PNNL-13501 - Pacific Northwest National Laboratory

still some key kinetic issues that have yet to be resolved.
Major areas of mechanistic uncertainty include
1) aromatics, 2) OH chemistry of hydrocarbons Cn, with
n>4, 3) O3 chemistry of hydrocarbons Cn with n>2, and
4) NO3 chemistry of alkenes. Paulson’s statements are
supported by Seinfeld’s view, also in JR Baker, Ed.
(1994, Chap. 2, 34-57), that the principal uncertainty in
volatile organic compounds/NOx chemistry lies in the
knowledge about the mechanisms of degradation of the
organic species (Figure 1).
Molecular
Mechanisms
& Rates
ATMOSPHERIC CHEMISTRY MODELS
Condensed
Chemical
Mechanistic
Models
• Uncertainties in reaction rates
• Postulated intermediates and pathways
• Temperature-dependent product yields
Three-Dimensional
Atmospheric
Chemistry &
Transport Models
Figure 1. Interplay between molecular reactions and
atmospheric chemistry and transport models
A revolution in science has taken place over the last 2
decades. This revolution is based on the utilization of
high performance computers to solve the complex
equations that describe natural phenomena. Both
software and hardware developments have contributed to
this revolution. The impact on chemistry and atmospheric
chemistry has already been seen. For example, modeling
has played a significant role in determining the effects of
ozone on the stratosphere and in the formulation of the
Montreal Protocol and its subsequent amendments. In the
area of electronic structure modeling, the methods of
computational quantum chemistry have developed to the
point that both novel insights and chemically accurate
data can be provided for many chemical species and
reactions, including those relevant to atmospheric
processes and combustion processes. Such computational
studies have contributed to the definitive assignment of
reaction pathways and mechanisms through direct
calculations of reaction enthalpies and rate constants.
Accurate computational studies have been important in
helping to experimentally identify novel species based on
calculated infrared and Raman vibrational spectra and
electronic absorption spectra. If one is striving to replace
experimental measurements by calculations, the mark is
for direct calculations of chemical accuracy, generally
defined as less than 1 kcal/mol in bond energies. Ab
initio determination of reaction rates are even more
demanding owing to the exponential dependence of the
rate on the activation energy and the fact that a factor of
236 FY 2000 Laboratory Directed Research and Development Annual Report
two in a rate constant has a very significant effect on the
overall reaction mechanism.
The objective of this work is to demonstrate the impact
that computational chemistry can have on atmospheric
modeling by providing mechanistic chemistry and
reaction rate information about the oxidation of volatile
organic compounds and their degradation byproducts. As
an initial step, we will identify and establish a
collaboration with a leading atmospheric modeling group
that will use the molecular mechanistic and kinetics data
in their atmospheric models. These contacts will put us in
a position to select a small number of model volatile
organic compounds reactions for which we will carry out
structure and kinetic calculations.
Results and Accomplishments
We used modern methods of computational chemistry to
characterize the elementary reaction pathways for OH
reacting with ethene, butadiene, isoprene, and alpha- and
beta-pinene. All of these systems play an important role
in volatile organic compounds oxidation. Selected
adducts and transition states for these systems have been
determined using medium level of theory (UHF with 6-
31G*). Higher levels of theory have been used for the
OH + ethene and OH + butadiene systems (MP2 with
aug-cc-pVDZ and aug-cc-pVTZ basis sets) (Figure 2).
For the simpler system OH + ethene, extensive
calculations have been carried out to map the important
regions of the potential energy surface that play a key role
in the first-principles calculation of reaction rates.
Addition of OH to C=C bonds have a very small
activation energy or no activation energy. Rate
calculations are very sensitive to the minimum energy
pathway minimum energy pathway and the shape of the
energy surface in the vicinity of the minimum energy
pathway. The computed VTST rates shown in Figure 3
correspond to scaled UCCSD(T) computed reaction
energies. While the agreement with experimental rate is
excellent at 400 K, the temperature dependence of the
rates is not well reproduced from the computations. The
discrepancy is attributed to inaccuracies in the calculation
of the partition functions that enter the rate expressions.
The inaccuracies arise from the low frequency reactions
modes as well as the level of electronic structure theory.
The overall methodology (level of accuracy required,
characterization of pathway) is being refined on the
smallest systems OH + ethene and OH + butadiene. The
computational protocol will be applicable to the more
complex systems with activation energies less than
2 cal/mol.