occurring radioactive substances, e.g., C-14, K-40, U,
and Th and some of their decay products.
NCE SeeNEW CHEMICAL ENTITY.
NDA(new drug application) The process of submit-
ting a new drug for approval. After a new drug appli-
cation (NDA) is received by the federal agency in
charge, it undergoes a technical screening generally
referred to as a completeness review and is evaluated to
ensure that sufficient data and information have been
submitted in each area to justify the filing.
neighboring-group participation The direct interac-
tion of the reaction center (usually, but not necessarily,
an incipient CARBENIUM CENTER) with a lone pair of
electrons of an atom or with the electrons of a SIGMAor
PI BONDcontained within the parent molecule but not
conjugated with the reaction center. A distinction is
sometimes made between n, sigma, and pi participation.
A rate increase due to neighboring group participa-
tion is known as anchimeric assistance. “Synartetic
acceleration” is the special case of anchimeric assis-
tance, ascribed to participation by electrons binding a
substituent to a carbon atom in a β-position relative to
the leaving group attached to the α-carbon atom.
According to the underlying model, these electrons then
provide a three-center bond (or bridge) “fastening
together” (as the word synarteticis intended to suggest)
the α- and β-carbon atoms between which the charge is
divided in the intermediate BRIDGED IONformed (and in
the TRANSITION STATE preceding its formation). The
term synartetic accelerationis not widely used.
See also INTRAMOLECULAR CATALYSIS; MULTICEN-
TER BOND.
Nernst equation An equation that correlates chemi-
cal energy and the electric potential of a galvanic cell or
battery. Links the actual reversible potential of an elec-
trode (measured in volts), E, at nonstandard conditions
of concentration or pressure, to the standard reversible
potential of the electrode couple, E0, which is a ther-
modynamic value. The Nernst equation is named after
the German physical chemist Walther Nernst.
net ionic equation A chemical equation used for a
reaction that lists only those species participating in the
reaction.neuron The basic data processing unit of the nervous
system; a specialized cell that carries information elec-
trically from one part of the body to another by spe-
cialized processes or extensions called dendrites and
axons. Widely branched dendrites carry nerve impulses
toward the cell body, while axons carry them away and
speed up transmitting nerve impulses (conduction)
from one neuron to another. Each neuron has a nucleus
within a cell body.neurotransmitter A chemical made of amino acids
and peptides that switch nerve impulses on or off across
the synapse between neurons. Excitatory neurotransmit-
ters stimulate the target cell, while inhibitory ones inhibit
the target cells. Examples of neurotransmitters are acetyl-
choline, dopamine, noradrenaline, and serotonin.
Acetylcholine is the most abundant neurotrans-
mitter in the body and the primary neurotransmitter
between neurons and muscles and controls the stom-
ach, spleen, bladder, liver, sweat glands, blood ves-
sels, heart, and others. Dopamine is essential to the
normal functioning of the central nervous system.
Noradrenaline, or norepinephrine, acts in the sympa-
thetic nervous system and produces powerful vaso-
constriction. Serotonin is associated with the sleep
cycle.neutralization The resulting reaction when an acid
reacts with a base to form salt and water.neutron An atomic particle found in the nuclei of
atoms that is similar to a PROTONin mass but has no
electric charge.
See alsoELECTRON.new chemical entity A compound not previously
described in the literature.NHOMO SeeSUBJACENT ORBITAL.192 NCE