1020 24 Magnetic Resonance Spectroscopy
EXAMPLE24.14
Predict the high-resolution proton NMR spectrum of 2-propanol.
Solution
The –OH proton will produce a line that is split into two lines by the proton on the –CH–
carbon, but the methyl protons are too distant to give significant splitting. The line of the
proton on the –CH– carbon is first split into seven lines by the six protons on the –CH 3
carbons, with relative intensities 1 : 6 : 15 : 20 : 15 : 6 : 1. Each of these lines will be split into
two lines by the single –OH proton, giving a total of 14 lines. The six –CH 3 protons produce
a doublet, because the only splitting is from the –CH– proton.
Figure 24.5 shows the high-resolution spectrum of 2-propanol dissolved in deuterated
chloroform. Because of solvent interaction, the doublet from the hydroxyl proton occurs
near 2 ppm instead of farther downfield. We discuss this effect later when we discuss
ethanol. The group of peaks near 4 ppm is from the –CH– proton. The 14 lines overlap
and are not completely resolved. The intense doublet near 1.3 ppm is from the six methyl
protons.The shielding term in Eq. (24.4-5), which gives rise to the chemical shift, is propor-
tional to the applied field, whereas the spin–spin coupling term is independent of the
applied field. If two instruments have different frequencies, say 60 MHz and 300 MHz,
the spin–spin splitting terms will be the same in terms of frequency in both spectra,
but the chemical shifts will be proportional to the magnetic field and thus larger in
the higher-frequency instrument. Comparison of the spectra of the same substance
from two such instruments can aid in interpretation of a spectrum with a number of
overlapping sets of lines.OH
CH 3 CHCH 310 9600 500 400 300 200 100 0
876543210100
90
80
70
60
50
40
30
20
10
0Figure 24.5 The High-Resolution Proton NMR Spectrum of 2-Propanol Dissolved in
Deuterated Chloroform.From Charles J. Pouchert and John R. Campbell,The Aldrich
Library of NMR Spectra, The Aldrich Chemical Co., 1974, Vol. I, p. 85.