TMS is the acronym for tetramethylsilane, formula Si(CH3)4,
whose protons are arbitrarily given a chemical shift of 0.0 ppm.
This is the 'standard' in 1H NMR spectroscopy and all
other proton resonances, called chemical shifts, are measured
with respect to the TMS, and depend on the
individual (electronic) chemical environment of the hydrogen atoms
in an organic molecule - butan-2-ol here.
The chemical shifts quoted in ppm on the diagram of
the H-1 NMR spectrum of butan-2-ol represent the peaks of the intensity of
the chemical shifts of (which are often groups of split lines at
high resolution) AND the relative integrated areas under the peaks
gives you the ratio of protons in the different chemical
environments of the butan-2-ol molecule.
Interpreting the
H-1 NMR spectrum of
butan-2-ol
In terms of spin-spin coupling from the possible proton magnetic orientations,
for butan-2-ol I
have only considered the interactions of
non-equivalent protons on adjacent carbon atoms
e.g. -CH2-CH3
protons.
For relatively simple molecules, the low
resolution H-1 NMR spectrum of butan-2-ol is a good starting point
(low resolution inset diagram above).
The hydrogen atoms (protons) of butan-2-ol occupy
5
different chemical environments so that the low resolution NMR
spectra should show 5 principal peaks of 5 different H-1 NMR chemical shifts (diagram above for
butan-2-ol).
CH3CH(OH)CH2CH3
Note the proton ratio 3:1:1:2:3 of the 5 colours of the protons
in the 5 chemically different environments
Chemical shifts (a) to (e) on the H-1 NMR
spectrum diagram for butan-2-ol.
Although there are 10 hydrogen atoms in the molecule,
there are only 5 possible different chemical
environments for the hydrogen atoms in butan-2-ol molecule.
The integrated signal proton ratio 3:1:1:2:3 observed
in the high resolution H-1 NMR spectrum, corresponds with
the structural formula of butan-2-ol.
The high resolution 1H NMR
spectrum of butan-2-ol
All low and high resolution spectra of
butan-2-ol
show 5 groups of proton resonances and in the 3:1:1:2:3 ratio expected from the
formula of butan-2-ol.
The ppm quoted on the diagram represent the peak
of resonance intensity for a particular proton group in the
molecule of butan-2-ol - since the peak' is at the apex of a band of
H-1 NMR resonances due to spin - spin coupling field splitting effects - see high resolution
notes on butan-2-ol below.
So, using the chemical shifts and applying the
n+1 rule to
butan-2-ol
and make some predictions using some colour coding! (In problem
solving you work the other way round!)
BUT, an important note about the hydroxyl group on butan-2-ol (for
pre-university students):
Unless the alcohol is completely free of
water (difficult), the hydrogen on the -O-H
hydroxyl group and any hydrogens on the adjacent carbon
don't interact to produce any spin-spin splitting. Therefore
the -OH peak shows up as a singlet and you don't usually
have to consider its effect on any hydrogen atoms, if
present on the adjacent carbon atom (C-OH),
and, neither do you have to consider the splitting effect of
adjacent C-H protons on the hydrogen of the OH group.
(a) 1H
Chemical shift 1.17 ppm for methyl protons: CH3CH(OH)CH2CH3
This 1H resonance is split
into a 1:1 doublet by the adjacent CH proton (n+1 = 2).
Evidence for the presence of a CH group
in the molecule of butan-2-ol
(b) 1H
Chemical shift 3.71 ppm for CH proton: CH3CH(OH)CH2CH3
This 1H resonance is split
into a 1:5:10:10:5:1 sextet by the adjacent CH3
and CH2 protons on either side (n+1 = 6).
Evidence for the presence of a CH3-CHx-CH2 grouping
in the molecule of butan-2-ol
(c) 1H
Chemical shift 1.46 ppm for CH2 protons: CH3CH(OH)CH2CH3
This 1H resonance is split
into a 1:4:6:4:1 quintet by the adjacent CH3
and CH protons on either side (n+1 = 5).
Evidence for the presence of a CH-CHx-CH3 group
in the molecule of butan-2-ol
(d) 1H
Chemical shift 0.93 ppm for methyl protons: CH3CH(OH)CH2CH3
This 1H resonance is split
into a 1:2:1 triplet by the adjacent CH2
protons (n+2 = 3).
Evidence for the presence of a ? group
in the molecule of butan-2-ol
(e) 1H
Chemical shift 2.37 ppm for the hydroxyl proton: CH3CH(OH)CH2CH3
This 1H resonance is observed
as a singlet - assuming no splitting effect from the CH
proton.
Evidence for the presence of an O-H or
an 'isolated' C-H group
(no adjacent C-Hx) in the molecule of butan-2-ol,
but there is no isolated CH proton in the molecule.
Note the decreasing effect on the 1H chemical shift as the
proton is further from the more electronegative oxygen atom in butan-2-ol.
Extra
note
on the OH proton resonance
If the alcohol is impure, containing water or
any source of labile protons, because water and the alcohol exchange protons
e.g.
R-O-H
+ H-O-H
R-O-H
+ H-O-H
This means the CH2 protons no
longer experience a 'simple' local field from one
singlet proton from two possible orientations, but, over
a finite period, experience the averaging effect of
exchanging protons.
This removes the spin - spin coupling effect and
the OH proton resonance just shows up as a singlet if the
butan-1-ol contains even a trace of water
(or acid).
This sort of exchange cannot happen with
the alkyl protons, but is common with molecules
containing a hydroxylic (OH) hydrogen atom like alcohols
and carboxylic acids.
Not only that, you also get proton transfer
between the alcohol molecules i.e.
R-O-H
+ H-O-R
R-O-H
+ H-O-R
which gives the same effect as traces of
water of acid.
So, in
butan-2-ol, all
you usually see in the H-1 NMR spectrum is the mutual splitting of the
CH, CH2
and CH3 proton resonances plus a singlet line
for the OH proton resonance.
