acid) and conjugated with one or more L-glutamate
units. Folate derivatives are important in DNA synthe-
sis and erythrocyte formation. Folate deficiency leads
to ANEMIA.
force constant A bond property that determines the
steepness of the potential energy parabola confining the
two atoms.
force-field calculations SeeMOLECULAR MECHANICS
CALCULATION.
formal charge A charge given to an atom in a
molecule or polyatomic ion derived from a specific set
of rules.
formation constant SeeSTABILITY CONSTANT.
formula An exact representation of the structure of a
molecule, ion, or compound showing the proportion of
atoms that comprise the material, e.g., H 2 O.
formula unit The smallest possible integer number
of different ions in an ionic compound.
formula weight The sum of the atomic weights of all
atoms found in one formula unit of an ionic com-
pound.
fossil fuel Mostly hydrocarbon material that is
derived from decay of organic materials under geologi-
cal conditions of high pressure and temperature (meta-
morphism), i.e., coal, petroleum, natural gas, peat, and
oil shale. Their combustion is considered part of the
global warming problem.fractional distillation Using a fractionating column
in DISTILLATIONto separate a liquid mixture into com-
ponent parts that have different boiling points. When a
mixture of liquids is boiled, the vapor will be richer in
the component with the lower boiling point, but it will
still be a mixture. By successive CONDENSATION and
revaporization steps (usually in a column called a frac-
tionating column), the vapor becomes increasingly rich
in the lower-boiling component and can eventually be
collected in pure form.fractional precipitation Removing ions by precipi-
tation in a solution while leaving other ions that have
similar properties in the solution.fractionation factor, isotopic The ratio (x 1 /x 2 )A/
(x 1 /x 2 )B, where x is the abundance, expressed as the
atom fraction of the isotope distinguished by the sub-
script numeral when the two isotopes are equilibrated
between two different CHEMICAL SPECIESA and B (or
between specific sites A and B in the same or different
chemical species). The term is most commonly met in
connection with deuterium solvent ISOTOPE EFFECTs,
when the fractionation factor φexpresses the ratio
φ= (xD/xH)solute/(xD/xH)solvent
for the exchangeable hydrogen atoms in the chemical
species (or sites) concerned. The concept is also appli-
cable to TRANSITION STATEs.fragmentation (1) The heterolytic cleavage of a
molecule according to the general reactiona–b–c–d–X →(a–b)++ cd + X–where a–b is an ELECTROFUGEand X is a NUCLEOFUGE
(which may emerge from the reaction in combined
form), and the middle group affords the unsaturated
fragment cd. For example,fragmentation 111