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transformation (1) (in chemistry) The con- frequency. The assembly of atoms at the transi-
version of a substrate into a particular prod- tion state has been called an activated complex.
uct, irrespective of reagents or mechanisms (It is not a complex according to the definition in
involved. For example, the transformation of this compendium.)
aniline (C H NH ) into N-phenylacetamide It may be noted that the calculations of reac-
2
6
5
(C H NHCOCH ) may be effected by use of tion rates by the transition state method and
5
6
3
acetyl chloride or acetic anhydride or ketene. A based on calculated potential-energy surfaces
transformation is distinct from a reaction, the full refer to the potential energy maximum at the
description of which would state or imply all the saddle point, as this is the only point for which
reactants and all the products. the requisite separability of transition state coor-
(2) (in mathematics) See function. dinates may be assumed. The ratio of the num-
ber of assemblies of atoms that pass through to
transient (chemical) species Relating to the products to the number of those that reach
a short-lived reaction intermediate. It can be the saddle point from the reactants can be less
defined only in relation to a timescale fixed by than unity, and this fraction is the transmission
the experimental conditions and the limitations coefficient κ. (There are reactions, such as the
of the technique employed in the detection of the gas-phase colligation of simple radicals, that do
intermediate. The term is a relative one. not require activation and which therefore do not
Transient species are sometimes also said to involve a transition state.)
be metastable. However, this latter term should
transition state theory A theory of the rates
be avoided, because it relates a thermodynamic
of elementary reactions which assumes a special
term to a kinetic property, although most
type of equilibrium, having an equilibrium con-
transients are also thermodynamically unstable ‡
with respect to reactants and products. stant K , existing between reactants and acti-
vated complexes. According to this theory the
transient phase (induction period) The rate constant is given by
period that elapses prior to the establishment of ‡
k = (k T /h)K
a steady state. Initially the concentration of a B
reactive intermediate is zero, and it rises to the
where k is the Boltzmann constant and h is the
A
steady-state concentration during the transient
Planck constant. The rate constant can also be
phase.
expressed as
‡
transition function See manifold. k = (k T/h) exp( S /R) exp(− H /RT )
‡ 0
0
B
‡ 0
transition state In theories describing ele- where S , the entropy of activation, is the
mentary reactions it is usually assumed that standard molar change of entropy when the acti-
there is a transition state of more positive molar vated complex is formed from reactants and
‡
Gibbs energy between the reactants and the prod- H , the enthalpy of activation, is the cor-
0
ucts through which an assembly of atoms (ini- responding standard molar change of enthalpy.
tially composing the molecular entities of the The quantities E (the energy of activation) and
a
reactants) must pass on going from reactants to ‡ 0
H are not quite the same, the relationship
products in either direction. In the formalism between them depending on the type of reaction.
of transition state theory the transition state of Also
an elementary reaction is that set of states (each
‡
0
characterized by its own geometry and energy) k = (k T/h) − exp(− G /RT )
B
in which an assembly of atoms, when randomly
‡
0
placed there, would have an equal probability of where G , known as the Gibbs energy of
forming the reactants or of forming the products activation, is the standard molar Gibbs energy
of that elementary reaction. The transition state change for the conversion of reactants into
is characterized by one and only one imaginary activated complex. A plot of standard molar

© 2003 by CRC Press LLC
© 2003 by CRC Press LLC