Polyatomic reactions, molecular potential

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. ... 

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. 

Valuable insight, particularly with regard to the effects of electronic excitation on reaction cross sections and reaction dynamics, has also been achieved without accurate knowledge of the actual potential surfaces, through the use of molecular-orbital correlation diagrams. Adiabatic correlation rules for neutral reactions involving polyatomic intermediates were developed by Shuler 478 These were adapted and extended for ion-neutral interactions by Mahan and co-workers.192,45 479,480 Electronic-state correlation diagrams have been used to deduce the qualitative nature of the potential surfaces that control ion-neutral reaction dynamics. The dynamics of the reaction N+(H2,H)NH+ and in particular the different behavior of the N + (3P) and N + ( Z)) states,123 for example, have been rationalized from such considerations (see Fig. 62). In this case the... 

F. J. Luque, M. Orozco, P. K. Bhadane and S. R. Gadre, Effect of solvation on the shapes, sizes, and anisotropies of polyatomic anions via molecular electrostatic potential topography An ab initio self-consistent reaction field approach, J. Chem. Phys., 100 (1994) 6718-6726. 

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 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. 

Photochemical Reactions.—The use of correlation diagrams in chemical dynamics has been discussed in a review article,510 and the problem of potential-surface crossing in diatomic511 and polyatomic molecules has been widely considered in several papers.512 A paper has appeared concerned with the quantum-mechanical expression to describe the relaxation processes in a chemically reacting gas under monochromatic (laser) excitation.513 Other quantum theories of molecular photodissociation have also been published.514 These are too extensive for detailed consideration here, but in general provide solvable models for small-molecule reactions. 

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... 

We are still in the situation where it is not possible to cover a polyatomic surface with ab initio points of sufficient accuracy that inaccuracies can be ignored the exceptions are sufficiently rare to prove the rule. For this reason potential functions which are based in part on empirical data will always be superior to those that are fully ab initio. However, it would be exceptional to derive a potential solely from empirical data. Spectroscopic and thermodynamic data generally only give information about minima on the surface often only the lowest minimum. Kinetic data generally only indicate barrier heights on reaction paths. Molecular beam scattering and equilibrium or transport gas phase data may provide sensitive tests of potentials but we cannot get the potential directly from the data. It is from the fusion of empirical and ab initio data that one obtains the best potential functions. This is, I believe, the reason for the success of LEPS fimctions [2], the DIM method [10] and the many-body expansion with empirical one and two-body terms [3]. 

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