nmr 3 Flashcards
why do we use H1 nmr aka proton nmrrr
bc its ubiquitous - its present in nearly all organic and inorganic metal complexes
spin 1/2 nuc gives well defined signals which helps with structural interpretation
abundant isotope - H1 is way more abundant than H2 or D
highly receptive // how sensitive it isif all other parameters are the same
what do we use as a referance in nmr
tetramethylsilane , SiMe4 TMS
what chemical shift does TMS have
its said to have a precession frequency, but the chemical shift is said to be 0.
what is the typical ranges of A CHEMICAL SHIFT in nmr spectrum
0-10 ppm
majority of compounds are found in this region.
H2 is also known as
deuterium
what type of solvent do we use in H1 nmr // general
we use deuterated solvents - solvents where most of the H’s are H2 (99.6%),, which are invisible in H1 nmr spec.
some contain inpurities (some H1) which shows up as an impurity signal.
give a deuterated solvent example and their impure versions.
CDCl3 = CHCl3
C6D6 = C6D5H
the H1’s will give a visible impure signal
why do we need to use a deuterated solvent
bc normally in a sample,, there are more H1’s in the solvent than the analyte.
meaning the sample will be analysed.
we use deuterated solvents so that only the H1’s are analysed by H1 nmr.
in CHDCl3,, what will the H couple to
it will couple to the deuterium.
which has a spin of (I=1)
and then 2nI+1 -> 3
so the impurity will be a triplet
the closer the electronegative group,,
the more e- it attracts,, the less shielded the H is,, the larger precession frequency it has,, the larger the chemical shift it has
order Ch, CH2 and CH3 in terms of which one has the greater chemical shift
CH
CH2
CH3
what are the effects of a functional group
theyre additive
so u have to add the effects of both fg and their effects to the chemical shift values
aldehyde chem shift
10ppm
benzene chem shift
8ppm
alkene chem shift
6ppm
heteroatom on same C to the H were measuring chemical shift
4ppm
heteroatom on adjacent c to the H that were measuring the chemical shift of
2ppm
normal alkane
1ppm
alcohol chemical shift range
5-2 ppm
why is the alcohol chem shift range so large
It has hydrogen bonding
H bonds are affected by concentration,,
diff conc = diff num of Hbonds = diff shielding = diff precession frequency = diff chemical shift
different conc = diff chemical shift = diff Hbonds
higher conc = higher ppm = more deshielded
how can we accurately measure the chemical shift of smt when if may have H bonds
DTO shake.
deuterium exchanges protons of OH to D,, no more H bonds,, invisible to H1 NMR spectra + the alcohol peak is removed from the H1 nmr spectrum.
what is OH bonding affected by
temp
concentration
diff temp with OH bonding
diff temp = diff H bonds = diff chemical shift
diff conc with OH bonding
higher conc = more Hbonds = diff shielding = diff chemical shift.
more shielded =
lower chemical shift
more e- to protect proton from EMS
what increases from right to lect on an nmr specta
the energy of the radiofrequency radiation
energy of radio frequency radiation on the rhs of the h1 nmr spectra
low energy RFR
energy on the lhs of the h1 nmr spectra
high radio frequency radiation
position of the signal on nmr spectrum is proportional to what
the radiofrequency radiation
when all arrows arrows are pointing up,, how is the applied magnetic field affected
when theyre all up,, aligned with it,, they strengthen the applied magnetic field
more precession,, higher chemical shift
what is integration
area under the curve
relative to the amount of protons that give that signal // indicates the relative number of hydrogen atoms.
how do we find integration
area under peaks
height of integration curve in mm / height of the smallest integration curve in mm!!!
symmetry has what kind of equilvalence
it has chemical equivalence
what is chemical equivalence
when nuclei have the same chemical environment
the nucs with the same chem environment have the same proximity to other nuclei in the molecule
what is magnetic equivalence
requires nuclei to have identical coupling partners.
identical chemical shifts
identical coupling constants
identical coupling partners
lowkey trick to see magnetic equivalence
see if one of the H’s has other H’s next to it
2 H’s,, are they magnetically equivalent?
do one of them have more H’s next to it?? aka are they coupled to 2 diff things
yes? -> probs not magnetically equivalent
no? -> probs magnetically equivalent.
how to think of magnetic equivalence
marriage + couples
if they are diff couples –> theyre not married aka they dont have magnetic equivalence.
what is 3 bond coupling
when the things that are coupled are 3 bonds apart
what is 4 bond coupling
when the things being coupled are 4 bonds apart
the larger the bond coupling theee
the smaller the magnitude = the smaller the hz
are molecules flat
nope
what is topicity
the sterochemical relationship between 2 apparently identical groups.
what does homotopic mean
when the moelcule is rotated,, it turns out to tbe the same
what does enantiotopic mean
enantiomers of eachother,,, identical signals in nmr.
not the same tho!! their wedges and dashes will be different. one will be r and one will be s
what does diastereotopic mean
2 chiral centres
different nmr signals
one of the chiral centres will switch!!
