Recent progress of X-ray crystal structure analysis enables us to “see” the dynamic processes in crystal. The crystalline-state reaction is one example by which chemical reaction pathway and structure of reaction intermediate can be studied. Two ways to “see” the reaction intermediates in crystal, i.e. the time-resolved measurement and freeze-trapping of reaction in-termediates, are mainly used for in situ observation of crystalline-state reaction. By using the freeze-trapping strategy, crystal structure of triplet carbene was analyzed. In this kind of study, special techniques should be applied carefully in every step in the irradiation of specimen crystal, X-ray diffraction measurement, and structure refinement.
