Polyatomic molecules potential energy surfaces . reaction

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... 

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. 

We first briefly describe the problems in constructing potential energy surfaces for the reactions of polyatomic molecules, we then go on to outline transition state theory, classical trajectory calculations, and mixed quantum classical methods for polyatomic molecules. After this we describe the various quantum scattering theories that have been developed for treating state-selected and state-to-state chemical reactions involving four or more atoms. We also refer to the reactions that have been studied using these treatments. 

It is quite straightforward to perform quasiclassical trajectory computations (QCT) on the reactions of polyatomic molecules providing a smooth global potential energy surface is available from which derivatives can be obtained with respect to the atomic coordinates. This method is described in detail in Classical Trajectory Simulations Final Conditions. Hamilton s equations are solved to follow the motion of the individual atoms as a function of time and the reactant and product vibrational and rotational states can be set or boxed to quantum mechanical energies. The method does not treat purely quantum mechanical effects such as tunneling, resonances. or interference but it can treat the full state-to-state, eneigy-resolved dynamics of a reaction and also produces rate constants. Numerous applications to polyatomic reactions have been reported. ... 

Based on these ideas, we will discuss the simple collision theory, which is attractive because of the relationship it provides between the orientation and the energy of the molecules involved in collisions and their reactivity. However, it is not actually satisfactory as a theoretical hypothesis for polyatomic systans, since it cannot be used quantitatively for the calculation of rates of chemical reactions. However, for triatomic systems whose potential energy surfaces (PESs) are weU defined, the calculation of trajectories using this approach provides an excellent method for calcnlating the reaction rates. Thus, we will restrict this type of calculation to simple systems. Eor polyatomic systems or reactions in... 

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). 

It is possible to use the 3-D surface with its corresponding 2-D contour diagram and potential energy profile to discuss the general reaction A + BC —> AB + C where A, AB, BC and C are all polyatomic molecules. 

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