reacting bond rules
For an internal motion of a molecular entity corresponding to progress over a transition state (energy maximum), any change that makes the motion more difficult will lead to a
new molecular geometry at the energy maximum, in which the motion has proceeded further.
Changes that make the motion less difficult will have the opposite effect. (This rule
corresponds to the Hammond principle).
For an internal motion of a molecular entity that corresponds to a vibration, any
change that tends to modify the equilibrium point of the vibration in a particular
direction will actually shift the equilibrium in that direction.
Effects on reacting bonds (bonds made or broken in the reaction) are the most significant.
The bonds nearest the site of structural change are those most strongly affected.
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1158