temperatures the system has sufficient energy to overcome the Ea.
this means that the methyl group rotates at a high frequency.
the frequency of this rotation is faster than the difference in frequencies between the 2 signals. this gives us a singlet even tho the Hs can be i. diff environments. the nmr takes an average of them
how can we slow down bond rotation which separates signals
u can lower the temperature
this provides less energy to the system
this makes it less likely for the bond rotation Ea to be reached
bonds will either stop rotating or slow rotation down. this gives us different nmr peaks
explain cyclohexane and nmr - think chair figure
there’s axial and equitorial H’s and these are in different environments - they interconvert tho!!!
at room temp they’re in equilibrium - lowering the temp makes peak split// separate
decoalesces meaning
when the temperature in the nmr is changed and the single peak turns into a broad little peak that u wouldn’t be able to notice normally
type of nmr where a cooling mechanism is equipped in the nmr
variable temperature nmr spec
what does coalescence mean
the merging // joining of elements to make a whole
basically when the temp is changed and the nmr shows one broad kinda big peak 🤟🤟🤟
what is the coalescence temperature
the temp at which the broad peak is produced when changing the temp.
the temp at which the two signals interchange ‼️‼️‼️‼️
what does decoalesce mean
if coalesce is to form a whole (aka the broad peak)
decoalesce is where the broad peak is separated into 2 peaks (the peaks we went to see basically)
is exchange always rapid at room temp
nope 🫶🫶
exchange is not always rapid at room temp - sometimes temp needs to be increased for it to rotate and for groups to interchange.
in dynamic nmr - is temp relative
yuppppp
temp is dynamic
high temp just means higher than room temp
low temp just means lower than room temp
what shape does PF5 have
trigonal bipyramidal!!!
bc 5
2 axial and 3 equitorial
what shape does PF5 have
trigonal bipyramidal!!!
bc 5
2 axial and 3 equitorial
what environments does PF5 have
PF5 has 2 different environments
the axial and the equitorial groups
we say it should have 2 different signals bc of this. 2 different signals for the two different environments
in PF5,, what does the equitorial F couple to in F nmr
F axial (x2)
P
AMX2 spin system
doublet or triplets
MX2 = 3 things,, so u split it 3 times,, and the X2 will have the same coupling constant so they overlap to give a triplet
in PF5,, what does the equitorial F couple to in F nmr
F axial (x2)
P
AMX2 spin system
doublet or triplets
MX2 = 3 things,, so u split it 3 times,, and the X2 will have the same coupling constant so they overlap to give a triplet
in F nmr,, what will the axial F couple to
F equitorial (x3)
P (x1)
AMX3 (split 4 times - 3 things will have the same coupling constant)
split from the M,, this gives a doublet of quartets!!!
in PF5,, what does the nmr spectrum show
it shows only 1 peak,,, even tho there’s 2 inequivalent F environments (axial and equitorial!!) bc they undergo barry pseudo rotation!!!
it still gives a doublet but this still counts as 1 peak 🫶🫶
in PF5,, how do the axial and equitorial interchange
they interchange using barry pseudo rotation!!!
basically just bending of the bonds
the axial bend and the equitorial straighten and this causes them to switch ‼️‼️‼️
in PF5,, how do the axial and equitorial interchange
they interchange using barry pseudo rotation!!!
basically just bending of the bonds
the axial bend and the equitorial straighten and this causes them to switch ‼️‼️‼️
in PF5 do the bonds rotate
nope!!!
the axis of the molecule rotated but the bonds don’t!! they just bend so the axial and equitorial can interchange - barry pseudo rotation
explain fluxionality
when there’s a change in the number of environments🫶🫶
explain exchange
the making and breaking of chemical bondsss ✨✨✨✨✨
barry pseudo rotation is fluxionality or exchange??
fluxionality!!
it changes the number of environments!!
what happens when fluxionality occursssss in coordination complexes
basically when there’s a rearrangement of ligands in the complex - causes there to be different environments.
WITHIN THE COORDINATION SPHERE 🫶🫶🫶🫶🫶
explain an example of intercolecular exchange in coordination compounds
exchange of coordinated ligands with uncoordinated ligands (eg in solution etc)
this could result in a different signal between the coordinated and uncoordinated ligand!!!
what can temp change
temp can change the coupling seen in molecules with dynamic behaviour
change in environment // ligand = things end up coupled to different things
why is spectra sometimes simpler in higher temps
observed coupling mirrors the apparent symmetry // number of environments.
aka they could be different environment - but high temp = fast rotation = it’s averaged into 1 peak rather than many little ones.
Rh (metal ion)
3 Ph3P ligands
what spectra will be used and what will couple to what
P spectra will be useddddd
so we look at the different P environments and what they’re coupled to.
2 P environments (one trans to the +) and 2 cis to it
AMX and AMX2
compounds that show high dynamic behaviour and high temps
simple spectra
faster rotation
different environments are averaged due to fast rotation. they’re perceived to be equivalent : so they’re not coupled to eachother ‼️‼️
gives 1 peak instead of the expected multiple peaks based on the different environments.
compounds that show high dynamic behaviour and high temps
simple spectra
faster rotation
different environments are averaged due to fast rotation. they’re perceived to be equivalent : so they’re not coupled to eachother ‼️‼️
gives 1 peak instead of the expected multiple peaks based on the different environments.
