Determining symmetry changes during a chemical reaction: the case of diazene isomerization

Analysis of reaction paths in terms of the Continuous Symmetry Measure (CSM) provides an alternative way to analyze the geometrical changes that take place during a reaction. Unique symmetry-profiles, describing the symmetry changes along the internal reaction coordinate were calculated for the cis-trans isomerization reaction of N₂H₂ and for its halogeno derivatives. A “symmetry transition point” is identified at the extremum point along the symmetry-profiles. At this point, the deviation of the molecule from the rotational symmetry of the reactant is the same as its deviation from the rotational symmetry of the product. In a second application we show that the CSM can be used as an alternative reaction coordinate. Calculations at the MP2 and DFT levels result in similar symmetry profiles.