ism of “transition state theory” the transition state of
an elementary reaction is that set of states (each charac-
terized by its own geometry and energy) in which an
assembly of atoms, when randomly placed there,
would have an equal probability of forming the reac-
tants or of forming the products of that elementary
reaction. The transition state is characterized by one
and only one imaginary frequency. The assembly of
atoms at the transition state has been called an ACTI-
VATED COMPLEX. (It is not a COMPLEXaccording to the
definition in this encyclopedia.)
It may be noted that the calculations of reaction
rates by the transition-state method and based on cal-
culated POTENTIAL-ENERGY SURFACEs refer to the
potential-energy maximum at the saddle point, as this
is the only point for which the requisite separability of
transition-state coordinates may be assumed. The ratio
of the number of assemblies of atoms that pass
through to the products to the number of those that
reach the saddle point from the reactants can be less
than unity, and this fraction is the “transmission coef-
ficient” κ. (There are also reactions, such as the gas-
phase COLLIGATIONof simple RADICALs, that do not
require “activation” and which therefore do not
involve a transition state.)
See alsoGIBBS ENERGY OF ACTIVATION; HAMMOND
PRINCIPLE; POTENTIAL-ENERGY PROFILE; TRANSITION
STRUCTURE.
transition-state analog A compound that mimics
the transition state of a substrate bound to an ENZYME.
transition structure A saddle point on a POTENTIAL-
ENERGY SURFACE. It has one negative force constant in
the harmonic force-constant matrix.
See alsoACTIVATED COMPLEX; TRANSITION STATE.
transmission coefficient SeeTRANSITION STATE.
transpiration The loss of water from a plant or tree
by way of the stomata.
transport control SeeMICROSCOPIC DIFFUSION CON-
TROL.
transuranium element A radioactive element with
atomic number greater than that of uranium (at. no. 92).
Up to and including fermium (at. no. 100), the transura-
nium elements are produced by the capture of neutrons.
These include neptunium (at. no. 93), plutonium (at. no.
94), americium (at. no. 95), curium (at. no. 96), berke-
lium (at. no. 97), californium (at. no. 98), einsteinium
(at. no. 99), fermium (at. no. 100), mendelevium (at. no.
101), nobelium (at. no. 102), and lawrencium (at. no.
103), rutherfordium, dubnium, and seaborgium (at. no.
104, 105, and 106, respectively), bohrium (at. no. 107),
hassium, meitnerium, ununnilium, unununium, and
ununbium (at. no. 108 through 112, respectively), unun-
quadium (at. no. 114), and ununhexium (at. no. 116).trapping The interception of a reactive molecule or
reaction intermediate so that it is removed from the
system or converted into a more STABLEform for study
or identification.triboluminescence Optical phenomenon in which
light is generated via friction, e.g., scratching a pepper-
mint in the dark. The term comes from the Greek
tribein(to rub) and the Latin lumin(light). Some min-
erals glow when you scratch them.triple bond A chemical bond consisting of three dis-
tinct covalent bonds linking two atoms in a molecule.triple point Temperature and pressure at which the
three phases of a substance are in equilibrium.tritium An isotope of hydrogen that has one proton
and two neutrons.trophic structure The distribution of the energy flow
and its relationships through the various trophic levels.tropism The movement of a plant toward (positive
tropism) or away (negative tropism) from an environ-
mental stimulus by elongating cells at different rates.
Phototropism is induced by light; gravitropism istropism 269