Encyclopedia of Chemistry

See alsoGENERAL ACID CATALYSIS; GENERAL BASE
CATALYSIS; PSEUDOCATALYSIS.

specific gravity The ratio of the density of a sub-
stance to the density of water.

specific heat The amount of heat per unit mass
required to move the temperature by 1°C. Every sub-
stance has its own specific heat. Measured in joules per
gram-degree Celsius (J/g°C), the specific heat of water
is 2.02 J/g°C in gas phase, 4.184 in liquid phase, and
2.06 in solid phase.

spectator ion Ions in a solution that do not take part
in a chemical reaction.

spectator mechanism A PRE-ASSOCIATION mecha-
nism in which one of the MOLECULAR ENTITIES, C, is
already present in an ENCOUNTER PAIRwith A during
formation of B from A, but does not assist the forma-
tion of B, for example:

The formation of B from A may itself be a
bimolecular reaction with some other reagent. Since
C does not assist the formation of A, it is described as
being present as a spectator, and hence such a mecha-
nism is sometimes referred to as a spectator mecha-
nism.
See alsoMICROSCOPIC DIFFUSION CONTROL.

spectral line Any of the lines in a spectrum due to
the emission or absorption of ELECTROMAGNETIC RADI-
ATIONat a discrete wavelength; represents the energy
difference between two energy levels. Different atoms
or molecules can be identified by the unique sequence
of spectral lines associated with them.

spectrochemical series A sequence of LIGANDs in
order of increasing ligand field strength.

spectrograph(spectrometer, spectroscope) Any instru-

ment that breaks down ELECTROMAGNETIC RADIATION

into its component frequencies and wavelengths for

study; a photograph or image of a spectrum.

spectrophotometer An instrument that measures the

intensity of light versus its wavelength.

spectrum The display of component wavelengths of

ELECTROMAGNETIC RADIATION.

spin adduct SeeSPIN TRAPPING.

spin counting SeeSPIN TRAPPING.

spin density The unpaired ELECTRON DENSITYat a

position of interest, usually at carbon, in a RADICAL. It

is often measured experimentally by ELECTRON PARAM-

AGNETIC RESONANCE(EPR, ESR [electron spin reso-

nance]) SPECTROSCOPY through HYPERFINE coupling

constants of the atom or an attached hydrogen.

See alsoRADICAL CENTER.

spin label A STABLE PARAMAGNETIC group that is

attached to a part of another MOLECULAR ENTITY

whose microscopic environment is of interest and may

be revealed by the ELECTRON PARAMAGNETIC RESO-

NANCEspectrum of the spin label. When a simple para-

magnetic molecular entity is used in this way without

covalent attachment to the molecular entity of interest,

it is frequently referred to as a “spin probe.”

spin-orbit coupling The interaction of the electron

spin MAGNETIC MOMENTwith the magnetic moment

due to the orbital motion of the electron.

spin probe SeeSPIN LABEL.

spin-spin coupling The interaction between the spin

MAGNETIC MOMENTs of different electrons and/or

nuclei. In NMR SPECTROSCOPYit gives rise to multiple

patterns and cross-peaks in two-dimensional NMR

A+C⇀↽Pre-association (AC))BC

encounter

complex

encounter

complex

→() →rrapid Product

spin-spin coupling 253