hypothetical) made up from the same atoms with the
same types of bonding. (The enthalpy of formation of
cyclopropane is 53.6 kJ mol–1, whereas the enthalpy of
formation based on three “normal” methylene groups,
from acyclic models, is –62 kJ mol–1. On this basis,
cyclopropane is destabilized by ca. 115 kJ mol–1 of
strain energy.)
See alsoMOLECULAR MECHANICS CALCULATION.
stratosphere The section of the atmosphere between
8 and 16 km to 50 km above the surface of the Earth.
Temperature generally increases with increasing height.
strict aerobe An organism that utilizes aerobic respi-
ration in that it can only survive in an atmosphere of
oxygen.
strong acid An acid that is completely dissociated
(ionized) in an aqueous (water) solution, e.g.,
hydrochloric acid (HCl), nitric acid (HNO 3 ).
strong base A metal hydroxide that completely dis-
sociates (ionizes) into water, e.g., potassium hydroxide
(KOH).
strong electrolyte A substance that conducts elec-
tricity well in a dilute aqueous solution and gives a 100
percent yield of ions when dissolved in water. A sub-
stance that is completely converted to ions when dis-
solved in water.
strong-field ligand LIGAND(a molecule or ion that
can bind another molecule) that exerts a strong crystal
or ligand electrical field and generally forms low-spin
complexes with metal ions when possible; causes a
large splitting of d-orbital energy of the metal ion in a
COORDINATION COMPLEX.
structural formula The structural formula of a com-
pound shows how the atoms and bonds are arranged in
the molecule; indicates the arrangement of the atoms in
space.
structural isomers Compounds that contain identical
numbers of identical kinds of atoms in different geomet-
ric arrangements differing in the sequence in which the
atoms or groups of atoms are bonded together.structure-activity relationship(SAR) The relation-
ship between chemical structure and pharmacological
activity for a series of compounds.structure-based design A DRUG design strategy
based on the three-dimensional structure of the target
obtained by X ray or NMR.structure-property correlations(SPC) All statisti-
cal mathematical methods used to correlate any struc-
tural property with any other property (intrinsic,
chemical, or biological) using statistical regression and
PATTERN RECOGNITIONtechniques.subjacent orbital The next-to-highest occupied
molecular orbital (NHOMO, also called HOMO) and the
second lowest unoccupied molecular orbital (SLUMO).
Subjacent orbitals are sometimes found to play an
important role in the interpretation of molecular interac-
tions according to the FRONTIER ORBITALapproach.sublimation The direct vaporization or transition of
a solid directly to a vapor without passing through the
liquid state.substance All forms of matter that have the same
chemical and physical properties.substituent An atom or GROUPof bonded atoms that
can be considered to have replaced a hydrogen atom
(or two hydrogen atoms in the special case of bivalent
groups) in a parent MOLECULAR ENTITY(real or hypo-
thetical).substituent electronegativity See ELECTRONEGA-
TIVITY.258 stratosphere