dextrorotatory Refers to an optically active sub-
stance or compound that rotates plane-polarized light
clockwise (to the right) when viewed in the direction of
the light source.
DFO SeeDESFERRIOXAMINE.
diagonal similarities Chemical similarities in the
periodic table. Refers to elements of period 2 and ele-
ments of period 3 (one group to the right), and is espe-
cially evident toward the left of the periodic table.
diamagnetic Substances having a negative MAGNETIC
SUSCEPTIBILITYare diamagnetic. They are repelled by a
magnetic field.
See alsoPARAMAGNETIC.
diamagnetism Weak repulsion by a magnetic field; a
property exhibited by materials to oppose applied mag-
netic fields.
diastereoisomerism Stereoisomerism other than
ENANTIOMERism. DIASTEREOISOMERS (or diastere-
omers) are stereoisomers not related as mirror images.
Diastereoisomers are characterized by differences in
physical properties and by some differences in chemi-
cal behavior toward ACHIRAL as well as chiral
reagents.
diastereoisomers (diastereomers) STEREOISOMERS
not related as mirror images.
See alsoDIASTEREOISOMERISM.
diatom The Bacillariophyceae or diatoms are unicel-
lular algae that are found in single, colonial, or fila-
mentous states. Under the microscope, they are often
beautifully symmetrical, as their cell walls, or frus-
tules, are composed of silica and are bivalved, one of
which overlaps the other, and the frustule is often
punctuated and ornamented. The two orders, Cen-
trales and Pennales, occupy two different environ-
ments. The centric diatoms (Centrales) are circular in
shape with radial symmetry and live mostly in marine
environments. The pennate diatoms (Pennales) are
elliptical in shape, have bilateral symmetry, and are
found in freshwater environments.
Deposits of fossil diatoms known as diatomaceous
earth have been mined and used for years in paints,
abrasives, and other products such as chalk. The
famous White Cliffs of Dover in England (rising to 300
feet) are composed of massive amounts of diatoms—
coccoliths—that were laid down some 790 million
years ago when Great Britain was submerged by a shal-
low sea.diatomic Containing two atoms in the molecule.dielectric constant A measure for the effect of a
medium on the potential energy of interaction
between two charges. It is measured by comparing the
capacity of a capacitor with and without the sample
present.dienophile The olefin component of a Diels-Alder
reaction.
See alsoCYCLOADDITION.differential scanning calorimetry(DSC) A process
that scans temperature and measures heat capacity of a
specimen. Records the energy required to keep a zero
temperature difference between a sample cell and a ref-
erence cell that are either heated or cooled at a con-
trolled rate.differential thermal analysis(DTA) Thermal anal-
ysis comprises a group of methods based on the deter-
mination of changes in chemical or physical properties
of a material as a function of temperature in a con-
trolled atmosphere. DTA is a technique for recording
differences in temperature between a substance and
some reference material against time or temperature
while the two specimens are subjected to identical tem-
perature controls in an environment that is heated or
cooled at a controlled rate.differential thermal analysis 75