The two CH 3 groups in isopropyl benzene(Fig. 8) are identical and their
resonance is split into a doublet by the adjacent CH, which is deshielded by the
ring current. The methine resonance is split into a septet by the six equivalent
methyl protons. The aromatic protons are almost equivalent as there is no elec-
tronegative group present to affect their chemical shifts. The integral ratios are
5:1:6.Carbon-13 spectra cover a much wider range of chemical shifts than proton
spectra, but the positions of resonances are generally determined by the same
factors. However, for ease of interpretation, they are often recorded as
decoupledspectra to eliminate the effects of coupling to adjacent protons which
would otherwise split the carbon-13 resonances according to the n+1rule and
Pascal’s triangle. Decoupled spectra consist of a single peak for each chemically
different carbon in the molecule, and spectral interpretation is confined to the
correlation of their chemical shifts with structure, augmented by reference to
chemical shift data and the spectra of known compounds. Proton coupling can
be observed under appropriate experimental conditions. The following is a
general approach.● Note the presence or absence of saturated structures, most of which give
resonances between 0 and 90 d/ppm.
● Note the presence or absence of unsaturated structuresin the region between
about 100 and 160 d/ppm. (N.B. Alkyne carbons are an exception, appearing
between 70 and 100 d/ppm.)
● Note any very low field resonances(160 to 220 d/ppm), which are associated
with carbonyland ethercarbons. Carboxylic acids, anhydrides, esters,
amides, acyl halidesand ethersare all found in the range 160 to 180 d/ppm,
whilst aldehydesand ketoneslie between 180 and about 220 d/ppm.Carbon-13
spectra
E13 – NMRS: interpretation of proton and carbon-13 spectra 267
500 400 300 200
(Hz)
100 08.0 7.0 6.0 5.0 4.0
d (ppm)3.0 2.0 1.0 0CHCH 3 CH 3IsopropylbenzeneFig. 8.^1 H spectrum of isopropyl benzene.