Chemical Dynamics

This program studies elementary chemical reactions, related non-reactive energy transfer processes, and coupled kinetics processes involved in combustion. Its basic approach is to combine a theoretical effort in the energetics and dynamics of chemical reactions with an experimental effort in dynamics and kinetics under chemically isolated conditions and also  under more complex conditions in flames. The theoretical effort, involving five staff members, embraces both large-scale applications of existing theoretical methods and the development of new methods that efficiently exploit advanced computer architectures. Both electronic structure techniques that determine intermolecular forces and dynamics techniques that determine molecular responses to these forces will be pursued.

Simulations of more complex combustion environments involving coupling kinetics are also being pursued. The experimental effort, involving five staff members,  encompasses state- resolved measurements in flow tubes at low temperatures, thermal reaction kinetics measurements in shock tubes at high tempertatures, photoionization measurements of thresholds and state-resolved product distributions, and in-situ x-ray scattering measurements of sooting flames.  Reaction rates, branching ratios (between different neutral products or between ionic and neutral products), product distributions, the effect of initial vibrational excitation on reactivity, ion-cycles for thermochemical information, and  the morphology and chemistry of soot formation can all be examined. The close coupling between theory and experiment brings a unique combination of expertise to bear on the study of chemical reactivity. This work is designed to provide a fundamental understanding of both major and trace reactions of importance in combustion.

Many of the projects of our group involve several group members and a mixture of expertise that complicates any attempt to organize our projects by authors or by categories.  Nonetheless, in the sections that follow, each of our ten staff will describe their contribution to the group's achievements.  To give a flavor of the group's accomplishments, I cite here  several illustrative achievements:

• Our group initiated and led a theoretical/experimental multi-national-laboratory collaboration that definitively showed that the heat of formation of the OH radical has  been overestimated in all standard thermochemical tables by approximately 0.5 kcal/mol. 
   
• Our group has concluded by systematic experimental measurements and supportive theoretical calculations that  the recombination rate of H+O₂ is an order of magnitude faster in water vapor than in other common buffer gases (e.g., rare gases, oxygen, nitrogen, or methane) because of long-range polar-polar electrostatic interactions.
   
• Our group, in collaboration with theoretical and experimental programs at other DOE laboratories, has  demonstrated that the addition-elimination process CH₃+O → H₂+HCO with a barrier but no saddle point and no steepest descent reaction path can still account for ~20% of the reaction branching ratio. This is the first  documentation of a reaction that cannot be modeled by reaction paths. 
   
• Utilizing state-of-the-art wave packet propagation techniques, the role of excited state and non-adiabatic dynamics in the O(¹D) + H₂ → OH + H reaction was investigated. Extensive calculations, including the ground and two excited electronic states predicted the ratio of the reactive cross sections for rotationally excited and cold H₂. The results disagreed with earlier experiments and motivated a new molecular beam  experiment that agreed quantitatively with the theoretical predictions.
   
• Our group has developed new ways to investigate the long-time dynamics of nonlinear master equations. This has allowed us to develop rate laws to describe association  kinetics and vibrational relaxation. Applications have been made to methyl recombination and the nonlinear vibrational relaxation of oxirane. In both cases, our rate laws model the  process correctly while standard rate laws break down when  reactant concentrations within inert buffer gases become a  few percent or higher.
   
• Our group, in collaboration with computational scientists in the Mathematics and Computer Science Division, has developed a new general way to iteratively solve matrix eigenvalue problems. The method, called SPAM, uses projection operators and a simple matrix that approximates the exact one to accelerate the Davidson iterative method (typically used in electronic structure calculations). The method is general to all eigenvalue problems where physical insight can produce a simple approximate matrix. 
   
• Our group, in collaboration with others in the Chemical Sciences and Engineering Division, has initiated a program of in-situ analysis of nano-scale soot within flames using small angle x-ray scattering (SAXS) at the Advanced Photon Source.  This effort, one of the first SAXS applications in the gas-phase, has discovered detailed structure in soot distributions in laminar flames and has led to development of a prototype detector to monitor transient (e.g., droplet) flames with a time resolution of ~10 µs. Such a detector will be useful in many other areas of chemistry. 
   
• Our group has carried out one of the most detailed state-to-state studies ever performed of vibrational autoionization in a polyatomic molecule, in this case ammonia. Of all the fundamental or combination normal mode excitations tried, initial excitation of the umbrella mode is found to be the most effective in promoting autoionization and the final products of the process involve a change in either electronic symmetry or rotational quantum number depending on the specific autoionizing level.

These accomplishments and others illustrate that our group has increasingly reached out beyond group boundaries to carry out fundamental studies in chemical reactivity. We have always had strong experimental-theoretical interactions within the group and an active collaboration with university programs. However, in the last several years we have collaborated more intimately than before with other parts of the national laboratory system. For example, our involvement within our division (in-situ analysis of nano-scale soot) is expected to be a long-term collaboration driven by a mutual interest in soot chemistry and a complementary background in experimental and theoretical expertise. Likewise, our involvement with computational scientists in other divisions is also long-term and a recognition of the fact that computational chemistry worldwide is one of the leading consumers of computer hardware resources and both a beneficiary and a source of advanced computer software. Our involvement with other national laboratories, especially the Combustion Research Facility at Sandia National Laboratory and the Environmental Molecular Science Laboratory at Pacific Northwest National Laboratory, reflects the complementary expertise that has become centered at those laboratories. The broader involvement by the group has not only furthered our combustion research program but has also won additional funding outside of Chemical Sciences. This additional funding includes Laboratory discretionary funding for the soot studies and Scientific Discovery through Advanced Computing (SciDAC) funding from the Mathematics, Information, and Computer Science (MICS) office in DOE. While different funding sources do not have identical missions, we believe the additional funding we receive will only augment and accelerate the Chemical Sciences-supported program in fundamental combustion research.  

In the future, our group intends to continue to pursue experimental and theoretical studies into the details of chemical reactivity manifested in combustion. We feel this is the "golden age" of combustion research in which effective coupling of experiment and theory can be achieved for increasingly complex chemical reactions that are prevalent but still poorly characterized within combustion. However, the increasing complexity of reactions we are studying and the broader collaborations the group has become involved in suggests future group interests not exclusively tied to gas-phase processes that have been our focus in the past. For example, the soot project will involve us in cluster and agglomeration kinetics that has both gas phase and gas-surface overtones. Furthermore, the experimental and theoretical techniques we develop for soot studies may well be applicable to studies of complex systems outside of combustion, such as molecular self-assembly or chelation kinetics. Another example of a broader study of chemical reactivity our group is involved in is a new collaboration with university researchers into reaction kinetics under carbon nanotube confinement. While all these activities are rooted in our experience and expertise in gas phase combustion research, the research itself is leading the group to a future in which broader issues of chemical reactivity can be addressed beyond the context of combustion but within the fundamental research agenda of Chemical Sciences.

Contact

Stephen T. Pratt, Group Leader
Chemical Dynamics
Chemical Sciences and Engineering Division
Argonne National Laboratory, Bldg. 200
phone: 630/252-4199
fax: 630/252-9292
e-mail: stpratt@anl.gov