Polyatomic reactions, molecular potential energy

ABSTRACT. The calculation and characterization of molecular potential energy surfaces for polyatomic molecules poses a daunting challenge even in the Age of Supercomputers. We have written a program, STEEP, which computes reaction paths (IRCs) for chemical reactions and characterizes the reaction valley centered on the IRC. This approach requires that only a swath of the potential surface be determined, a computationally tractable problem even for many-atom systems. We report ab initio reaction paths/valleys for two abstraction reactions the OH + H2 reaction, which is a simple, direct process and the H + HCO reaction which can proceed along two distinct pathways, a direct pathway and an addition-elimination pathway. We find that the reaction path/valley method provides many insights into the detailed dynamics of chemical reactions. 

Chang, Y.-T. and Miller, W.H. (1990) An Empirical Valence Bond Model for Constructing Global Potential Energy Surfaces for Chemical Reactions of Polyatomic Molecular Systems, J. Phys. Chem. 94, 5884-5888. 

The Born-Oppenheimer adiabatic approximation represents one of the cornerstones of molecular physics and chemistry. The concept of adiabatic potential-energy surfaces, defined by the Born-Oppenheimer approximation, is fundamental to our thinking about molecular spectroscopy and chemical reaction djmamics. Many chemical processes can be rationalized in terms of the dynamics of the atomic nuclei on a single Born Oppenheimer potential-energy smface. Nonadiabatic processes, that is, chemical processes which involve nuclear djmamics on at least two coupled potential-energy surfaces and thus cannot be rationalized within the Born-Oppenheimer approximation, are nevertheless ubiquitous in chemistry, most notably in photochemistry and photobiology. Typical phenomena associated with a violation of the Born-Oppenheimer approximation are the radiationless relaxation of excited electronic states, photoinduced uni-molecular decay and isomerization processes of polyatomic molecules. 

The computation of internal state densities and partition functions for polyatomic molecules is an essential task in the theoretical treatment of molecular gases. A first principles approach to the statistical thermodynamics of polyatomic gases requires the computation of the internal molecular energy levels based on an ab initio quantum mechanical (QM) determination of portions of the potential energy surface. Likewise, statistical theories of chemical reactions, such as Rice-Ramsberger-KasseUMarcus (RRKM) theory or transition state... 

A fascinating but challenging issue in molecular reaction dynamics is the characterization of reactive resonances in elementary chemical reactions. Since Liu and co-workers experimentally demonstrated the existence of the reactive resonances in the polyatomic reactions of F -f CH4/CHD3/CD4, research interest on the polyatomic reaction of F -I- CH4 and its isotope variants has continued to grow. On the theoretical side and for understanding the reaction mechanism, some attention is focused on the construction of a 12-dimensional ground potential energy surface of the polyatomic system while some is on implementation of dynamical (both QCT and quantum) calculations. ... 

Model for Constructing Global Potential Energy Surfaces for Chemical Reactions of Polyatomic Molecular Systems. 

J. N. Murrell, Potential energy surfaces for studying the reactions and molecular d3mamics of small polyatomic molecules. Specialist Periodical Reports Chem. Soc., Gas Kinetics and Energy Transfer 3 200 (1978). 

The diffusion cloud method can thus be seen to be a potentially useful technique for studying the reactions of laser-excited polyatomic molecules. Since the reactant sodium is monitored, the same technique can be used for a large number of molecular reactants. By measuring the laser power dependence of the reaction rate information can be obtained on both the vibrational energy requirements and the steady-state value of the reaction rates. 

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