Chapter 13
Nuclear Magnetic Resonance (NMR) Spectroscopy
Recap:
Mass Spectrometry – molecular formula and size
Infrared spectroscopy – functional groups present or absent
Ultraviolet spectroscopy – existence of a conjugated π-electron system and how many
NMR spectroscopy – map of carbon-hydrogen framework (put the pieces together)
For Organic chemistry, we’re interested in 1H and 13C NMR
Use tetramethylsilane (TMS) as our standard and set its peak at 0.00 ppm or 0.00 δ.
The position on the spectrum is called the chemical shift. Downfield is above 0.00 ppm
or to the left-hand side; upfield is below 0.00 ppm (negative number) or to the right-hand
side.
13
C NMR
In general: sp3-hybridized C’s absorb from 0-90 δ;
sp2-hybridized C’s absorb from 110-220 δ; sp-hybridized C’s absorb from 75-95 δ;
carbonyl C’s (C=O) absorb from 160-220 δ.
Broadband-decoupled: the H’s are also irradiated so that there is no spin-spin splitting
patterns. Results are that there is 1 unique peak for each unique C in the structure.
e.g. 1-bromobutane would show 4 peaks; 1-bromo-2-methylpropane would show 3
peaks.
Distortionless Enhancement by Polarization Transfer (DEPT-NMR): A series of
experiments are run such that the CH3, CH2, CH, and C carbons are distinquished from
one another. In the DEPT-90 spectrum, the CH carbons are identified. In the DEPT-135
spectrum, the CH3 and CH carbons are displaced as positive peaks, and the CH2 carbons
are displayed as negative peaks. The quaternary C’s (no H’s) are identified by noting
which C’s are present in the broadband experiment but never appear in the DEPT
experiments.
1
H NMR
In general: H’s on sp3-hybridized C’s absorb from 0-1.5 δ; H’s on sp2-hybridized C’s
absorb from 4.5-6.5 δ; H’s on sp-hybridized C’s absorb from 2.5-3.0 δ; H’s on aromatic
rings absorb from 6.5-8.0 δ; H’s on carbons bonded to electronegative atoms or adjacent
to carbons bonded to electronegative atoms will shift the absorbance downfield (higher
number).
Each unique type of hydrogen will have its unique chemical shift. However, the peaks
are not often single peaks. Spin-spin splitting is seen due to the neighboring H’s.
Use the “n + 1” rule to determine the splitting or coupling pattern. n = the number of H’s
on the adjacent carbon or carbons.
e.g. CH3 – CH2 – Br
CH3 hydrogens are adjacent to a carbon with 2 H’s; thus
the peak appears as a triplet (2+1)
CH2 hydrogens are adjacent to a carbon with 3 H’s; thus
the peak appears as a quartet (3+1)
The total area of the peaks will be in the correct ratio (2:3); since this will be hard to
determine by sight, the spectrum is usually “integrated” which means that the computer
will draw a line that can be measured to indicate the ratios. (Computer spits out the
number on a printout)
# of adjacent Hydrogens
Coupling or splitting pattern
Ratio
0
1 peak, singlet (s)
1
1
2 peaks, doublet (d)
1:1
2
3 peaks, triplet (t)
1:2:1
3
4 peaks, quartet (q)
1:3:3:1
4
5 peaks, quintet
6
7 peaks, septet
Hydrogens can show coupling from more than one set of H’s = complex pattern!

Chemically equivalent protons do not show spin-spin splitting

The signal of a proton that has n equivalent neighboring protons is split into a
multiplet of n + 1 peaks with coupling constant J.

Two groups of protons coupled to each other have the same coupling constant, J.