This species is also often (but not
always [5]) also called an activated complex. A transition structure, in strict usage,
is the saddle point (Fig. 2.8) on a theoretically calculated (e.g. Fig. 2.7) PES.
Normally such a surface is drawn through a set of points each of which represents
the enthalpy of a molecular species at a certain geometry; recall that free energy
differs from enthalpy by temperature times entropy. The transition structure is thus
a saddle point on an enthalpy surface. However, the energy of each of the calculated
points does not normally include the vibrational energy, and even at 0 K a molecule
has such energy (zero point energy: Fig. 2.2, and Section 2.5). The usual calculated
PES is thus a hypothetical, physically unrealistic surface in that it neglects vibra-
tional energy, but it should qualitatively, and even semiquantitatively, resemble the
vibrationally-corrected one since in considering relative enthalpies ZPEs at least
roughly cancel. In accurate work ZPEs are calculated for stationary points and
added to the “frozen-nuclei” energy of the species at the bottom of the reaction
coordinate curve in an attempt to give improved relative energies which represent
enthalpy differences at 0 K (and thus, at this temperature where entropy is zero, free
energy differences also; Fig. 2.19). It is also possible to calculate enthalpy and
entropy differences, and thus free energy differences, at, say, room temperature
(Section 5.5.2). Many chemists do not routinely distinguish between the two terms,
and in this book the commoner term, transition state, is used. Unless indicated
otherwise, it will mean a calculated geometry, the saddle point on a hypothetical
vibrational-energy-free PES.