Transient structures and chemical reaction dynamics

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

Advances in ultrafast gas-phase x-ray scattering

“…In contrast, scattering measurements provide direct access to all interatomic distances in the molecule. The development of ultrashort-pulse electron sources and x-ray free electron lasers (XFELs) have allowed the implementation of electron and x-ray scattering experiments in the ultrafast domain 2,3 . Scattering techniques have been used to image electrocyclic reactions 4,5,6 , measure coherent vibrational motions 7,8,9,10,11 , monitor photodissociation reactions 2,12 , determine the nature of electronically excited states 13,14,15 , and measure the dynamics of various other A c c e p t e d M a n u s c r i p t 2 molecular systems 16,17 .…”

Infrared Laser Excitation Controlled Reaction Acceleration in the Electronic Ground State

“…In contrast to photoreactions in the electronic excited state, in the ground state excess energies are much smaller and dynamics are typically slower, involving a multitude of coordinates. Unravelling these processes still poses considerable challenges to experiment and theory.…”

Approaching black-box calculations of pump-probe fragmentation dynamics of polyatomic molecules

“…It is used for elucidating reaction mechanisms and tracing transition states along with short-lived species. [1,2,3,4,5,6,7,8,9,10] In this technique one pulse (the pump) excites some inter-/intra-molecular dynamics, and a second delayed pulse (the probe) changes or visualizes the transient dynamics of the molecule, allowing for a scan of the intermediate states. Depending on the pump, the probe, and the observed signal, there is a large variety of different experimental methods to study the induced ultrafast dynamics in molecules: they differ in how the induced pump-probe delay signal is observed.…”

“…Depending on the pump, the probe, and the observed signal, there is a large variety of different experimental methods to study the induced ultrafast dynamics in molecules: they differ in how the induced pump-probe delay signal is observed. For example, the response can be measured with ion/electron spectroscopy [3] by measuring the yields of different electrons and fragments from the molecule; absorption spectroscopy that measures the amount of absorbed photons of certain kind; [4,11,12,13] ultrafast diffraction/microscopy, which investigates the changes in scattering patterns, [8,9,10] etc. An understanding of these types of experiments can be obtained through the framework of explicitly simulating the underlying processes.…”