Chemical Dynamics in the Gas Phase

Chemical Dynamics in the Gas Phase

Program Overview

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 can not 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 the Carbon Chemistry
group within the 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 in the research summaries to
follow 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 with
the Carbon Chemistry group within our division 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 discretionary (LDRD) 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.

Flash Photolysis-Shock Tube Studies (J. Michael)
Theoretical Studies of Potential Energy Surfaces
(L. Harding)
Theoretical Studies of Potential Energy Surfaces
and Computational Methods (R. Shepard)
Time-Resolved Infrared Absorption Studies of the
Dynamics of Radical Reactions (G. Macdonald)
Quantum Dynamics of Chemical Reactions (S. Gray)
Photoionization Studies of Transient and
Metastable Species
(B. Ruscic)
Optical Probes of Atomic and Molecular Decay
Processes (S. Pratt)
Theoretical Kinetics Studies (A. Wagner)
Geometric Approach to Multiple-Time-Scale
Kinetics (M. Davis)
Elementary Reaction Rate Measurements at
High Temperatures (J. Hessler)

Staff

VeriFlex

Past Research

Parallel MRSDCI Code Developments (R. Shepard)

Return to Research Areas

Glassblowing

Interfacial Processes

Radiation and Photochemistry

Photosynthesis
Biological Materials Growth Facility

Atomic Physics

Coordination Chemistry

f-Electron Interactions

Actinide Facility

Computational Materials and Electrochemical Processes