5 research outputs found
Theoretical Analysis of the Effect of Cî—»C Double Bonds on the Low-Temperature Reactivity of Alkenylperoxy Radicals
Biodiesel contains a large proportion
of unsaturated fatty acid
methyl esters. Its combustion characteristics, especially its ignition
behavior at low temperatures, have been greatly affected by these
Cî—»C double bonds. In this work, we performed a theoretical
analysis of the effect of Cî—»C double bonds on the low-temperature
reactivity of alkenylperoxy radicals, the key intermediates from the
low-temperature combustion of biodiesel. To understand how double
bonds affect the fate of peroxy radicals, we selected three representative
peroxy radicals from heptane, heptene, and heptadiene having zero,
one, and two double Cî—»C bonds, respectively, for study. The
potential energy surfaces were explored at the CBS-QB3 level, and
the reaction rate constants were computed using canonical/variational
transition state theories. We have found that the double bond is responsible
for the very different bond dissociation energies of the various types
of C–H bonds, which in turn affect significantly the reaction
kinetics of alkenylperoxy radicals
Role of Spin-Triplet Polycyclic Aromatic Hydrocarbons in Soot Surface Growth
Using
density functional theory, a possible pathway of soot surface
growth is studied in the low-temperature, postflame region in which
spin-triplet polycyclic aromatic hydrocarbon (PAH) molecules with
a small singlet–triplet energy gap react with unsaturated aliphatics
such as acetylene via the carbon-addition-hydrogen-migration (CAHM)
reaction. Results show that a PAH-core-aliphatic-shell structure is
formed and the mass growth rate of this triplet soot surface growth
reaction is one order of magnitude larger than that of the surface
hydrogen-abstraction-carbon-addition (HACA) reaction at temperatures
below 1500 K
Benchmark Calculations for Bond Dissociation Enthalpies of Unsaturated Methyl Esters and the Bond Dissociation Enthalpies of Methyl Linolenate
It is important to
determine an appropriate computational method
for obtaining accurate thermochemical properties of large biodiesel
molecules such as methyl linolenate. In this study, we use Kohn–Sham
density functional theory (DFT) and coupled cluster theory to calculate
bond dissociation enthalpies (BDEs) of seven fragment molecules of
methyl linolenate, in particular, propene, methyl formate, <i>cis</i>-3-hexene, 1,4-pentadiene, 1-pentene, butane, and methyl
butanoate. The results are compared to BDEs obtained from experiments
and to Oyeyemi et al.’s multireference averaged coupled pair
functional (MRACPF2) calculations. We found that with extrapolation
to the complete basis set (CBS) limit, the BDEs derived from coupled
cluster calculations with single, double, and triple excitations (CCSDT)
and from CCSDT with a perturbative treatment of connected quadruple
excitations, CCSDT(2)<sub>Q</sub>/CBS, are closer to the available
experimental values than those obtained by MRACPF2 for propene and
methyl formate. The CCSDT/CBS calculations were chosen as the reference
for validating the DFT methods. Among the density functionals, we
found that M08-HX has the best performance with a mean unsigned deviation
(MUD) from CCSDT/CBS of only 1.0 kcal/mol, whereas the much more expensive
MRACPF2 has an MUD of 1.1 kcal/mol. We then used the most successfully
validated density functionals to calculate the BDEs of methyl linolenate
and compared the results with the MRACPF2 BDEs. The present study
identifies several Kohn–Sham exchange-correlation functionals
that should be useful for modeling ester combustion, especially the
M08-HX, M06-2X, M05-2X, M08-SO, and MPWB1K global-hybrid meta functionals,
the M11 and MN12-SX range-separated-hybrid meta functionals, the ωB97
range-separated hybrid gradient approximation functional, and the
SOGGA11-X global-hybrid gradient approximation functional
Role of Carbon-Addition and Hydrogen-Migration Reactions in Soot Surface Growth
Using
density functional theory and master equation modeling, we
have studied the kinetics of small unsaturated aliphatic molecules
reacting with polycyclic aromatic hydrocarbon (PAH) molecules having
a diradical character. We have found that these reactions follow the
mechanism of carbon addition and hydrogen migration (CAHM) on both
spin-triplet and open-shell singlet potential energy surfaces at a
rate that is about ten times those of the hydrogen-abstraction-carbon-addition
(HACA) reactions at 1500 K in the fuel-rich postflame region. The
results also show that the most active reaction sites are in the center
of the zigzag edges of the PAHs. Furthermore, the reaction products
are more likely to form straight rather than branched aliphatic side
chains in the case of reacting with diacetylene. The computed rate
constants are also found to be independent of pressure at conditions
of interest in soot formation, and the activation barriers of the
CAHM reactions are linearly correlated with the diradical characters
Dimerization of Polycyclic Aromatic Hydrocarbons in Soot Nucleation
A possible pathway of soot nucleation,
in which localized π
electrons play an important role in binding the polycyclic aromatic
hydrocarbon (PAH) molecules having multiradical characteristics to
form stable polymer molecules through covalent bonds, is studied using
density functional and semiempirical methods. Results show that the
number of covalent bonds formed in the dimerization of two identical
PAHs is determined by the radical character, and the sites to form
bonds are related to the aromaticity of individual six-membered ring
structure. It is further shown that the binding energy of dimerization
increases linearly with the diradical character in the range relevant
to soot nucleation