The role of seven-membered ring in the photoisomerization and photodissociation of small aromatic molecules

Photoisomerization and photodissociation of toluene, xylene, 4-methyl-pyridine, and aniline under collision-free conditions are reviewed. For many years, it was thought that all the isomerization of aromatic molecules could be described in terms of ring permutation. One important characteristic of ring permutation is that the atoms belonging to the alkyl or amino groups attached to the aromatic ring are not involved in the exchange with the atoms within the aromatic ring. Only the relative positions of these alkyl or amino groups are changed. However, the observation of new dissociation products from toluene, xylene, 4-methyl-pyridine, and aniline suggests that a small fraction of these molecules undergo another kind of isomerization prior to dissociation. This isomerization involves the exchange between six-membered ring and seven-membered ring structures. It provides a new pathway for aromatic molecules to undergo totally different structural change. The significance of this isomerization is that the carbon atoms (or nitrogen atom) and hydrogen atoms belonging to the alkyl group (or amino group) attached to the aromatic ring are involved in an exchange with those atoms in the aromatic ring during the isomerization. The structural change and competition between isomerization and bond dissociation are discussed.