Field Of Chemical Dynamics Research Articles

Covariance-Map Imaging: A Powerful Tool for Chemical Dynamics Studies.

Over the past decade or so, the state-of-the-art in the field of chemical reaction dynamics has progressed from studies of few-atom systems to wide-ranging investigations into a variety of photoinduced and collision-induced processes in much larger molecules. Many of these studies are of direct relevance to a wide audience of chemists, spanning fields such as atmospheric chemistry, astrochemistry, synthetic chemistry, and chemical biology. Key to this work has been the technique of velocity-map imaging, which allows complete product scattering distributions to be recorded for the process of interest. Recent advances in camera technology have enabled the development of multimass velocity-map imaging, in which the scattering distributions of all reaction products can be recorded in a single measurement. In addition to the scattering distributions of individual reaction products, the data set now contains information on correlations between the scattering distributions of two or more fragments. These correlations can be revealed using the technique of statistical covariance, yielding an approach known as covariance-map imaging. This review will introduce the reader to covariance mapping and will describe various applications of the technique within the field of chemical dynamics. The underlying concepts will be illustrated through a series of simple simulations, before moving on to describe a number of recent experimental studies in which covariance mapping has been used to obtain mechanistic insight and information on molecular structure on the femtosecond time scale.

Benchmarking the polyatomic reaction dynamics of X+methane

With recent developments of sophisticated experimental techniques and advanced theoretical methods/computations, the field of chemical dynamics has reached the point that theory-experiment comparisons can be made at a quantitative level in very fine details for a prototypical A+BC system. As the system becomes larger, more degrees of freedom are involved and the complexity increases exponentially. At the same time, the multifaceted nature of polyatomic systems also opens up the possibilities for observing many new chemistry and novel phenomena—a land of opportunities. For the past 15 years or so my laboratory has delved into the reaction dynamics of methane+X (X: F, Cl, O(3P), and OH). This effort shifts the paradigm in the field of reaction dynamics by making the title reaction a benchmark polyatomic system. In this account, I shall disclose my thinking behind some of the key concepts and methods we introduced and how the unexpectedly discovered phenomena led to other uncharted territories. Those findings not only enrich our understanding of the specific reactions we studied at the most fundamental level and inspire the theoretical developments, but also shape our thinking and lay the foundation for future explorations of different aspects of the multifaceted nature of polyatomic reactivity.

Ultrafast Chemical Dynamics in Time Domain Through Fluorescence Spectroscopy

While absorption and emission spectroscopy have always been used to detect and characterize molecules and molecular complexes, the availability of ultrashort laser pulses and associated computer-aided optical detection techniques allowed study of chemical processes directly in the time domain at unprecedented time scales, through appearance and disappearance of fluorescence from participating chemical species. Application of such techniques to chemical dynamics in liquids, where many processes occur with picosecond and femtosecond time scales lead to the discovery of a host of new phenomena that in turn led to the development of many new theories. Experiment and theory together provided new and valuable insight into many fundamental chemical processes, like isomerization dynamics, electron and proton transfer reactions, vibrational energy and phase relaxation, photosynthesis, to name just a few. In this article, we shall review a few of such discoveries in attempt to provide a glimpse of the fascinating research employing fluorescence spectroscopy that changed the field of chemical dynamics forever.

Dynamical resonances in chemical reactions

Gas-phase chemical reactions occur when one reactant collides with another followed by rearrangements of the atoms in the activated complex. This process is typically described by the reaction coordinate leading from the reactants to products via a transition state. In general, the transition state is only a whistle-stop for the reactive flux. However, an activated complex can sometimes be transiently trapped in the transition-state region as metastable quantum states. Such dynamical resonances can profoundly influence the dynamics of a chemical reaction. As a result, it has attracted much attention in the field of chemical dynamics over the past 30 years. Although signatures of dynamical resonances have been detected in several reactions [1–4], only the F + H2(HD) resonances have been elucidated [1–3]. It is thus exciting when a research team led by Profs Xueming Yang and Dong Hui Zhang in State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, unequivocally proved the existence of dynamical resonances in the Cl + HD (v = 1) → DCl + H reaction and elucidate them clearly through the state-of-the-art crossed molecular beams experiment and quantum dynamics calculations [5]. Experimentally, the first application of stimulated Raman pumping of HD molecules to the vibrationally excited v= 1 state in a crossed molecular beams study, in combination with the so-called differential cross-sections backward scattering spectroscopy method developed in their own laboratory, is Reaction coordinate Feshbach resonance pathway Shape resonance pathway

An improved Lobatto discrete variable representation by a phase optimisation and variable mapping method

The Lobatto discrete variable representation (LDVR) proposed by Manoloupolos and Wyatt (1988) has unique features but has not been generally applied in the field of chemical dynamics. Instead, it has popular application in solving atomic physics problems, in combining with the finite element method (FE-DVR), due to its inherent abilities for treating the Coulomb singularity in spherical coordinates. In this work, an efficient phase optimisation and variable mapping procedure is proposed to improve the grid efficiency of the LDVR/FE-DVR method, which makes it not only be competing with the popular DVR methods, such as the Sinc-DVR, but also keep its advantages for treating with the Coulomb singularity. The method is illustrated by calculations for one-dimensional Coulomb potential, and the vibrational states of one-dimensional Morse potential, two-dimensional Morse potential and two-dimensional Henon–Heiles potential, which prove the efficiency of the proposed scheme and promise more general applications of the LDVR/FE-DVR method.

Synchronous concerted multiple-body photodissociation of oxalyl chloride explored by ab initio-based dynamics simulations

Photo-induced multiple body dissociation is of fundamental interest in chemistry and physics. A description of the mechanism associated with n-body (n ≥ 3) photodissociation has proven to be an intriguing and yet challenging issue in the field of chemical dynamics. Oxalyl chloride, (ClCO)2, is the sole molecule reported up to date that can undergo four-body dissociation following absorption of a single UV photon, with a rich history of mechanistic debate. In the present work, the combined electronic structure calculations and dynamics simulations have been performed at the advanced level, which provides convincing evidence for resolving the mechanistic debate. More importantly, synchronous and asynchronous concertedness were explored for the first time for the (ClCO)2 photodissociation, which is based on the simulated time constants for the C-C and C-Cl bond fissions. Upon photoexcitation of (ClCO)2 to the S1 state, the adiabatic C-C or C-Cl fission takes place with little possibility. The four-body dissociation to 2Cl((2)P) and 2CO((1)Σ) was determined to a dominant channel with its branch of ∼0.7, while the three-body dissociation to ClCO((2)A(')) + CO((1)Σ) + Cl((2)P) was predicted to play a minor role in the (ClCO)2 photodissociation at 193 nm. Both the four-body and three-body dissociations are non-adiabatic processes, which proceed in a synchronous concerted way as a result of the S1 → S0 internal conversion. There is a little possibility for two-body dissociation to occur in the S0 and S1 states.

Modern development of the reaction coordinate concept

The explicit mathematical definitions of reaction path curves and reaction coordinates are discussed. Applications of these formulations in the field of chemical dynamics and in calculations of transition states are considered.