nance (NMR) used to increase the in-
tensities of resonance lines. The large
population differences between the
states given by the Boltzmann distri-
bution gives rise to strong intensities
of resonance lines. In the nuclear
Overhauser effect, spin relaxation is
used to transfer population differ-
ence from one type of nucleus to an-
other type of nucleus so that the
intensities of the resonance lines of
the second type of nucleus are in-
creased. An example of such an en-
hancement, which is widely used, is
between protons and^13 C, in which
case it is possible to attain enhance-
ment of about three times. In the
NOE proton, irradiation is used to
produce the enhancement of the^13 C
line. Another use of the NOE is to de-
termine the distance between pro-
tons.nucleic acidA complex organic
compound in living cells that con-
sists of a chain of *nucleotides. There
are two types: *DNA (deoxyribonu-
cleic acid) and *RNA (ribonucleic
acid).
A- Information on IUPAC nomenclature
nucleonA proton or a neutron.
nucleon number (mass number)
Symbol A. The number of *nucleons
in an atomic nucleus of a particular
nuclide.nucleophileAn ion or molecule
that can donate electrons. Nucle-
ophiles are often oxidizing agents
and Lewis bases. They are either neg-
ative ions (e.g. Cl–) or molecules that
have electron pairs (e.g. NH 3 ). In or-
ganic reactions they tend to attack
positively charged parts of a mol-
ecule. Compare electrophile.nucleophilic additionA type of
addition reaction in which theÜrst
step is attachment of a *nucleophileto a positive (electron-deÜcient) part
of the molecule. *Aldehydes and *ke-
tones undergo reactions of this type
because of polarization of the car-
bonyl group (carbon positive).
nucleophilic substitutionA type
of substitution reaction in which a
*nucleophile displaces another group
or atom from a compound. For exam-
ple, in
CR 3 Cl + OH–→CR 3 OH + Cl–
the nucleophile is the OH–ion. There
are two possible mechanisms of nu-
cleophilic substitution. In SN1 reac-
tions, a positive carbonium ion is
Ürst formed:
CR 3 Cl →CR 3 ++ Cl–
This then reacts with the nucleophile
CR 3 ++ OH–→CR 3 OH
The CR 3 +ion is planar and the OH–
ion can attack from either side. Con-
sequently, if the original molecule is
optically active (the three R groups
are different) then a racemic mixture
of products results.
The alternative mechanism, the
SN2 reaction, is a concerted reaction
in which the nucleophile approaches
from the side of the R groups as the
other group (Cl in the example)
leaves. In this case the conÜguration
of the molecule is inverted. If the
original molecule is optically active,
the product has the opposite activity,
an effect known as Walden inversion.
The notations SN1 and SN2 refer to
the kinetics of the reactions. In the
SN1 mechanism, the slow step is the
Ürst one, which is unimolecular (and
Ürst order in CR 3 Cl). In the SN2 reac-
tion, the process is bimolecular (and
second order overall).
nucleosideAn organic compound
consisting of a nitrogen-containing
*purine or *pyrimidine base linked
to a sugar (ribose or deoxyribose). An
example is *adenosine. Compare nu-
cleotide.nucleic acid 380n