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Organic Chemistry, Compound Detection 487

G. MALDI–MS is very useful for studying reaction kinetics and chem-
ical equilibria as well as quantitatively analyzing mix-
Matrix-assisted laser desorption-ionization mass spec-
tures. For example, it was found that in the reduction of
trometry (MALDI–MS) is a practically effective tool for
5,5-dimethylcyclohexen-2-one with LiAlH 4 two products
the measurement of natural and synthetic polymers. The
were obtained, 5,5-dimethylcyclohexen-2-ol and 15% of
ability to ionize a broad range of materials, high sensi-
5,5-dimethylcyclohexanol. This impurity was not previ-
tivity, broad mass range, fast sample preparation, and the
ously detected by other methods.
absence of fragmentation are principal characteristics of
It was only from FTNMR that chemical kinetics and
the technique.
structural analyses of the acid-catalyzed rearrangement of
The MALDI–MS instrument, directs short duration
trans-methyl chrysanthemate to methyl lavandulyl esters
pulses of laser light at a prepared mixture of a sample
could be accurately analyzed.
and a matrix chemical. The laser energy volatilizes a por-
Simplification of NMR spectra of more complex
tion of the matrix/polymer sample (as individual polymer
molecules such as natural products and polymers can be
molecules) which are then ionized. These individual ions
done in three ways:
are accelerated to a fixed energy in an electrostatic field
and directed into a field-free flight tube. The ions then
1. Selective decoupling spin–spin interactions by the
impact into an ion detector, and the time intervals be-
double resonance technique. The sample is simultane-
tween the pulse and laser and impact on the detector are
ously irradiated with two radio-frequency fields; one fre-
measured.
quency gives the resonance pattern of interest, and the
MALDI–MS has found applications in the mass de-
other frequency is the resonance frequency of the coupled
termination of molecules ranging from small peptides,
nuclei.Whenthesecondprotonisirradiatedwithsufficient
oligosaccharides, and oligonucleotides, to large proteins
intensity, the spin–spin coupling collapses giving simple
and synthetic polymers. Polymeric constituents in coal
spectra.
ranging from several hundred molecular weight to 16,000
2. Deuterium exchange of protons. Substitution of
have been mass analyzed by MALDI, and synthetic poly-
deuterium for hydrogen often gives spectra having no
mers of MW up to 70,000 have been reported.
observable coupling to deuterium because of its small
magnetogyric ratio. Protons attached to heteroatoms are
easily exchanged by deuterium such as OH in alcohols,
H. Nuclear Magnetic Resonance Spectroscopy
SH in mercaptans, and NH in amines. The use of reagents
It was over 35 years ago that NMR spectroscopy became containing deuterium such as LiAlD 4 ,D 2 SO 4 , NaBD 4 ,
attractive to organic chemists when Knight reported in CD 3 COCl, D 2 O and CD 3 ND 2 · DCl are just a number of
1949 that the precise frequency of energy absorption de- reagents that can be used in synthesizing complex struc-
pends on the chemical environment of hydrogens. The fre- tures in which the NMR spectra would be much simpler. It
quency of radiation required for NMR absorption depends shouldberealizedthatselectivedecouplingcanthenbeap-
on the isotope and its chemical environment: the number plied to deuterated compounds, thus producing even sim-
of absorption peaks for magnetic nuclei and the inten- pler spectra. Simple proteins can be examined in this way.
sity of the absorption peaks is proportional to the number 3. Complexation of the organic molecule by certain
of nuclei. Before the advent of Fourier transform, NMR paramagnetic metal ions can result in useful changes in
spectra were measured by continuously varying the field- the NMR spectrum. It has been shown that certain
frequency ratio seen by the analytical sample and is re- transition elements such as europium and praesodymium
ferred to as the continuous-wave or slow passage method. induce large shifts in the proton NMR spectra of organic
During the past 15 years FTNMR spectrometers have ap- compounds that coordinate to the metal, for example,
peared which have the advantage over continuous-wave molecules that have certain functional groups that
NMR in that an increased sensitivity for an equal amount contain oxygen or nitrogen. The chloroform soluble tris
of instrument time is obtained, because the entire spectrum (dipivalomethanato)europium (III) complex, Eu(DPM) 3 ,
is observed with each pulse. has been used as a shift reagent to simplify the proton
NMR is a valuable technique in molecular elucida- NMR spectra of compounds, including alcohols, steroids
tion and verification. In some instances, the NMR spec- and terpenoids such as the racemic mixture of the cis-δ-
trum is sufficient for identifying relatively simple un- lactone of cis chrysanthemic acid (Fig. 12). In this instance
known compounds while in other applications struc- the singlets of the methyl moieties are clearly shifted in
tural information from NMR spectra compliments that the presence of the enantiomer (−)-2,2,2-trifluoro-1-(9-
of other chemical and spectroscopic methods. FTNMR anthryl)ethanol or Pirkle’s reagent (Fig. 13).