Gas-phase chemical reactions occur when one reactant collides with another followed by rearrangements of the atoms in the activated complex.his process is typically described by the reaction coordinate leading from the reactants to products via a transition state.In general,the transition state is only a whistle-stop for the reactive lux.However,an activated complex can sometimes be transiently trapped in Gas-phase chemical reactions occur when one reactant collides with another followed by rearrangements of the atoms in the activated complex. Process is typically described by the reaction coordinate leading from the reactants to products via a transition state. General, the transition state is only a whistle-stop for the reactive lux. Host, an activated complex can sometimes be transiently trapped in