Other contributions to the barrier
·
We must be careful, though
- the energy we calculate for these stationary points on the potential energy
surface (a minimum and a transition state) are energies of static
structures.
· The geometry is specified exactly, and the energy for that geometry is calculated - this energy is the energy of the nuclei and the electrons that surround them.
·
However, we know that a
harmonic oscillator cannot be at rest according to quantum mechanics, so really
we should add the zero-point vibrational energy to the (electronic + nuclear)
energy found from the potential energy surface.
·
We have already seen that
we can use the calculated vibrational frequencies to find the zero-point energy
of a molecule.
·
Although calculating just
the electronic energy barrier can be very useful, really, we should include the
vibrational energy of the molecules in calculating the barrier to
reaction.