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Abstract

To study transition structures that are formed in real time during a chemical reaction process, it is necessary to use ultrafast methods to follow the structural dynamics of molecular systems. For many decades, optical methods have been used to study the electronic states and time course giving rise to structural intermediates of chemical reactions with an ever higher time resolution. Although in some cases optical methods show many details, ultimately the results of these observations give only indirect information about the structure of the chemical reaction intermediates. Experimental observation of the behaviour of matter in the space-time continuum on ultrashort time scale is the necessary first step to explain and, subsequently, to control the nonequilibrium processes and functionality of the systems under study, to trace the relationship between the elements of the triad 'Structure–Dynamics–Function' to fully understand material properties. The results of these studies provide the necessary new information for testing theoretical approaches to the description of nonequilibrium chemical dynamics of molecular systems. The study of the time sequence of ultrafast processes occurring during the evolution of intermediate structures in the course of chemical reactions requires the integration of information that can be obtained by using complementary diffraction and spectroscopic methods based on various physical phenomena. Integration of data from ultrafast diffraction and spectroscopy makes it possible to investigate timescales prior to the onset of dissipation in which the coherent dynamics of matter can be observed. The use of quantum chemical calculations has reached a level of sophistication that makes it possible to explain the results of these experimental studies, the features of nonadiabatic behaviour of intermediate structures, and transition states of chemical reactions of molecular systems. This review analyzes the achievements of this rapidly developing field of modern chemistry, femtochemistry, to observe the primary events directing chemistry. The bibliography includes 580 references.