W0376

Structure Analysis of Reaction Intermediates Trapped in Crystalline Lattice. Yuji Ohashi, Masaki Kawano and Akifumi Takayama, Department of Chemistry and Materials Science, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8551, Japan.

Many organic reactions proceed by multi step processes in which intermediate molecular species are involved in the conversion from one molecule to another. Such intermediate molecules, in general, are so unstable at room temperature that the structure analysis has been considered to be very difficult. However, the unstable species produced by photo irradiation may be trapped in the crystalline lattice at low temperatures and the structures can be analyzed by X-ray technique.

The first example is a triplet carbene. When a single crystal of 2,2’,4,4’,6,6’-hexachlorodiphenyldiazomethane was irradiated with 365 nm light from a high-pressure mercury lamp at low temperatures below 80 K for 2 h, the crystal turned from yellow to red retaining the single crystal form. The crystal structure after irradiation indicated that a dinitrogen molecule and the hexachlorodiphenylcarbene were produced in the crystalline lattice. The salient feature of the carbene structure is the carbenic angle of the central carbon (142(2)°), which is wider by about 15° than the corresponding angle of the original diazomethane. The second example is a nitrene. When the crystal of 2-azidobiphenyl was irradiated with light (>420 nm) for 5 h, the crystal color changed from colorless to brown with retention of the single crystal form. The structure after irradiation revealed that the molecule is changed to a dinitrogen molecule and a 2-biphenylnitrene due to the cleavage of the azido group. These results made clear the reaction mechanism.