c18 JWBS043-Rogers September 13, 2010 11:29 Printer Name: Yet to Come
298 EXPERIMENTAL DETERMINATION OF MOLECULAR STRUCTUREIn NMR, resonance is found by varying the magnetic field until the energy separa-
tion matches an input frequency. At a resonance frequency, protons are both absorbing
and emitting radiation. In addition to a powerful electromagnet and radiation source,
an NMR spectrograph is equipped with a receiver to detect and record the reso-
nance emission frequencies from the sample. A record of the various resonances in a
molecule (there will normally be more than one) is its NMR spectrum.
Examination of the details of molecular structure by NMR is possible because of
chemical shifts. The field under which a given proton acts is primarily the external
field, but it is slightlyshieldedby its surroundings. The electron density around H
(or other nuclei like^13 C) is a function of its chemical environment, particularly the
electronegativity of neighboring atoms. Thus a CH 2 group absorbs and emits radiation
at a different field strength than a CH 3 group. Chemical shielding is normally recorded
in relative terms to minimize differences from one experimental setup to another. One
compound, tetramethylsilane TMS, is usually selected as a reference point and other
resonances are reported relative to it in units of parts per million difference between
the resonance frequency brought about by hydrogens in the sample and those in TMS.
The chemical shift is denotedδ(ppm). Tables of the approximate chemical shift of
various groups are available.
Methyl hydrogens have a chemical shift of about 1 (ppm), CH 2 hydrogens have
δ∼=2 (ppm), and COOH hydrogens haveδ∼=10 (ppm). NMR spectra, like IR spec-
tra, are unique and serve as “fingerprints” of compounds. If a pure unknown has
an NMR spectrum that is identical to an authentic sample, the substances are identi-
cal. NMR is, however, more than an expensive way of carrying out qualitative analysis.
NMR spectra provide information on the internal details of the molecules examined.
Ethanol CH 3 CH 2 OH shows three peaks at low resolution because of the three
distinct kinds of protons CH 3 ,CH 2 , and OH. The peaks are about equally spaced,
but they can be identified because the area under each peak is directly proportional
to the number of protons producing the peak, in the ratio 3:2:1.18.8.1 Spin–Spin Coupling
Spin–spin coupling refers to the minute interaction between the spin field of a proton
and the spin fields of adjacent protons. Normally, a proton with one immediate
neighbor can couple with the adjacent spin in two ways:↑↓and↓↑.^1 A proton with
two neighbors can couple in three ways:↑↑,↑↓ ↓↑, and↓↓. A proton with three
neighbors can couple in four ways. We arrive at a peak multiplicity ofn+1, where
nis the number of neighboring protons. Arrow diagrams show that the intensity of
the split peaks is in the ratio 1:1, 1:2:1, 1:3:3:1, and so on (Fig. 18.9).
Extrapolating these simple examples to more complicated molecules gives an idea
of the use of NMR in experimental studies of molecular structure. Very powerful
magnetic fields are necessary to separate proton and other nuclear peaks in compli-
cated molecular structures. Combination of NMR with the times necessary for various(^1) The case of ethanol is an exception because of fast proton transfer at the OH site, yielding only one peak.