Chemistry Reference
In-Depth Information
The impact of NMR was revolutionary (Zandvoort 1985, Anet 1996,
Diehl 1996, Feeney 1996) also in bringing a host of new questions to the
fore. These concerned, to mention only a few, nonequivalent groups of
atoms and distinctions among stereoisomers, fluxionality, nonbonded
atomic contacts. NMR gave a boost to the study of stereochemical rela-
tionships in molecules, which the contemporary conformational analysis
had brought to the fore in the 1950s and 1960s (Barton 1972, Eliel 1990).
The A-60 was the instrument of change. Built in Palo Alto, Califor-
nia, by Varian Associates, it was a hands-on, routine instrument that
graduate students in chemistry could operate. Precalibrated charts, to-
gether with an internal lock on a water sample tube, allowed for a single
sweep to record a spectrum (Anon. 1961). The A-60 brought NMR to the
masses. Chemists took to it like fish to water (Jackman 1996).
The NMR tube replaced the test tube. This assertion echoes that by
Victor Hugo when he claimed so perceptively, in
Notre Dame de Paris
,
in the superb 'ceci tuera cela' digression, that when Gutenberg invented
movable types, the topic killed the cathedral and its imagery, which had
been so important earlier in educating Christians.
For centuries, chemists were associated with test tubes, not only in
the popular imagination. The test tube was used primarily for qualitative
analysis. Adding a few drops of a reagent would trigger a response, may-
be a color change, a turbidity, or a precipitation, from which the chemist
would infer the likely presence of a component in the liquid mixture, be
it a ketone or sulfate anions.
With the NMR tube, chemists were offered a lighter and subtler per-
turbation. The reagent now consisted only of radiofrequencies (RF).
Their resonant absorption by the sample in the NMR tube yielded a com-
prehensive inventory of the groups of atoms present. The NMR tube did
not alone replace the test tube, because it came at the heels of other spec-
troscopic cells, such as for electronic absorption (UV-visible) or in vibra-
tional (IR and Raman) spectroscopy.
Moreover, NMR was revolutionary in provoking a
Gestalt
switch (to
borrow Kuhn's phraseology) in the chemists' perceptions. Earlier, mole-
cules were identified by their functional groups, carbonyls, hydroxyls,
and the like, which were like beacons (as reflected in chemical nomen-
clature, in its affixes and priorities). Now, especially at the beginning of
