Encyclopedia of Chemistry

Such parameters have been based on numerous differ-
ent physicochemical quantities, e.g., RATE CONSTANTs,
solvatochromic shifts in ultraviolet/visible spectra, sol-
vent-induced shifts in infrared frequencies, etc. Some
solvent parameters are purely empirical in nature, i.e.,
they are based directly on some experimental mea-
surement. It may be possible to interpret such a
parameter as measuring some particular aspect of sol-
vent-solute interaction, or it may be regarded simply
as a measure of solvent POLARITY. Other solvent
parameters are based on analyzing experimental
results. Such a parameter is considered to quantify
some particular aspect of solvent capability for inter-
action with solutes.
See also DIMROTH-REICHARDT ET PARAMETER;
GRUNWALD-WINSTEIN EQUATION; KAMLET-TAFT SOL-
VENT PARAMETERS; KOPPEL-PALM SOLVENT PARAME-
TERS; SOLVOPHOBICITY PARAMETER; Z-VALUE.

solvent polarity SeePOLARITY.

solvent-separated ion pair SeeION PAIR.

solvolysis Generally, reaction with a solvent or with
a LYONIUM IONor LYATE ION, involving the rupture of
one or more bonds in the reacting solute. More specifi-
cally, the term is used for ELIMINATION, FRAGMENTA-
TION, and SUBSTITUTION REACTIONs in which a solvent
species is the NUCLEOPHILE(“alcoholysis” if the solvent
is an alcohol, etc.).

solvophobicity parameter A SOLVENT PARAMETER
defined by

Sp= 1–M/M(hexadecane)

derived from the GIBBS ENERGYof transfer (∆tGo) of a
series of solutes from water to numerous aqueous-
organic mixtures and to pure solvents:

∆tGo(to solvent) = MRT+ D

where RTis a solute parameter, and M and D charac-
terize the solvent. The M values are used to define a
solvent solvophobic effect so that Spvalues are scaled
from unity (water) to zero (hexadecane).

SOMO A singly occupied molecular orbital (such as

the half-filled HOMO of a radical).

See alsoFRONTIER ORBITALS; HOMO.

sonication Irradiation with (often ultra-) sound

waves, e.g., to increase the rate of a reaction or to pre-

pare vesicles in mixtures of surfactants and water.

sorbital An orbital is the area in space about an

atom or molecule in which the probability of finding

an electron is greatest. There is one s-orbital for each

shell (orbital quantum number l= 0), three p-orbitals,

and five d-orbitals. A spherically symmetrical atomic

orbital.

soret band A very strong absorption band in the

blue region of the optical absorption spectrum of a

HEMEprotein.

SPC SeeSTRUCTURE-PROPERTY CORRELATIONS.

special salt effect The initial steep rate increase

observed in the KINETIC ELECTROLYTE EFFECTon cer-

tain SOLVOLYSISreactions upon addition of some non-

common ion salts, especially LiClO 4.

speciation Refers to the chemical form or compound

in which an element occurs in both nonliving and living

systems. It may also refer to the quantitative distribu-

tion of an element. In biology, it refers to the origina-

tion of a new species.

See alsoBIOAVAILABILITY.

species SeeCHEMICAL SPECIES.

specific catalysis The acceleration of a reaction by a

unique CATALYST, rather than by a family of related

substances. The term is most commonly used in con-

nection with specific hydrogen-ion or hydroxide-ion

(LYONIUM IONor LYATE ION) catalysis.

252 solvent polarity