How to guess a transition state structure (continued)
· If the structures
of the reactant and product are known, can construct a model TS as a ‘midpoint’
between the two.
· For example,
for the reaction H-N-C ® N-C-H, the TS
model could be specified from the internal coordinates for the reactant and
product.
· The H-N-C
angle in the reactant is 180°, whereas it
is 0° in the product. In the TS model,
therefore, a value of H-N-C = 90° could be used as a first guess.
· The bond
lengths (interatomic distances) could also be chosen to be intermediate between
the reactant and product.
Need to be careful in the choice of
coordinates- trying to create a model TS with Cartesian coordinates would cause
problems here (hydrogen between N and C!).