ETHANOL
The most simple NMR output, or spectrum, for a very simple molecule is shown in this picture on the left. It shows a 1D spectrum for ethanol.
In very simple terms it shows three different peaks representing the three different ‘proton environments’ present in the molecule. These all have different ‘frequencies’.
All protons have broadly similar ‘frequencies’, but there are subtle differences between them, and we may exploit this. These are due to local electronic environment.
Generally speaking, the more electronegative the neighbouring atoms, the greater the extent of de-shielding, and hence the ‘greater’ the frequency. In the above instance, the proton of the hydroxyl group is clearly nearest the most electronegative neighbour. This explains why it appears way to the left of the other protons. Conversely the protons of the methyl group experience the least de-shielding due to a lack electron-withdrawing neighbours (it’s safely tucked away from the oxygen atom, hidden by a methylene group), and it appears furthest to the right hand side.
NMR Interpretation of hexaborane12.8 MHz. range (right)
A simple example of the chemical shift is the "a" and "b" doublets (pairs of peaks).
"A" and "b" are two groups of boron atoms in different electronic environments,
which place the "a" doublet to the right of the "b" doublet in the spectrum.
The relative peak heights of the "a" and "b" doublets suggest a 1:5 ratio of borons
in one environment to borons in another environment.
This suggests, combined with the molecular weight of the molecule:
which in turn suggests a pyramidal structure, which does appear in the atomic diagram.
40 MHz. range (left)
"a" suggests a hydrogen at the apex, which is in the atomic diagram sticking straight up.
"b" suggests hydrogens bonded to one boron each, which are in the
atomic diagram sticking straight out from the edges.
"c" suggests other bridge hydrogens, which is to say hydrogens in the
middle of a boron-hydrogen-boron bonding arrangement like a hydrogen bridge between two boron shores,
which are in a ring around the atomic diagram.
(Williams1959)
History
X-ray diffraction was needed to be sure of the structure.(Hirshfeld1958)
More complex molecules are more challenging, although in some
cases common subgroups of atoms produce characteristic spectral patterns, which can be recognized.
References
(Williams1959) Wiliams, R.E., Gibbins, S.G., and Shapiro, I., J. Chem. Phys, 30, 333 (1959).
(Hirshfeld1958) Hirshfeld, F. L., Eriks, K., Dickerson, R. E., Lippert, E. L., and Lipscomb, W. N.,
"Molecular and Crystal Structure of B6H10,” J. Chem. Phys. 28, 56 (1958).
The two papers above, reviewed in: Eaton GR, Lipscomb, WN. 1969.
NMR Studies of Boron Hydrides and Related Compounds. W. A. Benjamin, Inc.
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COMPLICATED NMR
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