present in all atoms. The specific radius of each elec-
tron shell is determined by the energy level of the elec-
trons in that shell. The inner shell of any atom has a
capacity of two electrons, and any further shells have a
capacity of eight electrons. A shell is designated by the
value of the principal quantum number n.
shielding In the context of NMR spectroscopy,
shielding is the effect of the electron shells of the
observed and the neighboring nuclei on the external
magnetic field. The external field induces circulations
in the electron cloud. The resulting magnetic moment is
oriented in the opposite direction to the external field,
so that the local field at the central nucleus is weak-
ened, although it may be strengthened at other nuclei
(deshielding).
The phenomenon is the origin of the structural
dependence of the resonance frequencies of the nuclei.
See alsoCHEMICAL SHIFT.
shielding effect The shielding of the effect of protons
in the nucleus on the outer-shell electrons by the elec-
trons in the filled sets of the sand porbitals.
side chain A part of an amino acid that confers its
identity and can range from a single hydrogen atom to
groups of 15 or more atoms. Each of the 20 amino
acids differs from the others by having a unique side
chain, and the different identity and arrangement of the
atoms in the side chains give different physical and
chemical properties to each amino acid. May also refer
to groups attached to the main backbone of linear
polymers.
siderophore Generic term for Fe(III)-complexing
compounds released into the medium by bacteria for
the purpose of scavenging iron.
See alsoSCAVENGER.
sigma, pi(σ, π) The terms are symmetry designa-
tions, with pi molecular orbitals being antisymmetric
with respect to a defining plane containing at least one
atom (e.g., the molecular plane of ethene), and sigma
molecular orbitals being symmetric with respect to the
same plane. In practice, the terms are used both in this
rigorous sense (for orbitals encompassing the entire
molecule) and also for localized two-center orbitals or
bonds, and it is necessary to make a clear distinction
between the two usages.
In the case of two-center bonds, a pi bond has a
nodal plane that includes the internuclear bond axis,
whereas a sigma bond has no such nodal plane. (A
delta bond in organometallic or inorganic molecular
species has two nodes.) RADICALs are classified by anal-
ogy into sigma and pi radicals.
Such two-center orbitals may take part in molecu-
lar orbitals of sigma or pi symmetry. For example, the
methyl group in propene contains three C–H bonds,
each of which is of local sigma symmetry (i.e., with-
out a nodal plane including the internuclear axis), but
these three “sigma bonds” can in turn be combined to
form a set of group orbitals, one of which has pi sym-
metry with respect to the principal molecular plane
and can accordingly interact with the two-center
orbital of pi symmetry (pi bond) of the double-
bonded carbon atoms to form a molecular orbital of
pi symmetry.
Such an interaction between the CH 3 group and
the double bond is an example of what is called HYPER-
CONJUGATION. This cannot rigorously be described as
“sigma-pi conjugation,” since sigma and pi here refer
to different defining planes, and interaction between
orbitals of different symmetries (with respect to the
same defining plane) is forbidden.sigma (σ) adduct The product formed by the
ATTACHMENTof an ELECTROPHILIC or NUCLEOPHILIC
entering group or of a RADICALto a ring carbon of an
aromatic species so that a new sigma bond is formed
and the original CONJUGATIONis disrupted. (This has
generally been called a σ-complex, but “adduct” is
more appropriate than “complex” according to the
definitions given.) The term may also be used for anal-
ogous adducts to unsaturated (and conjugated) systems
in general.
See alsoMEISENHEIMER ADDUCT.sigma (σ) bond A covalent bond where most of the
electrons are located in between the nuclei. (The distri-sigma (σ) bond 245