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
