X-Ray Crystal Structure Analyses of Unstable Triplet Carbenes and Nitrenes and Comparison with their Reactivities

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In order to understand the mechanism of chemical reactions, it is essential to obtain the reaction intermediates and to analyze their characteristics. Although the spectroscopic studies have been made clear the energy change of the excited molecules in the time scale of a few femtosecond, the structures of the excited molecule have been obscure till now, since it has been considered to be impossible to trap the excited molecules in a crystalline lattice and to analyze the structures by X-rays. In the present work, we intended to analyze the structures of unstable species such as triplet carbenes and nitrenes. The crystal of the reactant molecule, diazomethane or phenylazide, was cooled below 80 K on the diffractometer using the nitrogen-gas-flow method and was exposed to the high-pressure mercury lamp using a guide tube. The diffraction data were collected after the irradiation at low temperatures. In a difference Fourier map, the new peaks appeared, which were assigned the produced ca … More rbene or nitrene, although only 10% reactant molecules were converted to the unstable species. The structures are in good agreement with those obtained from the theoretical calculation. For the nitrene, it was difficult to obtain the precise structure, since the produced nitrene overlapped on the reactant molecule in the crystalline lattice. The host-guest complex was produced using dibenzylamine as a host. The irradiated crystal was analyzed by the vacuum camera set up at Spring-8. The precise structure of the nitrene was obtained from the host-guest complex. Moreover, the crystal of a platinum complex was exposed to a xenon lamp at 80K. The diffraction data before and during the irradiation were collected and both of the structures were analyzed. The lattice constant clearly changed during the irradiation. The structure during the irradiation clearly indicated that the Pt-Pt distance is shortened at the excited state in accordance with the molecular orbital theory. A11 the above results suggest that the X-ray crystallography will play an important role in analyzing the unstable species in near future. Less